a profit maximization scheme with guaranteed quality of service in

A Profit Maximization Scheme with GuaranteedQuality of Service in Cloud Computing

Jing Mei, Kenli Li,Member, IEEE, Aijia Ouyang, and Keqin Li, Fellow, IEEE

Abstract—As an effective and efficient way to provide computing resources and services to customers on demand, cloud computing

has become more and more popular. From cloud service providers’ perspective, profit is one of the most important considerations,

and it is mainly determined by the configuration of a cloud service platform under given market demand. However, a single long-term

renting scheme is usually adopted to configure a cloud platform, which cannot guarantee the service quality but leads to serious

resource waste. In this paper, a double resource renting scheme is designed firstly in which short-term renting and long-term renting

are combined aiming at the existing issues. This double renting scheme can effectively guarantee the quality of service of all requests

and reduce the resource waste greatly. Secondly, a service system is considered as anM/M/m+D queuing model and the performance

indicators that affect the profit of our double renting scheme are analyzed, e.g., the average charge, the ratio of requests that need

temporary servers, and so forth. Thirdly, a profit maximization problem is formulated for the double renting scheme and the optimized

configuration of a cloud platform is obtained by solving the profit maximization problem. Finally, a series of calculations are conducted

to compare the profit of our proposed scheme with that of the single renting scheme. The results show that our scheme can not only

guarantee the service quality of all requests, but also obtain more profit than the latter.

Index Terms—Cloud computing, guaranteed service quality, multiserver system, profit maximization, queuing model, service-level

agreement, waiting time

Ç

1 INTRODUCTION

AS an effective and efficient way to consolidate comput-ing resources and computing services, clouding com-

puting has become more and more popular [1]. Cloudcomputing centralizes management of resources and serv-ices, and delivers hosted services over the Internet. Thehardware, software, databases, information, and all resour-ces are concentrated and provided to consumers on-demand [2]. Cloud computing turns information technologyinto ordinary commodities and utilities by the the pay-per-use pricing model [3], [4], [5]. In a cloud computing environ-ment, there are always three tiers, i.e., infrastructure pro-viders, services providers, and customers (see Fig. 1 and itselaboration in Section 3.1). An infrastructure provider main-tains the basic hardware and software facilities. A serviceprovider rents resources from the infrastructure providersand provides services to customers. A customer submits itsrequest to a service provider and pays for it based on theamount and the quality of the provided service [6]. In thispaper, we aim at researching the multiserver configurationof a service provider such that its profit is maximized.

Like all business, the profit of a service provider in cloudcomputing is related to two parts, which are the cost andthe revenue. For a service provider, the cost is the rentingcost paid to the infrastructure providers plus the electricitycost caused by energy consumption, and the revenue is theservice charge to customers. In general, a service providerrents a certain number of servers from the infrastructureproviders and builds different multiserver systems for dif-ferent application domains. Each multiserver system is toexecute a special type of service requests and applications.Hence, the renting cost is proportional to the number ofservers in a multiserver system [2]. The power consumptionof a multiserver system is linearly proportional to the num-ber of servers and the server utilization, and to the square ofexecution speed [7], [8]. The revenue of a service provider isrelated to the amount of service and the quality of service.To summarize, the profit of a service provider is mainlydetermined by the configuration of its service platform.

To configure a cloud service platform, a service providerusually adopts a single renting scheme. That’s to say, theservers in the service system are all long-term rented.Because of the limited number of servers, some of theincoming service requests cannot be processed immedi-ately. So they are first inserted into a queue until they canhandled by any available server. However, the waiting timeof the service requests cannot be too long. In order to satisfyquality-of-service requirements, the waiting time of eachincoming service request should be limited within a certainrange, which is determined by a service-level agreement(SLA). If the quality of service is guaranteed, the service isfully charged, otherwise, the service provider serves therequest for free as a penalty of low quality. To obtain higherrevenue, a service provider should rent more servers fromthe infrastructure providers or scale up the server execution

� J. Mei, K. Li, and A. Ouyang are with the College of Information Scienceand Engineering, Hunan University, and National SupercomputingCenter in Changsha, Changsha 410082, Hunan, China.E-mail: [email protected], [email protected].

� Keqin Li is with the College of Information Science and Engineering,Hunan University, and National Supercomputing Center in Changsha,Changsha 410082, Hunan, China, and the Department of ComputerScience, State University of New York, New Paltz, NY 12561.E-mail: [email protected].

Manuscript received 4 Jan. 2014; revised 4 Jan. 2015; accepted 19 Jan. 2015.Date of publication 5 Feb. 2015; date of current version 9 Oct. 2015.Recommended for acceptance by C. Xu.For information on obtaining reprints of this article, please send e-mail to:[email protected], and reference the Digital Object Identifier below.Digital Object Identifier no. 10.1109/TC.2015.2401021

3064 IEEE TRANSACTIONS ON COMPUTERS, VOL. 64, NO. 11, NOVEMBER 2015

0018-9340� 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

speed to ensure that more service requests are processedwith high service quality. However, doing this would leadto sharp increase of the renting cost or the electricity cost.Such increased cost may counterweight the gain from pen-alty reduction. In conclusion, the single renting scheme isnot a good scheme for service providers. In this paper, wepropose a novel renting scheme for service providers, whichnot only can satisfy quality-of-service requirements, but alsocan obtain more profit. Our contributions in this paper canbe summarized as follows.

� A novel double renting scheme is proposed for ser-vice providers. It combines long-term renting withshort-term renting, which can not only satisfy qual-ity-of-service requirements under the varying sys-tem workload, but also reduce the resource wastegreatly.

� A multiserver system adopted in our paper is mod-eled as an M/M/m+D queuing model and the perfor-mance indicators are analyzed such as the averageservice charge, the ratio of requests that need short-term servers, and so forth.

� The optimal configuration problem of service pro-viders for profit maximization is formulated and twokinds of optimal solutions, i.e., the ideal solutionsand the actual solutions, are obtained respectively.

� A series of comparisons are given to verify the per-formance of our scheme. The results show that theproposed double-quality-guaranteed (DQG) rentingscheme can achieve more profit than the comparedsingle-quality-unguaranteed (SQU) renting schemein the premise of guaranteeing the service qualitycompletely.

The rest of the paper is organized as follows. Section 2reviews the related work on profit aware problem in cloudcomputing. Section 3 presents the used models, including thethree-tier cloud computing model, the multiserver systemmodel, the revenue and cost models. Section 4 proposes ourDQG renting scheme and formulates the profit optimizationproblem. Section 5 introduces themethods of finding the opti-mal solutions for the profit optimization problem in two sce-narios. Section 6 demonstrates the performance of theproposed scheme through comparison with the traditionalSQU renting scheme. Finally, Section 7 concludes thework.

