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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