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Turning off cachingTurning off cachingPosted Aug 8, 2013 12:13 UTC (Thu) by Funcan (subscriber, #44209)In reply to: Turning off caching by epa Parent article: A survey of memory management patches
man fadvise64 and look for the POSIX_FADV_SEQUENTIAL flag
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Turning off caching Posted Aug 8, 2013 14:58 UTC (Thu) by sbohrer (subscriber, #61058) [Link]
POSIX_FADV_SEQUENTIAL? I don't think that does what the previous poster asked but I've been surprised by what the various fadvise flags _actually_ do before. POSIX_FADV_NOREUSE sounds like it might avoid caching pages but the man page I'm looking at claims that in 2.6.18+ it is a no-op.
I am certain that POSIX_FADV_DONTNEED drops pages from the page cache but it doesn't work for future pages. In other words you have to periodically call it on pages you've previously read or have written which is somewhat annoying. The other gotcha for writes is that POSIX_FADV_DONTNEED doesn't drop dirty pages from the page cache it only initiates writeback so you have to call it twice for each possibly dirty page range if you really want those pages dropped. I currently use this for write-once files or files that I know will no longer be in the page cache by the next time I'm going to need them.
Turning off caching Posted Aug 11, 2013 1:28 UTC (Sun) by giraffedata (subscriber, #1954) [Link] I don't know exactly what Linux's current page replacement policy is, but this problem of sequential read of a file too big to fit in cache pushing other stuff out of cache as it goes, called a cache sweep, was solved long ago. The kernel should detect that this is happening and stop caching that file before it does much harm, and I presume that it does. That would explain why Linux doesn't do anything special with POSIX_FADV_NOREUSE.I know that even before modern cache sweep protection was invented, Linux avoided much of the pain by using version of second-chance, so that these pages, since they were referenced only once, would be the first to be evicted and most of the pages that would actually be referenced again would remain.
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