16.3: I/O Buffering

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What is a Buffer

A data buffer (or just buffer) is a region of a physical memory storage used to temporarily store data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such as a microphone) or just before it is sent to an output device (such as speakers). However, a buffer may be used when moving data between processes within a computer. This is comparable to buffers in telecommunication. Buffers can be implemented in a fixed memory location in hardware—or by using a virtual data buffer in software, pointing at a location in the physical memory. In all cases, the data stored in a data buffer are stored on a physical storage medium. A majority of buffers are implemented in software, which typically use the faster RAM to store temporary data, due to the much faster access time compared with hard disk drives. Buffers are typically used when there is a difference between the rate at which data is received and the rate at which it can be processed, or in the case that these rates are variable, for example in a printer spooler or in online video streaming. In the distributed computing environment, data buffer is often implemented in the form of burst buffer that provides distributed buffering service.

Uses of Buffers

Buffers are often used in conjunction with I/O to hardware, such as disk drives, sending or receiving data to or from a network, or playing sound on a speaker. A line to a rollercoaster in an amusement park shares many similarities. People who ride the coaster come in at an unknown and often variable pace, but the roller coaster will be able to load people in bursts (as a coaster arrives and is loaded). The queue area acts as a buffer—a temporary space where those wishing to ride wait until the ride is available. Buffers are usually used in a FIFO (first in, first out) method, outputting data in the order it arrived.

Buffers can increase application performance by allowing synchronous operations such as file reads or writes to complete quickly instead of blocking while waiting for hardware interrupts to access a physical disk subsystem; instead, an operating system can immediately return a successful result from an API call, allowing an application to continue processing while the kernel completes the disk operation in the background. Further benefits can be achieved if the application is reading or writing small blocks of data that do not correspond to the block size of the disk subsystem, allowing a buffer to be used to aggregate many smaller read or write operations into block sizes that are more efficient for the disk subsystem, or in the case of a read, sometimes to completely avoid having to physically access a disk.

Types of Buffers

There are several common types of buffering used. We will talk about just a couple.

Single I/O Buffer

The simplest form of buffering is using a single buffer implementation. As a user process executes, it issues an I/O request, This causes the OS to assign a buffer in the system portion of main memory to the I/O operation. The OS then send the request to the proper device. The I/O device responds by sending data in response to the request. The OS simply writes the data into the buffer the OS had previously created. When te buffer gets full, or at some OS specified timed interval, the data is transferred to the process in a single transfer.

This process can operate in the other direction as well. The process can be creating data that needs to be stored on a disk drive - or to some other output device. As the process creates the data, it is stored in the buffer, until the device is free to accept the data, at which time, the OS sends the contents of the buffer to the output device.

Double I/O Buffer

This concept simply adds a second buffer to be used to increase efficiency. When the first buffer gets full, the OS immediately switches to the other buffer. As the second buffer is filled, the OS is also moving the data out of the first buffer. So, by the time the first buffer is needed again it is empty, and the OS then begins filling it again, and the OS empties the other buffer.

Again, this concept works with either a read request or a write request from the user process. The example aboe is showing a read request, but the same process takes place on a write request.

Circular Buffer

A circular buffer first starts out empty and has a set length. In the diagram below is a 7-element buffer:

Figure \(\PageIndex{1}\): Circular Buffer - Empty. ("Circular_buffer_-_empty.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

Assume that 1 is written in the center of a Circular Buffer (the exact starting location is not important in a Circular Buffer):

Figure \(\PageIndex{1}\): Circular Buffer 1. ("Circular_buffer_-_XX1XXXX.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

Then assume that two more elements are added to the Circular Buffer — 2 & 3 — which get put after 1:

Figure \(\PageIndex{1}\): Circular Buffer 123. ("Circular_buffer_-_XX123XXXX.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

If two elements are removed, the two oldest values inside of the Circular Buffer would be removed. Circular Buffers use FIFO (First In, First Out) logic. In the example 1 & 2 were the first to enter the Circular Buffer, they are the first to be removed, leaving 3 inside of the Buffer.

Figure \(\PageIndex{1}\): Circular Buffer 3. ("Circular_buffer_-_XXXX3XX.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

If the buffer has 7 elements, then it is completely full:

Figure \(\PageIndex{1}\): Circular Buffer 6789345. ("Circular_buffer_-_6789345.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

A property of the circular buffer is that when it is full and a subsequent write is performed, then it starts overwriting the oldest data. In the current example, two more elements — A & B — are added and they overwrite the 3 & 4:

Figure \(\PageIndex{1}\): Circular Buffer 6789AB5. ("Circular_buffer_-_6789AB5.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

Alternatively, the routines that manage the buffer could prevent overwriting the data and return an error or raise an exception. Whether or not data is overwritten is up to the semantics of the buffer routines or the application using the circular buffer.

Finally, if two elements are now removed then what would be returned is not 3 & 4 but 5 & 6 because A & B overwrote the 3 & the 4 yielding the buffer with:

Figure \(\PageIndex{1}\): Circular Buffer 789AB. ("Circular_buffer_-_X789ABX.svg" by I, Cburnett, Wikimedia is licensed under CC BY-SA 3.0)

Uses

Adapted from:
"Data buffer" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0
"Circular buffer" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0