From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 14 Mar 2007 22:33:17 +0100 From: Andreas Mohr Subject: Re: [PATCH] mm/filemap.c: unconditionally call mark_page_accessed Message-ID: <20070314213317.GA22234@rhlx01.hs-esslingen.de> References: <20070312142012.GH30777@atrey.karlin.mff.cuni.cz> <20070312143900.GB6016@wotan.suse.de> <20070312151355.GB23532@duck.suse.cz> <20070312173500.GF23532@duck.suse.cz> <20070313185554.GA5105@duck.suse.cz> <1173905741.8763.36.camel@kleikamp.austin.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1173905741.8763.36.camel@kleikamp.austin.ibm.com> Sender: owner-linux-mm@kvack.org Return-Path: To: Dave Kleikamp Cc: Ashif Harji , linux-mm@kvack.org, Nick Piggin , Jan Kara , linux-kernel@vger.kernel.org, akpm@linux-foundation.org List-ID: 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org