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Memory Residency Testing

Flash uses the mincore() system call, which is available in most modern UNIX systems, to determine if mapped file pages are memory resident. In operating systems that don't support this operation but provide the mlock() system call to lock memory pages (e.g., Compaq's Tru64 UNIX, formerly Digital Unix), Flash could use the latter to control its file cache management, eliminating the need for memory residency testing.

Should no suitable operations be available in a given operating system to control the file cache or test for memory residency, it may be possible to use a feedback-based heuristic to minimize blocking on disk I/O. Here, Flash could run the clock algorithm to predict which cached file pages are memory resident. The prediction can adapt to changes in the amount of memory available to the file cache by using continuous feedback from performance counters that keep track of page faults and/or associated disk accesses.

Peter Druschel