Messages in this thread | | | Date | Wed, 14 Mar 2007 22:33:17 +0100 | From | Andreas Mohr <> | Subject | Re: [PATCH] mm/filemap.c: unconditionally call mark_page_accessed |
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Hi,
On Wed, Mar 14, 2007 at 03:55:41PM -0500, Dave Kleikamp wrote: > On Wed, 2007-03-14 at 15:58 -0400, Ashif Harji wrote: > > This patch unconditionally calls mark_page_accessed to prevent pages, > > especially for small files, from being evicted from the page cache despite > > frequent access. > > I guess the downside to this is if a reader is reading a large file, or > several files, sequentially with a small read size (smaller than > PAGE_SIZE), the pages will be marked active after just one read pass. > My gut says the benefits of this patch outweigh the cost. I would > expect real-world backup apps, etc. to read at least PAGE_SIZE.
I also think that the patch is somewhat problematic, since the original intention seems to have been a reduction of the number of (expensive?) mark_page_accessed() calls, but this of course falls flat on its face in case of permanent single-page accesses or accesses with progressing but very small read size (single-byte reads or so), since the cached page content will expire eventually due to lack of mark_page_accessed() updates; thus this patch decided to call mark_page_accessed() unconditionally which may be a large performance penalty for subsequent tiny-sized reads.
I've been thinking hard how to avoid the mark_page_accessed() starvation in case of a fixed, (almost) non-changing access state, but this seems hard since it'd seem we need some kind of state management here to figure out good intervals of when to call mark_page_accessed() *again* for this page. E.g. despite non-changing access patterns you could still call mark_page_accessed() every 32 calls or so to avoid expiry, but this would need extra helper variables.
A rather ugly way to do it may be to abuse ra.cache_hit or ra.mmap_hit content with a if ((prev_index != index) || (ra.cache_hit % 32 == 0)) mark_page_accessed(page); This assumes that ra.cache_hit gets incremented for every access (haven't checked whether this is the case). That way (combined with an enhanced comment properly explaining the dilemma) you would avoid most mark_page_accessed() invocations of subsequent same-page reads but still do page status updates from time to time to avoid page deprecation.
Does anyone think this would be acceptable? Any better idea?
Andreas Mohr
P.S.: since I'm not too familiar with this area I could be rather wrong after all... - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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