i/o buffering operating system

7/28/2019 I/O Buffering Operating System

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PERATING SYSTEM


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a) To cope with the speed mismatch between producer (or sender) and

consumer (or receiver) of a data stream.

b) To adapt between the devices having different data-transfer size.

c) To support copy semantics for application I/O.

A process of storing data in memory area calledBuffers

while data is being transferred between two devices or

between a device and an application.

I/O BUFFERING


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NO BUFFERING

Process must read/write a device a byte/word a time.

Each individual system call adds significant overhead. Process must what until each I/O is complete.

i) Blocking/waking adds to overhead

ii) Many short run of a process is inefficient.

What happens if buffer is paged out to disk

i) Could lose data while buffer is paged in

ii) Could lock buffer in memory (needed for DMA), however many processes doing

I/O reduce RAM available for paging.

iii) Can cause deadlock as RAM is limited

resource.


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USER LEVEL BUFFERING

User process can process one block of data while next block is read in.

Swapping can occur since input is taking place in system memory, not user

memory.

Operating system keeps track of assignment of system buffers to user processes.

Assume:

T is transfer time for a block from device

C is computation time to process incoming block

M is time to copy kernel buffer to user buffer

Computation and transfer can be done in parallel.


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USER LEVEL BUFFERING

Process specifies a memory buffer that incoming data is placed in until it fills.

Filling can be done by interrupt service routine.

Only a single system call, and block/wakeup per data buffer.

Much more efficient.

When is buffer available for re-use?

Either process must block until potential slow device drains buffer

OR deal with asynchronous signals indicating buffer drained


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SINGLE BUFFERING

~ The simplest type of buffering ~

Operating system assigns a buffer in Kernels memory for an I/O request .

Stream Oriented

i) Used a line at time.

ii) User input from a terminal with carriage return signaling the end of the line.

iii) Output to the terminal is one line at a time.

Block-oriented

i) Input transfers made to buffer

ii) Block moved to user space when needed.

iii) Another block is moved into the buffer. (Read ahead)


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DOUBLE BUFFERING

Use two system buffers instead of one.

A process can transfer data to or from one buffer while the operating

system empties or fills the other buffer.

Computation and Memory copy can be done in parallel with transfer.

May be insufficient for really burst traffic

Lots of application writes between long periods of computation

Long periods of application computation while receiving data

Might want to read-ahead more than a single block for disk


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CIRCULAR BUFFERING

More than two buffers are used.

Each individual buffer is one unit in a circular buffer.

Used when I/O operation must keep up with process.

Double buffering may be inadequate if the process performs rapid bursts of I/O.

The problem can often be alleviated by using more than two buffers.

When more than two buffers are used, the collection of buffers is itself referred

to as a circular buffer with each individual buffer being one unit in the circular

buffer.


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UTILITY BUFFERING

Buffering is a technique that smoothest out peaks in I/O demand. However, no

amount of buffering will allow an I/O device to keep pace with a process

indefinitely when the average demand of the process is greater than the I/O

device can service. If offline operation after the data has been read and the

CPU is operated on it the magnetic tapes are instructed to store the next input

into some reserve memory area called buffer.


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UTILITY BUFFERING Buffering is done for 3 reasons

- To cope with the speed mismatch between producer (or sender) and consumer (or

receiver) of a data stream

- To adapt between devices having different data-transfer size

- To support copy semantics for application I/O

The time taken to complete a job is faster but the CPU is more utilized as the

access time from memory to the CPU is faster than the access time from any I/O

device to the CPU.

Buffering overlaps I/O of a job with jobs computations. In a multiple

programming environment, when there is a variety of I/O activity and a variety

of process activity to service, buffering is one tool that can increase the efficiency

the OS and the performance of individual processes.


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i/o buffering operating system

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