2 RELATED WORK

In this section, we review recent works relevant to the profitof cloud service providers. Profit of service providers isrelated with many factors such as the price, the market

demand, the system configuration, the customer satisfactionand so forth. Service providers naturally wish to set a higherprice to get a higher profit margin; but doing so woulddecrease the customer satisfaction, which leads to a risk ofdiscouraging demand in the future. Hence, selecting a rea-sonable pricing strategy is important for service providers.

The pricing strategies are divided into two categories,i.e., static pricing and dynamic pricing. Static pricing meansthat the price of a service request is fixed and known inadvance, and it does not change with the conditions. Withdynamic pricing a service provider delays the pricing deci-sion until after the customer demand is revealed, so that theservice provider can adjust prices accordingly [9]. Staticpricing is the dominant strategy which is widely used inreal world and in research [2], [10], [11]. Ghamkhari andMohsenian-Rad [11] adopted a flat-rate pricing strategy andset a fixed price for all requests, but Odlyzko in [12] arguedthat the predominant flat-rate pricing encourages waste andis incompatible with service differentiation. Another kind ofstatic pricing strategies are usage-based pricing. For exam-ple, the price of a service request is proportional to the ser-vice time and task execution requirement (measured by thenumber of instructions to be executed) in [10] and [2],respectively. Usage-based pricing reveals that one can useresources more efficiently [13], [14].

Dynamic pricing emerges as an attractive alternative tobetter cope with unpredictable customer demand [15].Mac�ıas and Guitart [16] used a genetic algorithm to itera-tively optimize the pricing policy. Amazon EC2 [17], [18]has introduced a ”spot pricing” feature, where the spotprice for a virtual instance is dynamically updated to matchsupply and demand. However, consumers dislike prices tochange, especially if they perceive the changes to be”unfair” [19], [20]. After comparison, we select the usage-based pricing strategy in this paper since it agrees with theconcept of cloud computing mostly.

The second factor affecting the profit of service providersis customer satisfaction which is determined by the qualityof service and the charge. In order to improve the customersatisfaction level, there is a service-level agreement betweena service provider and the customers. The SLA adopts aprice compensation mechanism for the customers with lowservice quality. The mechanism is to guarantee the servicequality and the customer satisfaction so that more custom-ers are attracted. In previous research, different SLAs areadopted. Ghamkhari and Mohsenian-Rad [11] adopted astepwise charge function with two stages. If a servicerequest is handled before its deadline, it is normallycharged; but if a service request is not handled before itsdeadline, it is dropped and the provider pays for it due topenalty. In [2], [10], [21], charge is decreased continuouslywith the increasing waiting time until the charge is free. Inthis paper, we use a two-step charge function, where theservice requests served with high quality are normallycharged, otherwise, are served for free.

Since profit is an important concern to cloud service pro-viders, many works have been done on how to boost theirprofit. A large body of works have recently focused onreducing the energy cost to increase profit of service pro-viders [22], [23], [24], [25], and the idle server turning offstrategy and dynamic CPU clock frequency scaling are adopted

Fig. 1. The three-tier cloud structure.

MEI ET AL.: A PROFIT MAXIMIZATION SCHEMEWITH GUARANTEED QUALITY OF SERVICE IN CLOUD COMPUTING 3065

to reduce energy cost. However, only reducing energy costcannot obtain profit maximization. Many researchers inves-tigated the trade-off between minimizing cost and maximiz-ing revenue to optimize profit. Both [11] and [26] adjustedthe number of switched on servers periodically using differ-ent strategies and different profit maximization modelswere built to get the number of switched on servers. How-ever, these works did not consider the cost of resourceconfiguration.

Chiang and Ouyang [27] considered a cloud server sys-tem as an M/M/R/K queuing system where all servicerequests that exceed its maximum capacity are rejected. Aprofit maximization function is defined to find an optimalcombination of the server size R and the queue capacity Ksuch that the profit is maximized. However, this strategyhas further implications other than just losing the revenuefrom some services, because it also implies loss of reputa-tion and therefore loss of future customers [3]. In [2], Caoet al. treated a cloud service platform as an M/M/m model,and the problem of optimal multiserver configuration forprofit maximization was formulated and solved. This workis the most relevant work to ours, but it adopts a single rent-ing scheme to configure a multiserver system, which cannotadapt to the varying market demand and leads to low ser-vice quality and great resource waste. To overcome thisweakness, another resource management strategy is used in[28], [29], [30], [31], which is cloud federation. Using federa-tion, different providers running services that have comple-mentary resource requirements over time can mutuallycollaborate to share their respective resources in order tofulfill each one’s demand [30]. However, providers shouldmake an intelligent decision about utilization of the federa-tion (either as a contributor or as a consumer of resources)depending on different conditions that they might face,which is a complicated problem.

In this paper, to overcome the shortcomings mentionedabove, a double renting scheme is designed to configure acloud service platform, which can guarantee the servicequality of all requests and reduce the resource wastegreatly. Moreover, a profit maximization problem is formu-lated and solved to get the optimal multiserver configura-tion which can product more profit than the optimalconfiguration in [2].

3 THE MODELS

In this section, we first describe the three-tier cloud comput-ing structure. Then, we introduce the related models usedin this paper, including a multiserver system model, a reve-nue model, and a cost model.

3.1 A Cloud System Model

The cloud structure (see Fig. 1) consists of three typicalparties, i.e., infrastructure providers, service providers andcustomers. This three-tier structure is used commonly inexisting literatures [2], [6], [10].

In the three-tier structure, an infrastructure provider thebasic hardware and software facilities. A service providerrents resources from infrastructure providers and preparesa set of services in the form of virtual machine (VM). Infra-structure providers provide two kinds of resource renting

schemes, e.g., long-term renting and short-term renting. Ingeneral, the rental price of long-term renting is muchcheaper than that of short-term renting. A customer submitsa service request to a service provider which delivers serv-ices on demand. The customer receives the desired resultfrom the service provider with certain service-level agree-ment, and pays for the service based on the amount of theservice and the service quality. Service providers pay infra-structure providers for renting their physical resources, andcharge customers for processing their service requests,which generates cost and revenue, respectively. The profitis generated from the gap between the revenue and the cost.

3.2 A Multiserver Model

In this paper, we consider the cloud service platform as amultiserver system with a service request queue. Fig. 2gives the schematic diagram of cloud computing [32].

In an actual cloud computing platform such as AmazonEC2, IBM blue cloud, and private clouds, there are manywork nodes managed by the cloud managers such as Euca-lyptus, OpenNebula, and Nimbus. The clouds provideresources for jobs in the form of virtual machine. In addi-tion, the users submit their jobs to the cloud in which a jobqueuing system such as SGE, PBS, or Condor is used. Alljobs are scheduled by the job scheduler and assigned to dif-ferent VMs in a centralized way. Hence, we can consider itas a service request queue. For example, Condor is a special-ized workload management system for compute-intensivejobs and it provides a job queueing mechanism, schedulingpolicy, priority scheme, resource monitoring, and resourcemanagement. Users submit their jobs to Condor, and Con-dor places them into a queue, chooses when and where torun them based upon a policy [33], [34]. Hence, it is reason-able to abstract a cloud service platform as a multiservermodel with a service request queue, and the model iswidely adopted in existing literature [2], [11], [35], [36], [37].

