14.1. Overview

All files opened for random access can share the global file cache. In addition, files can be privately cached; however, private file caches can be manipulated only from a program.

When a device MODULE makes a request for a buffer, the file cache is searched. If the buffer is found, it is returned; otherwise a new buffer is read. Thus, an I/O operation is eliminated for every buffer request satisfied by the file cache. This significantly improves performance on I/O-bound machines. For example, the MAINSAIL compiler running on one system showed a 5-to-1 improvement in wall clock performance with the caching disk MODULE.

Maintenance of the cache is governed by three parameters: the minimum number of buffers in the file cache (requestedMinSize), the maximum number of buffers in the file cache (requestedMaxSize), and the file cache hit percentage to be maintained (requestedHitPercent). Buffers are added to the file cache as necessary until requestedMinSize buffers exist, after which the hit percentage governs further addition of buffers. Each time a request is made for a non-resident buffer, the actual hit percentage is compared with requestedHitPercent. If the actual hit percentage is less than requestedHitPercent (minus five percent) and the number of buffers is less than requestedMaxSize, a new buffer is created. The current buffer is added to the cache, making it larger. If the number of buffers is between requestedMinSize and requestedMaxSize and the actual hit percentage is greater than requestedHitPercent (minus five percent), the least recently accessed buffer is removed from the cache, written if necessary, and returned as empty. Again, the current buffer is added to the list, but since one has been removed, the file cache size remains constant.

XIDAK reserves the right to change the implementation of the file cache without notice.

14.2. How to Use GCCHP

Example 14–1 illustrates how GCCHP may be used. The global file cache parameters are first changed so that the minimum number of buffers is 30, the maximum number of buffers is 256, and the hit percentage is 100. The compiler, which opens random access files, is then run and GCCHP is invoked again to display the actual hit percentage and maximum number of buffers allocated for the file cache. The current global file cache parameters remain unchanged.