Chapter 8. Buffering

8.1. Buffering overview

I/O is the process of transferring data between a program and an external device. The process of optimizing I/O consists primarily of making the best possible use of the slowest part of the path between the program and the device.

The slowest part is usually the physical channel, which is often slower than the CPU or a memory-to-memory data transfer. The time spent in I/O processing overhead can reduce the amount of time that a channel can be used, thereby reducing the effective transfer rate. The biggest factor in maximizing this channel speed is often the reduction of I/O processing overhead.

A buffer is a temporary storage location for data while the data is being transferred. A buffer is often used for the following purposes:

• Small I/O requests can be collected into a buffer, and the overhead of making many relatively expensive system calls can be greatly reduced.

  A collection buffer of this type can be sized and handled so that the actual physical I/O requests made to the operating system match the physical characteristics of the device being used. For example, a 42-sector buffer, when read or written, transfers a track of data between the buffer and the DD-49 disk; a track is a very efficient transfer size.

• Many data file structures, such as the f77 and cos file structures, contain control words. During the write process, a buffer can be used as a work area where control words can be inserted into the data stream (a process called blocking). The blocked data is then written to the device. During the read process, the same buffer work area can be used to examine and remove these control words before passing the data on to the user (deblocking).

• When data access is random, the same data may be requested many times. A cache is a buffer that keeps old requests in the buffer in case these requests are needed again. A cache that is sufficiently large and/or efficient can avoid a large part of the physical I/O by having the data ready in a buffer. When the data is often found in the cache buffer, it is referred to as having a high hit rate. For example, if the entire file fits in the cache and the file is present in the cache, no more physical requests are required to perform the I/O. In this case, the hit rate is 100%.

• Running the disks and the CPU in parallel often improves performance; therefore, it is useful to keep the CPU busy while data is being moved. To do this when writing, data can be transferred to the buffer at memory-to-memory copy speed and an asynchronous I/O request can be made. The control is then immediately returned to the program, which continues to execute as if the I/O were complete (a process called write-behind). A similar process can be used while reading; in this process, data is read into a buffer before the actual request is issued for it. When it is needed, it is already in the buffer and can be transferred to the user at very high speed. This is another form or use of a cache.

Buffers are used extensively on UNICOS and UNICOS/mk systems. Some of the disk controllers have built-in buffers. The kernel has a cache of buffers called the system cache that it uses for various I/O functions on a system-wide basis. The Cray IOS uses buffers to enhance I/O performance. The UNICOS logical device cache (ldcache) is a buffering scheme that uses a part of the solid-state storage device (SSD) or buffer memory resident (BMR) in the IOS as a large buffer that is associated with a particular file system. The library routines also use buffers.

The I/O path is divided into two parts. One part includes the user data area, the library buffer, and the system cache. The second part is referred to as the logical device, which includes the ultimate I/O device and all of the buffering, caching, and processing associated with that device. This includes any caching in the disk controller and the operating system.

Users can directly or indirectly control some buffers. These include most library buffers and, to some extent, system cache and ldcache. Some buffering, such as that performed in the IOS, or the disk controllers, is not under user control.

A well-formed request refers to I/O requests that meet the criteria for UNICOS systems; a well-formed request for a disk file requires the following:

• The size of the request must be a multiple of the sector size in bytes. For most disk devices, this will be 4096 bytes.

• The data that will be transferred must be located on a word boundary.

• The file must be positioned on a sector boundary. This will be a 4096-byte sector boundary for most disks.