In the three-tier structure, a cloud service provider servescustomers’ service requests by using a multiserver systemwhich is rented from an infrastructure provider. Assumethat the multiserver system consists of m long-term rentedidentical servers, and it can be scaled up by temporarily

Fig. 2. The schematic diagram of cloud computing.


renting short-term servers from infrastructure providers.The servers in the system have identical execution speed s(Unit: billion instructions per second). In this paper, a multi-server system excluding the short-term servers is modeledas an M/M/m queuing system as follows (see Fig. 3). Thereis a Poisson stream of service requests with arrival rate �,i.e., the interarrival times are independent and identicallydistributed (i.i.d.) exponential random variables with mean1=�. A multiserver system maintains a queue with infinitecapacity. When the incoming service requests cannot beprocessed immediately after they arrive, they are firstlyplaced in the queue until they can be handled by any avail-able server. The first-come-first-served (FCFS) queuingdiscipline is adopted. The task execution requirements(measured by the number of instructions) are independentand identically distributed exponential random variables rwith mean r (Unit: billion instructions). Therefore, the exe-cution times of tasks on the multiserver system are also i.i.d.exponential random variables x ¼ r=s with mean x ¼ r=s(Unit: second). The average service rate of each server is cal-culated as m ¼ 1=x ¼ s=r, and the system utilization isdefined as r ¼ �=mm ¼ �=m� r=s.

Because the fixed computing capacity of the service sys-tem is limited, some requests would wait for a long timebefore they are served. According to the queuing theory, wehave the following theorem about the waiting time in an M/M/m queuing system.

Theorem 3.1. The cumulative distribution function (cdf) of thewaiting timeW of a service request is

FW ðtÞ ¼ 1� pm

1� re�mmð1�rÞt; (1)

where

pm ¼ ðmrÞmm!

Xm�1k¼0

ðmrÞkk!þ ðmrÞmm!ð1� rÞ

" #�1:

Proof. We have known that the probability distributionfunction (pdf) of the waiting timeW of a service request is

fW ðtÞ ¼ ð1� PqÞuðtÞ þmmpme�ð1�rÞmmt;

where Pq ¼ pm=ð1� rÞ and uðtÞ is a unit impulse func-tion [2], [38]. Then, FW ðtÞ can be obtained by straightfor-ward calculation. tu

3.3 Revenue Modeling

The revenue model is determined by the pricing strategyand the server-level agreement (SLA). In this paper,the usage-based pricing strategy is adopted, since cloud

computing provides services to customers and charges themon demand. The SLA is a negotiation between service pro-viders and customers on the service quality and the price.Because of the limited servers, the service requests that can-not be handled immediately after entering the system mustwait in the queue until any server is available. However, tosatisfy the quality-of-service requirements, the waiting timeof each service request should be limited within a certainrange which is determined by the SLA. The SLA is widelyused bymany types of businesses, and it adopts a price com-pensation mechanism to guarantee service quality and cus-tomer satisfaction. For example, China Post gives a servicetime commitment for domestic express mails. It promisesthat if a domestic express mail does not arrive within a dead-line, the mailing charge will be refunded. The SLA is alsoadopted by many real world cloud service providers such asRackspace [39], Joyent [40], Microsoft Azure [41], and so on.Taking Joyent as an example, the customers order SmartMachines, Smart Appliances, and/or virtual machines fromJoyent, and if the availability of a customer’s services is lessthan 100 percent, Joyent will credit the customer 5 percentof the monthly fee for each 30 minutes of downtime up to100 percent of the customer’s monthly fee for the affectedserver. The only difference is that its performance metric isavailability and ours is waiting time.

In this paper, the service level is reflected by the waitingtime of requests. Hence, we define D as the maximum wait-ing time here that the service requests can tolerate, in otherwords, D is their deadline. The service charge of each taskis related to the amount of a service and the service-levelagreement. We define the service charge function for a ser-vice request with execution requirement r and waiting timeW in Eq. (2),

Rðr;W Þ ¼ ar; 0 �W � D;0; W > D;

�(2)

where a is a constant, which indicates the price per one bil-lion instructions (Unit: cents per one billion instructions).When a service request starts its execution before waiting afixed time D (Unit: second), a service provider considersthat the service request is processed with high quality-of-service and charges a customer ar. If the waiting time of aservice request exceeds deadline D, a service provider mustserve it for free. Similar revenue models have been used inmany existing research such as [2], [11], [42].

According to Theorem 1, it is easy to know that the prob-ability that the waiting time of a service request exceeds itsdeadlineD is

P ðW � DÞ ¼ 1� FW ðDÞ ¼ pm

1� re�mmð1�rÞD: (3)

3.4 Cost Modeling

The cost of a service provider consists of two major parts,i.e., the rental cost of physical resources and the utility costof energy consumption. Many existing research such as[11], [43], [44] only consider the power consumption cost.As a major difference between their models and ours, theresource rental cost is considered in this paper as well, sinceit is a major part which affects the profit of service

Fig. 3. The multiserver system model, where service requests are firstplaced in a queue before they are processed by any servers.


providers. A similar cost model is adopted in [2]. Theresources can be rented in two ways, long-term renting andshort-term renting, and the rental price of long-term rentingis much cheaper than that of short-term renting. This is rea-sonable and common in the real life. In this paper, weassume that the long-term rental price of one server for unitof time is b (Unit: cents per second) and the short-termrental price of one server for unit of time is g (Unit: centsper second), where b < g.

The cost of energy consumption is determined by theelectricity price and the amount of energy consumption. Inthis paper, we adopt the following dynamic power model,which is adopted in the literature such as [2], [7], [45], [46]:

Pd ¼ NswCLV2f; (4)

where Nsw is the average gate switching factor at each clockcycle, CL is the loading capacitance, V is the supply voltage,and f is the clock frequency [45]. In the ideal case, the rela-tionship between the clock frequency f and the supply volt-

age V is V / ff for some constant f > 0 [46]. The serverexecution speed s is linearly proportional to the clock fre-quency f , namely, s / f . Hence, the power consumption is

Pd / NswCLs2fþ1. For ease of discussion, we assume that

Pd ¼ bNswCLs2fþ1 ¼ �sa where � ¼ bNswCL and a ¼ 2fþ 1.

