Question

Operating systems read from disk more than what a program actually requests, because a program is likely to need nearby information in the future. In my application, when I fetch an item from disk, I would like to show an interval of information around the element. There’s a trade off between how much information I request and show, and speed. However, since the OS already reads more than what I requested, accessing these bytes already in memory is free. What API can I use to find out what’s in the OS caches?

Alternatively, I could use memory mapped files. In that case, the problem reduces to finding out whether a page is swapped to disk or not. Can this be done in any common OS?

EDIT: Related paper http://www.azulsystems.com/events/mspc_2008/2008_MSPC.pdf

Answer 1

You can indeed use your second method, at least on Linux. mmap() the file, then use the mincore() function to determine which pages are resident. From the man page:

int mincore(void *addr, size_t
length, unsigned char *vec);

mincore() returns a vector that
indicates whether pages of the calling
process’s virtual memory are resident
in core (RAM), and so will not cause a
disk access (page fault) if
referenced. The kernel returns
residency information about the
pages starting at the address addr,
and continuing for length bytes.

There’s of course a race condition here – mincore() can tell you that a page is resident, but it might then be swapped out just before you access it. C’est la vie.