vax virtual

7/30/2019 Vax Virtual

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Virtual Memory Management in

the VAX/VMS Operating System

Henry Levy and Peter Lipman, DEC

Presenter Abhijeet Mahule


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Introduction

VAX-11/780 and VAX-11/750 were DECs firstimplementation of the 32-bit architecture

The Operating System on them was a

VAX/VMS Built on the experience of the PDP-11 system

Intended to provide a single environment for

all VAXbased applications These applications included real-time, time

shared or batch.


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Motivation

VAX/VMS had to operate on a family ofprocessors

These processors had different performancecharacteristics

The physical memory capacities ranged from250KB to more than 8M bytes

Memory management system had to be capable

of handling changing demands of timesharingsystem

Also they had to allow the predictableperformance of batch/real-time systems


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VAX-11 hardware

Basic entity in the VAX-11 system is the process

Each process has a byte-addressable 32-bit virtual

address space

This address space is divided into 512-byte pages

The 32-bits of an address contain 21-bit virtual

page number and a 9-bit byte offset within the

page

Page is the basic unit of mapping and protection


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VAX-11 hardware

The upper two bits of the virtual address divide theprocess address space into a number of functionalregions or spaces

Bit 31 Bit 30 Region/Space

0 0 Program region (P0)

0 1 Control Region (P1)

1 0 System Space

1 1 Unused Bits 9 29 (21 bits) refer to the Virtual Page number

Bits 0 8 (9 bits) refer to byte offset within the page


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VAX-11 hardware System Space

The high address half of the address space (bit

31 = 1) is system space and is shared by all

processes in the system

Thus, a system space virtual address

generated by any process will access the same

physical memory location

Only half of the system space is utilized in this

architecture


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VAX-11 hardware Process Space

The low-address half of the address space (bit31 = 0) is known as process space and isunique to each process

Process space is divided into two regions bybit 30 of the virtual address

The low-address half, known as P0 is the

program region The high-address half, known as P1 is the

control region


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VAX-11 hardware - Regions

Each region System, Program and Control, is

defined by a page table

A VAX-11 page table is a contiguous array of

32-bit page table entries

Each page table entry or PTE contains:

A valid bit (PTE ) indicates whether thepage table entry contains mapping info


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Page Table Entry (PTE) fields contd

A protection field (PTE ) that indicates

what privilege is required to read or write the

page

A modify bit ( PTE ) that indicates

whether write access has occurred to the page

A field used by the Operating System (PTE

)

The physical page frame number (PTE )

that locates the page in physical memory


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VAX-11 registers

Each page table is defined by two hardwareregisters: base address register and lengthregister

The System space page table is located byreference to the system page table base register,which contains its physical address

The P0 and P1 page tables for each process are

located in system space section of the addressspace

P0 and P1 page table base registers containvirtual addresses


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Use of address space by VAX/VMS

All the three address regions are used for

specific purposes

All executable code and data including some

process-specific data structures and process

page tables, are stored in system region

First few pages of system space, called vector

region, contain pointers to executive service

routines


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VAX/VMS is a collection of procedures that

exist in the address space of each process

These procedures can be called explicitly or

implicitly to perform services on behalf of a

process.

The operating system does not have a

separate address context.


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The P0 (program) region contains the usersexecutable program

The users program can dynamically grow intohigher-addressed sections of P0 space

VAX/VMS uses the P1 (Control) region to storeprocess-specific data

P1 is also used to store the program image forcommand interpreter

The users stack is located in low address part ofP1 region


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Memory management implementation

The typical concerns in a paging system were:

Effect of one or more heavily paging programs on

other programs in the system

High cost of program start up and restart time invirtual memory environments

Increased disk workload caused by paging

Processor time consumed by searching page lists

To overcome these problems, VAX/VMS is divided

into two basic components: pager and swapper


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Pager:

It is an operating system procedure that executes asresult of page fault

It executes within the context of the faulting process It is responsible for loading and removing process

pages into and out of memory

Swapper:

It is a separate process responsible for loading andremoving entire process into and out of memory


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A limit is placed on the amount of physical

memory a process may occupy

The set of pages currently in memory for a

process is called the processs resident set and isdescribed by a pager data structure

When a new program is started, its resident set is

initially empty As the program executes, the pager loads the

page whenever a non-resident page is referenced


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When the resident-set limit is reached, thefaulting process must release a page for eachnewly faulted page added to its resident set.

FIFO replacement algorithm is used to selectthe page to be removed

When a page is removed from a processs

resident set, it is placed on one of two pagelists: the free page list or the modified pagelist


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If the modify bit is zero in the page table entry,

the page is added to the tail of the free list

If the modify bit is one in the page table entry,

the page is queued on the tail of the modified

page list.

If a process faults a page that is on either list,

the page is returned to the processs resident

set


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Clustering of pages

The page size chosen for VAX/VMS is 512 bytes

which can increase I/O overload due to small size

VAX/VMS reduces the paging overload by reading

and writing several pages at a time, which isknown as clustering

The cluster size is the maximum number of pages

the pager will attempt to load at once. A user can specify a default cluster size when a

program is linked


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Write Optimizations

VAX/VMS delays the modified-page writerequests and gains these optimizations:

If the page on the modified page list is referenced, it isreturned to the process at minimum cost

When a write request must be performed, manypages can be written at once

Pages can be arranged on the paging file so thatclustering on read requests is possible

Due to delayed writes, many page writes are avoidedbecause either the pages are faulted again or programterminates


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Swapper

Entire resident sets can be swapped betweenmemory and the backing store.

This is handled by a process called swapper whichis executed whenever it is awakened by theoperating system

Whenever a process is removed from thememory, its entire resident set is written to aswap file, along with some process specific

operating system data. The swapper operation may be performed in

several pieces


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Swapper

The swapper writes all resident-set pages to acontiguous section of the swap file

When a process not in memory becomes able toexecute, it is read in by the swapper

The swapper allocates pages for the resident set,page tables, and operating system datastructures.

When a new process is created, the swapperswaps in a process shell, which provides theinitial environment in which a program can beexecuted


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Conclusions

VAX/VMS takes a slightly unorthodox approach tovirtual memory management

Three major mechanisms have been used to enhancethe performance of the OS:-

Caching of pages: reduces the number of disk I/Otransactions

Clustering: provides transfer efficiency of large pagesalong with the fragmentation characteristics of smallpages

Use of process-local replacement along with swapping:to reduce the effect of one processs paging activitieson anothers


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Thank You!

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