In this paper, we set NswCL ¼ 7:0, b ¼ 1:3456 and f ¼ 0:5.Hence, a ¼ 2:0 and � ¼ 9:4192. The value of power con-sumption calculated by Pd ¼ �sa is close to the value of theIntel Pentium M processor [47]. It is reasonable that a serverstill consumes some amount of static power [8], denoted asP � (Unit: Watt), when it is idle. For a busy server, the aver-age amount of energy consumption per unit of time isP ¼ �sa þ P � (Unit: Watt). Assume that the price of energyis d (Unit: cents per Watt).

4 A QUALITY-GUARANTEED SCHEME

The traditional single resource renting scheme cannot guaran-tee the quality of all requests but wastes a great amount ofresources due to the uncertainty of systemworkload. To over-come the weakness, we propose a double renting scheme asfollows, which not only can guarantee the quality of servicecompletely but also can reduce the resourcewaste greatly.

4.1 The Proposed Scheme

In this section, we first propose the double-quality-guaranteed resource renting scheme which combines long-term renting with short-term renting. The main computingcapacity is provided by the long-term rented servers due totheir low price. The short-term rented servers provide theextra capacity in peak period. The detail of the scheme isshown in Algorithm 1.

The proposed DQG scheme adopts the traditional FCFSqueueing discipline. For each service request entering thesystem, the system records its waiting time. The requestsare assigned and executed on the long-term rented serversin the order of arrival times. Once the waiting time of arequest reaches D, a temporary server is rented from infra-structure providers to process the request. We consider thenovel service model as an M/M/m+D queuing model [48],[49], [50]. The M/M/m+D model is a special M/M/m queuing

model with impatient customers. In an M/M/m+D model,the requests are impatient and they have a maximal tolera-ble waiting time. If the waiting time exceeds the tolerablewaiting time, they lose patience and leave the system. Inour scheme, the impatient requests do not leave the systembut are assigned to temporary rented servers.

Algorithm 1. Double-Quality-Guaranteed Scheme

1: A multiserver system with m servers is running and wait-ing for the events as follows

2: A queue Q is initialized as empty3: Event – A service request arrives4: Search if any server is available5: if true then6: Assign the service request to one available server7: else8: Put it at the end of queue Q and record its waiting time9: end if10: End Event11: Event—A server becomes idle12: Search if the queue Q is empty13: if true then14: Wait for a new service request15: else16: Take the first service request from queue Q and assign it

to the idle server17: end if18: End Event19: Event – The deadline of a request is achieved20: Rent a temporary server to execute the request and release

the temporary server when the request is completed21: End Event

Since the requests with waiting timeD are all assigned totemporary servers, it is apparent that all service requests canguarantee their deadline and are charged based on the work-load according to the SLA. Hence, the revenue of the serviceprovider increases. However, the cost increases as well dueto the temporarily rented servers. Moreover, the amount ofcost spent in renting temporary servers is determined by thecomputing capacity of the long-term rented multiserver sys-tem. Since the revenue has been maximized using ourscheme, minimizing the cost is the key issue for profit maxi-mization. Next, the tradeoff between the long-term rentalcost and the short-term rental cost is considered, and an opti-mal problem is formulated in the following to get the optimallong-term configuration such that the profit is maximized.

4.2 The Profit Optimization Problem

Assume that a cloud service platform consists of m long-term rented servers. It is known that part of requests needtemporary servers to serve, so that their quality can beguaranteed. Denoted by pextðDÞ the steady-state probabilitythat a request is assigned to a temporary server, or put dif-ferently, pextðDÞ is the long-run fraction of requests whosewaiting times exceed the deadline D. pextðDÞ is differentfrom FW ðDÞ. In calculating FW ðDÞ, all service requests,whether exceed the deadline, will be waiting in the queue.However, in calculating pextðDÞ, the requests whose waitingtimes are equal to the deadline will be assigned to the tem-porary servers, which will reduce the waiting time of the


following requests. In general, pextðDÞ is much less thanFW ðDÞ. Refer to [50], we can known that pextðDÞ is:

pextðDÞ ¼ ð1� rÞð1� FW ðDÞÞ1� rð1� FW ðDÞÞ : (5)

That is to say, there are about �pextðDÞ service requests inone unit of time which need short-term rented servers.Fig. 4 gives the probability versus different deadline where� ¼ 5:99, r ¼ 1, m ¼ 6 and s ¼ 1. Hence, the cost on short-term rented servers in one unit of time is calculated as:

Cshort ¼ �pextðDÞ rsðg þ dP Þ; (6)

where rs is the average execution time of each request.

Among the requests entering the service system, aboutpextðDÞ percentage requests are not executed by the m long-term rented servers. Hence, the system utilization of the mservers is rð1� pextðDÞÞ. Since the power for speed s is �sa,the average amount of energy consumed by a long-termrented server in one unit of time is Plong ¼ rð1� pextðDÞÞ�sa þ P �. Hence, the cost of the long-term rented serv-ers in one unit of time is calculated as:

Clong ¼ mðbþ dPlongÞ: (7)

The following theorem gives the expected charge to a ser-vice request.

Theorem 4.1. The expected charge to a service request is ar.

Proof. Because the waiting timeW of each request is less thanor equal toD, the expected charge to a service request withexecution requirement r is ar according to the SLA. Since ris a random variable, ar is also random variable. It isknown that r is an exponential random variable withmean

r, so its probability distribution function is frðzÞ ¼ 1r e�z=r:

The expected charge to a service request isZ 1

0

frðzÞRðr; zÞdz ¼Z 1

0

1

re�z=razdz

¼ a

r

Z 1

0

e�z=rzdz ¼ �aZ 1

0

zde�z=r

¼ �a ze�z=r��10�Z 1

0

e�z=rdz� �

¼ �a ze�z=r��10þ re�z=r

��10

h i

¼ ar:

(8)

The theorem is proven. tu

The profit of a service provider in one unit of time isobtained as

Profit ¼ Revenue� Clong � Cshort; (9)

where Revenue ¼ �ar;

Clong ¼ mðbþ dðrð1� pextðDÞÞ�sa þ P �ÞÞ;and

Cshort ¼ �pextðDÞ rsðg þ dð�sa þ P �ÞÞ:

We aim to choose the optimal number of fixed servers mand the optimal execution speed s to maximize the profit:

Profitðm; sÞ ¼ �ar� �pextðDÞ rsðg þ dð�sa þ P �ÞÞ

�mðbþ dðrð1� pextðDÞÞ�sa þ P �ÞÞ:(10)

Fig. 5 gives the graph of function Profitðm; sÞ where� ¼ 5:99, r ¼ 1, D ¼ 5, a ¼ 15, P � ¼ 3, a ¼ 2:0, � ¼ 9:4192,b ¼ 1:5, g ¼ 3, and d ¼ 0:3.

From the figure, we can see that the profit of a serviceprovider is varying with different server size and differentexecution speed. Therefore, we have the problem of select-ing the optimal server size and/or server speed so that theprofit is maximized. In the following section, the solutionsto this problem are proposed.

5 OPTIMAL SOLUTION

In this section, we first develop an analytical method tosolve our optimization problem. Using the analyticalmethod, the ideal optimal solutions are obtained. Becausethe server size and the server speed are limited and discrete,we give an algorithmic method to get the actual solutionsbased on the ideal optimal ones.

5.1 An Analytical Method for Ideal Solutions

We firstly solve our optimization problem analytically,assuming that m and s are continuous variables. To thisend, a closed-form expression of pextðDÞ is needed. In thispaper, we use the same closed-form expression as [2], which

isPm�1

k¼0ðmrÞkk! � emr. This expression is very accurate when

m is not too small and r is not too large [2]. Since Stirling’s

approximation of m! isffiffiffiffiffiffiffiffiffiffi2pmp ðme Þm, one closed-form expres-

sion of pm is

Fig. 4. The probability of waiting time exceedingD. Fig. 5. The function Profitðm; sÞ.


pm � 1�rffiffiffiffiffiffiffiffiffiffi2pmp ð1�rÞðererÞmþ1

;

and

pextðDÞ � ð1� rÞe�mmð1�rÞD

1þ ffiffiffiffiffiffiffiffiffiffi2pmp ð1�rÞðererÞm�re�mmð1�rÞD :

For convenience, we rewrite pextðDÞ � ð1�rÞK1K2�rK1

, where

K1 ¼ e�mmð1�rÞD; and K2 ¼ 1þ ffiffiffiffiffiffiffiffiffiffi2pmp ð1� rÞF; where F ¼

ðer=erÞm:In the following, we solve our optimization problems

based on above closed-form expression of pextðDÞ.

5.1.1 Optimal Size

Given �, r, a, P �, a, b, g, d, �,D, and s, our objective is to findm such that Profit is maximized. To maximize Profit, mmust be found such that

@Profit

@m¼ � @Clong

@m� @Cshort

@m¼ 0;

where

@Clong

@m¼ bþ dP � � d�r�sa�1

@pextðDÞ@m

;

and

@Cshort

@m¼ �ðg þ dP �Þ r

s

@pextðDÞ@m

þ �rd�sa�1@pextðDÞ

@m:

Since

lnF ¼ m lnðer=erÞ ¼ mðr� ln r� 1Þ;and

@r

@m¼ � �r

m2s¼ � r

m;

we have

1

F

@F

@m¼ ðr� ln r� 1Þ þm 1� 1

r

� �@r

@m¼ � ln r;

and

@F

@m¼ �F ln r:

Then, we get

@K1

@m¼ �mDK1;

and

@K2

@m¼

ffiffiffiffiffiffiffiffiffiffi2pmp

F1

2mð1þ rÞ � ln rð1� rÞ

� �:

Furthermore, we have

@pextðDÞ@m

¼ 1

ðK2�rK1Þ2h r

mK1ðK2�K1Þ

þ ðr�1ÞmDK1K2�ð1þrÞK1

2mðK2�1Þ

þ ð1�rÞK1ðln rÞðK2�1Þi:

We cannot get a closed-form solution to m, but we canget the numerical solution to m. Since @Profit=@m is not anincreasing or decreasing function of m, we need to find thedecreasing region of m, and then use the standard bisectionmethod. If there are more than one maximal values, theyare compared and the maximum is selected. When usingthe bisection method to find the extreme point, the iteration

accuracy is set as a unified value 10�10.In Fig. 6, we demonstrate the net profit in one unit of

time as a function of m and � where s ¼ 1, r ¼ 1, and theother parameters are same as with those in Fig. 5.We notice that there is an optimal choice of m such thatthe net profit is maximized. Using the analytical method,the optimal value of m such that @Profit=@m ¼ 0 is4.8582, 5.8587, 6.8590, 7.8592 for � ¼ 4.99, 5.99, 6.99, 7.99,respectively. When the number of servers m is less thanthe optimal value, the service provider needs to rentmore temporary servers to execute the requests whosewaiting times are equal to the deadline; hence, the extracost increases, even surpassing the gained revenue. As mincreases, the waiting times are significantly reduced, butthe cost on fixed servers increases greatly, which alsosurpasses the gained revenue too. Hence, there is an opti-mal choice of m which maximizes the profit.

In Fig. 7, we demonstrate the optimal size and maxi-mal profit in one unit of time as a function of s and �. Itmeans, for each combination of s and �, we find the opti-mal number of servers and the maximal profit. Theparameters are same as those in Fig. 6. From the figureswe can see that a higher speed leads to a less number ofservers needed for each �, and different � values have dif-ferent optimal combinations of speed and size. In addi-tion, the greater the � is, the more the maximal profit canbe obtained.

5.1.2 Optimal Speed

Given �, r, a, P �, a, b, g, d, �, D, and m, our objective is tofind s such that Profit is maximized. To maximize Profit, smust be found such that

@Profit

@s¼ � @Clong

@s� @Cshort

@s¼ 0;

where

@Clong

@s¼ d��rsa�2

�ða� 1Þð1� pextðDÞÞ � s

@pextðDÞ@s

�;

Fig. 6. Net profit versusm and �.


and

@Cshort

@s¼ r�ðg þ dP �Þ

s2

�s@pextðDÞ

@s� pextðDÞ

�

þ �rd�sa�2�s@pextðDÞ

@sþ ða� 1ÞpextðDÞ

�:

Since

@r

@s¼ � �r

ms2¼ � r

s;

and

1

F

@F

@s¼ m 1� 1

r

� �@r

@s;

we have

@F

@s¼ m

sð1� rÞF:

Now, we get

@K1

@s¼ �DK1

m

r;

and

@K2

@s¼


ðrþmð1� rÞ2ÞFs:

Furthermore, we have

@pextðDÞ@s

¼ 1

ðK2 � rK1Þ2h rsK1ðK2 �K1Þ

þ ðr� 1ÞK1


ðrþmð1� rÞ2ÞFs

þ ðr� 1ÞDK1K2m

r

i:

Similarly, we cannot get the closed-form expression of s,so we can use the same method to find the numerical solu-tion of s. In Fig. 8, we demonstrate the net profit in one unitof time as a function of s and �, wherem ¼ 6. The rest param-eters are the same as that in Figs. 6 and 7.We notice that thereis an optimal choice of s such that the net profit ismaximized.Using the analytical method, the optimal value of s such thatrespectively. When the servers run at a slower speed thanthe optimal speed, the waiting times of service requests willbe long and exceed the deadline. So, the revenue is small andthe profit is not optimal. When s increases, the energy con-sumption as well as the electricity cost increases. Hence, theincreased revenue is much less than the increased cost. As aresult, the profit is reduced. Therefore, there is an optimalchoice of s such that the net profit is maximized.

In Fig. 9, we demonstrate the optimal speed and maximalprofit in one unit of time as a function of m and �. Theparameters are same as that in Figs. 6, 7, and 8. From the fig-ures we can see that if the number of fixed servers is great,the servers must run at a lower speed, which can lead to anoptimal profit. In addition, the optimal speed of servers isnot faster than 1.2, that is because the increased electricitycost surpasses the increased cost that rents extra servers.The figure also shows us that different � values have differ-ent optimal combinations of speed and size.

5.1.3 Optimal Size and Speed

Given �, r, a, P �, a, b, g, d, �, D, our third problem is to findm and s such that Profit is maximized. Hence, we need tofind m and s such that @Profit=@m ¼ 0 and @Profit=@s ¼ 0,where @Profit=@m and @Profit=@s have been derived in thelast two sections. The two equations are solved by using thesame method as [2]. In Fig. 10, we demonstrate the net profitin one unit of time as a function of m and s. Here � is 5.99,and r ¼ 1. The optimal value is m ¼ 6:2418 and s ¼ 0:9386,which result in the maximal profit 58.0150. In Fig. 11, wedemonstrate the maximal profit in one unit of time in differ-ent combinations of � and r. The figure shows that the

Fig. 7. Optimal size and maximal profit versus s and �.

Fig. 8. Net profit versus s and �.


service providers can obtain more profit when the servicerequests are with greater � and r.

5.2 An Algorithmic Method for Actual Solutions

In above subsection, the optimal solutions find using theanalytical method are ideal solutions. Since the number ofrented servers must be integer and the server speed levelsare discrete and limited in real system, we need to find theoptimal solutions for the discrete scenarios. Assume thatS ¼ fsij1 � i � ng is a discrete set of n speed levels withincreasing order. Next, different situations are discussedand the corresponding methods are given as follows.

5.2.1 Optimal Size

Assume that all servers run at a given execution speed s.Given �, r, a, P �, a, b, g, d, �, and D, the first problem is

to find the number of long-term rented servers m suchthat the profit is maximized. The method is shown inAlgorithm 2.

Algorithm 2. Finding the Optimal Size

Input: s, �, r, a, P �, a, b, g, d, �, andDOutput: the optimal number Opt size of fixed servers1: Profit max 02: find the server size m using the analytical method in

Section 5.1.13: m�l bmc,m�u dme4: Profitl Profitðm�l ; sÞ, Profitu Profitðm�u; sÞ5: if Profitl > Profitu then6: Profit max Profitl7: Opt size m�l8: else9: Profit max Profitu10: Opt size m�u11: end if

5.2.2 Optimal Speed

Assume that the service provider rents m servers. Given �,r, a, P �, a, b, g, d, �, and D, the second problem is to find theoptimal execution speed of all servers such that the profit ismaximized. The method is shown in Algorithm 3.

Algorithm 3. Finding the Optimal Speed

Input:m, �, r, a, P �, a, b, g, d, �, andDOutput: the optimal server speed Opt speed1: Profit max 02: find the server speed s using the analytical method in

Section 5.1.23: s�l si, s

�u siþ1 if si < s � siþ1

4: Profitl Profitðm; s�l Þ, Profitu Profitðm; s�uÞ5: if Profitl > Profitu then6: Profit max Profitl7: Opt speed s�l8: else9: Profit max Profitu10: Opt speed s�u11: end if

5.2.3 Optimal Size and Speed

In this subsection, we solve the third problem, which is tofind the optimal combination of m and s such that the profit

Fig. 9. Optimal speed and maximal profit versusm and �.

Fig. 10. Net profit versusm and s.

Fig. 11. Maximal profit versus � and r.


is maximized. Given �, r, a, P �, a, b, g, d, �, and D, themethod is shown in Algorithm 4.

Algorithm 4. Finding the Optimal Size and Speed

Input: �, r, a, P �, a, b, g, d, �, andDOutput: the optimal number Opt size of fixed servers and the

optimal execution speed Opt speed of servers1: Profit max 02: find the server size m and speed s using the analytical

method in Section 5.1.33: m�l bmc,m�u dme4: find the optimal speed s�l and s�u using Algorithm 3 with

server sizem�l andm�u, respectively

5: Profitl Profitðm�l ; s�l Þ, Profitu Profitðm�u; s�uÞ6: if Profitl � Profitu then

7: Profit max Profitu

8: Opt size m�u , Opt speed s�u9: else10: Profit max Profitl11: Opt size m�l , Opt speed s�l12: end if

5.3 Comparison of Two Kinds of Solutions

In Tables 1, 2, and 3, the ideal optimal solutions and theactual optimal solutions are compared for three differentcases. Table 1 compares the ideal optimal size and theactual optimal size under the given server speed. Table 2compares the ideal optimal speed and the actual optimalspeed under the given server size. In Table 3, two kindsof solutions are compared for different combinations of �and r. Here, m can be any positive integer, and the avail-able speed levels are S ¼ f0:2; 0:4; . . . ; 2:0g. According tothe comparisons we can see that the ideal maximal profitis greater than the actual maximal profit. In the tables,we also list the relative difference (RD) between the idealoptimal profit and the actual optimal profit, which is cal-culated as

RD ¼ Idep �ActpActp

;

where Idep and Actp are the maximal profit in ideal andactual scenarios. From the results we know that the rela-tive difference is always small except some cases inTable 2. That is because a small difference of speedwould lead to a big difference of profit when the serversize is large.

6 PERFORMANCE COMPARISON

Using our resource renting scheme, temporary servers arerented for all requests whose waiting time are equal to thedeadline, which can guarantee that all requests are servedwith high service quality. Hence, our scheme is superior tothe traditional resource renting scheme in terms of the ser-vice quality. Next, we conduct a series of calculations tocompare the profit of our renting scheme and the rentingscheme in [2]. In order to distinguish the proposed schemeand the compared scheme, the proposed scheme is renamedas Double-Quality-Guaranteed renting scheme and thecompared scheme is renamed as single-quality-unguaran-teed renting scheme in this paper.

6.1 The Compared Scheme

Firstly, the average charge of the using the SQU rentingscheme is analyzed.

Theorem 6.1. The expected charge to a service request using theSQU renting scheme is

arð1� Pqe�ð1�rÞmmDÞ:

Proof. Recall that the probability distribution function of thewaiting timeW of a service request is

fW ðtÞ ¼ ð1� PqÞuðtÞ þmmpme�ð1�rÞmmt:

TABLE 1Comparison of the Two Methods for Finding the Optimal Size

Given Speed 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

IdealSolution

Optimal Size 29.1996 14.6300 9.7599 7.3222 5.8587 4.8827 4.1854 3.6624 3.2555 2.9300Maximal Profit 11.5546 45.5262 54.6278 57.5070 57.8645 56.9842 55.3996 53.3498 51.0143 48.4578

ActualSolution

Optimal Size 29 15 10 7 6 5 4 4 3 3Maximal Profit 11.5268 45.4824 54.6014 57.3751 57.8503 56.9727 55.3259 53.0521 50.8526 48.4513

Relative Difference 0.2411% 0.0964% 0.0483% 0.2299% 0.0246% 0.0202% 0.1332% 0.5612% 0.3180% 0.01325%

TABLE 2Comparison of the Two Methods for Finding the Optimal Speed

Given Size 5 7 9 11 13 15 17 19 21 23

IdealSolution

Optimal Speed 1.1051 0.8528 0.6840 0.5705 0.4895 0.4288 0.3817 0.3440 0.3132 0.2875Maximal Profit 57.3742 57.7613 56.0783 53.3337 49.9896 46.2754 42.3167 38.1881 33.9366 29.5933

ActualSolution

Optimal Speed 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.4 0.4 0.4Maximal Profit 57.0479 57.3751 54.7031 53.1753 48.4939 45.4824 42.2165 37.4785 32.6795 27.8795

Relative Difference 0.5721% 0.6732% 2.5140% 0.2979% 3.0843% 1.7435% 0.2373% 1.8934% 3.8470% 6.1474%


Since W is a random variable, so Rðr;WÞ is also a ran-dom variable. The expected charge to a service requestwith execution requirement r is

RðrÞ ¼ Rðr;WÞ¼

Z 1

0

fW ðtÞRðr; tÞdt

¼Z D

0

ð1� PqÞuðtÞ þmmpme�ð1�rÞmmt

h iardt

¼ ð1� PqÞarþmmpmar1� e�ð1�rÞmmD

ð1� rÞmm

¼ arð1� Pqe�ð1�rÞmmDÞ:

Therefore, the expected charge to a service request isthe expected value of RðrÞ:

RðrÞ ¼Z 1

0

frðzÞRðzÞdz

¼Z 1

0

1

re�z=razð1� Pqe

�ð1�rÞmmDÞdz

¼ a

rð1� Pqe

�ð1�rÞmmDÞZ 1

0

e�z=rzdz

¼ arð1� Pqe�ð1�rÞmmDÞ:

The theorem is proven. tuBy the above theorem, the profit in one unit of time using

the SQU renting scheme is calculated as:

�arð1� Pqe�ð1�rÞmmDÞ �mðbþ dðr�sa þ P �ÞÞ: (11)

Using the SQU renting scheme, a service provider mustrent more servers or scale up the server speed to maintain ahigh quality-guaranteed ratio. Assumed that the requiredquality-guaranteed ratio of a service provider is c and thedeadline of service requests isD. By solving equation

FW ðDÞ ¼ 1� pm

1� re�mmð1�rÞD � c

with givenm or s, we can get the corresponding s orm suchthat the required quality-guaranteed ratio is achieved.

6.2 Profit Comparison under DifferentQuality-Guaranteed Ratio

Let � be 5.99 and the other parameters be the same as thosein Section 5. In the first example, for a given number of serv-ers, we compare the profit using the SQU renting schemewith quality-guaranteed ratio 100, 99, 92, 85 percent andthe optimal profit using our DQG renting scheme. Becausethe quality-guaranteed ratio 100 percent cannot beachieved using the SQU renting scheme, hence, we set99:999999% � 100%. The results are shown in Fig. 12. Fromthe figure, we can see that the profit obtained using the pro-posed scheme is always greater than that using the SQUrenting scheme, and the five curves reach the peak at differ-ent sizes. In addition, the profit obtained by a service pro-vider increases when the qualtiy-guaranteed ratio increasesfrom 85 to 99 percent, but decreases when the ratio isgreater than 99 percent. That is because more servicerequests are charged with the increasing ratio from 85 to99 percent; but once the ratio is greater than 99 percent, thecost to expand the server size is greater than the revenueobtained from the extra qualtiy-guaranteed requests, hence,the total profit is reduced.

In the second example, we compare the profit of theabove five scenarios under the given server speed. Theresults are given in Fig. 13. The figure shows the trend ofprofit when the server speed is increasing from 0.1 to 2.9.From the figure, we can see that the curves increase firstly

TABLE 3Comparison of the Two Methods for Finding the Optimal Size and the Optimal Speed

r 0.50 0.75 1.00 1.25 1.50 1.75 2.00

� ¼ 4:99

IdealSolution

Optimal Size 2.5763 3.8680 5.1608 6.4542 7.7480 9.0420 10.3362Optimal Speed 0.9432 0.9422 0.9413 0.9406 0.9399 0.9394 0.9388Maximal Profit 24.0605 36.0947 48.1539 60.1926 72.2317 84.3121 96.3528

ActualSolution

Optimal Size 3 4 5 6 7 9 10Optimal Speed 1.0 1.0 1.0 1.0 1.0 1.0 1.0Maximal Profit 23.8770 35.7921 48.0850 60.1452 72.0928 83.9968 96.2230

Relative Difference 0.7695% 0.8454% 0.14355% 0.0789% 0.1927% 0.3754% 0.1349%

� ¼ 5:99

IdealSolution

Optimal Size 3.1166 4.6787 6.2418 7.8056 9.3600 10.9346 12.4995Optimal Speed 0.9401 0.9393 0.9386 0.9380 0.9375 0.9370 0.9366Maximal Profit 28.9587 43.4364 57.9339 72.4121 86.9180 101.3958 115.9086

ActualSolution

Optimal Size 3 4 6 7 9 10 12Optimal Speed 1.0 1.0 1.0 1.0 1.0 1.0 1.0Maximal Profit 28.9158 43.1208 57.8503 72.2208 86.7961 101.2557 115.7505

Relative Difference 0.1484% 0.7317% 0.1445% 0.2649% 0.1405% 0.1384% 0.1365%

Fig. 12. Profit versusm and different quality-guaranteed ratios.


and reach the peak at certain speed, and then decrease alongwith the increasing speed on the whole. The figure verifiesthat our proposed scheme can obtain more profit than theSQU renting scheme. Noticed that the changing trends ofthe curves of the SQU renting scheme with 100, 99, 92, and85 percent quality-guaranteed ratio are interesting. Theyshow an increasing trend at the beginning and thendecrease during a small range of speed repeatedly. Thereason is analyzed as follows. When the server speed ischanging within a small speed range, in order to satisfy therequired deadline-guaranteed ratio, the number of serversrented by a service provider keeps unchanged. At the begin-ning, the added revenue is more than the added cost, so theprofit is increasing. However, when the speed becomesgreater, the energy consumption increases, leading to thetotal increased cost surpassing the increased revenue,hence, the profit decreases.

In the third example, we explore the changing trend ofthe profit with different D, and the results are shown asFig. 14. Fig. 14a gives the numerical results when the serverspeed is fixed at 0:7, and Fig. 14b shows the numericalresults when the number of servers is fixed at 5. We analyzethe results as follows.

From Fig. 14a, we can see that the profit obtained usingthe SQU renting scheme increases slightly with the incre-ment of D. That is because the service charge keeps con-stant but the extra cost is reduced when D is greater. As aconsequence, the profit increases. The second phenome-non from the figure is that the curves of SQU 92 percentand SQU 85 percent have sharp drop at some points andthen ascend gradually and smoothly. The reasons areexplained as follows. When the server speed is fixed,enough servers are needed to satisfy the given quality-guaranteed ratio. By calculating, we know that the num-ber of required servers is the same for all D values in acertain interval. For example, [5,7] and [8,25] are twointervals of D for the curve of SQU 92 percent, and therequired servers are 10 and 9, respectively. For all Dwithin the same interval, their costs are the same witheach other. Whereas, their actual quality-guaranteedratios are different which get greater with the increasingD. Hence, during the same interval, the revenue getsgreater as well as the profit. However, if the deadlineincreases and enters a different interval, the quality-guaranteed ratio sharply drops due to the reduced serv-ers, and the lost revenue surpasses the reduced cost,hence, the profit sharply drops as well. Moreover, we can

also see that the profit of SQU 100 percent is much lessthan the other scenarios. That is because when the qual-ity-guaranteed ratio is great enough, adding a small reve-nue leads to a much high cost.

From Fig. 14b, we can see that the curves of SQU92 percent and SQU 85 percent descend and ascendrepeatedly. The reasons are same as that of Fig. 14a. Thedeadlines within the same interval share the same mini-mal speed, hence, the cost keeps constant. At the sametime, the revenue increases due to the increasing quality-guaranteed ratio. As a consequence, the profit increases.At each break point, the minimal speed satisfying therequired quality-guaranteed ratio gets smaller, whichleads to a sharp drop of the actual quality-guaranteedratio. Hence, the revenue as well as the profit drops.

6.3 Comparison of Optimal Profit

In order to further verify the superiority of our proposedscheme in terms of profit, we conduct the following compar-ison between the optimal profit achieved by our DQG rent-ing scheme and that of the SQU renting scheme in [2]. Inthis group of comparisons, � is set as 6.99, D is 5, r is vary-ing from 0.75 to 2.00 in step of 0.25, and the other parame-ters are the same as Section 5. In Fig. 15, the optimal profitand the corresponding configuration of two rentingschemes are presented. From Fig. 15a we can see that theoptimal profit obtained using our scheme is always greaterthan that using the SQU renting scheme. According to thecalculation, our scheme can obtain 4.17 percent more profiton the average than the SQU renting scheme. This shows

Fig. 13. Profit versus s and different quality-guaranteed ratios.

Fig. 14. Profit versusD and different quality-guaranteed ratios.


that our scheme outperforms the SQU renting scheme interms of both of quality of service and profit. Figs. 15b and15c compare the server size and speed of the two schemes.The figures show that using our renting scheme the capacityprovided by the long-term rented servers is much less thanthe capacity using the SQU renting scheme. That is becausea lot of requests are assigned to the temporary servers usingour scheme, and less servers and slower server speed areconfigured to reduce the waste of resources in idle period.In conclusion, our scheme can not only guarantee the ser-vice quality of all requests, but also achieve more profitthan the compared one.

7 CONCLUSIONS

In order to guarantee the quality of service requests andmaximize the profit of service providers, this paper has pro-posed a novel Double-Quality-Guaranteed renting schemefor service providers. This scheme combines short-termrenting with long-term renting, which can reduce theresource waste greatly and adapt to the dynamical demandof computing capacity. An M/M/m+D queueing model isbuild for our multiserver system with varying system size.And then, an optimal configuration problem of profit maxi-mization is formulated in which many factors are taken intoconsiderations, such as the market demand, the workloadof requests, the server-level agreement, the rental cost ofservers, the cost of energy consumption, and so forth. Theoptimal solutions are solved for two different situations,which are the ideal optimal solutions and the actual optimalsolutions. In addition, a series of calculations are conductedto compare the profit obtained by the DQG renting schemewith the single-quality-unguaranteed renting scheme. Theresults show that our scheme outperforms the SQU schemein terms of both of service quality and profit.

In this paper, we only consider the profit maximizationproblem in a homogeneous cloud environment, because theanalysis of a heterogenous environment is much more com-plicated than that of a homogenous environment. However,we will extend our study to a heterogenous environment inthe future.

ACKNOWLEDGMENTS

The authors thank the anonymous reviewers for their valu-able comments and suggestions. The research was partiallyfunded by the Key Program of National Natural ScienceFoundation of China (Grant Nos. 61133005, 61432005), the

National Natural Science Foundation of China (Grant Nos.61370095, 61472124, 61173013, 61202109, and 61472126).Kenli Li is the corresponding author.

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Jing Mei received the BSc degree in informa-tion and computer science from Hunan NormalUniversity in 2009. She is currently workingtoward the PhD degree in Hunan University,China. Her research interest includes proces-sor allocation and resource management,energy-efficient scheduling for parallel and dis-tributed computing systems, and profit optimi-zation in cloud computing.

Kenli Li received received the PhD degree incomputer science from Huazhong University ofScience and Technology, China, in 2003. He wasa visiting scholar at the University of Illinois atUrbana Champaign from 2004 to 2005. He iscurrently a full professor of computer science andtechnology at Hunan University and the deputydirector in National Supercomputing Center,Changsha. His major research includes parallelcomputing, cloud computing, and DNA comput-ing. He has published more than 130 papers in

international conferences and journals, such as IEEE-TC, IEEE-TPDS,IEEE-TSP, JPDC, ICPP, CCGrid, FGCS. He is currently or has servedon the editorial boards of IEEE Transactions on Computers, Interna-tional Journal of Pattern Recognition and Artificial Intelligence, He is anoutstanding member of the CCF and a member of the IEEE.


Aijia Ouyang received the ME degree from theDepartment of Computer Science and Technol-ogy, Guangxi University for Nationalities, China,in 2010. He is currently working toward the PhDdegree in the College of Information Scienceand Engineering, Hunan University, China. Hisresearch interests include parallel computing,cloud computing, and artificial intelligence.He has published research articles in interna-tional conference and journals of intelligencealgorithms and parallel computing.

Keqin Li is a SUNY Distinguished Professor ofcomputer science. His current research interestsinclude parallel computing and high-performancecomputing, distributed computing, energy-efficient computing and communication, hetero-geneous computing systems, cloud computing,big data computing, CPU-GPU hybrid and coop-erative computing, multicore computing, storageand file systems, wireless communication net-works, sensor networks, peer-to-peer file sharingsystems, mobile computing, service computing,

Internet of things and cyber-physical systems. He has published morethan 340 journal articles, book chapters, and refereed conferencepapers, and has received several Best Paper Awards. He is currently orhas served on the editorial boards of IEEE Transactions on Parallel andDistributed Systems, IEEE Transactions on Computers, IEEE Transac-tions on Cloud Computing, Journal of Parallel and Distributed Comput-ing. He is a fellow of the IEEE.

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