Messages in this thread | | | Date | Mon, 12 Oct 2009 17:39:21 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH] mm: make VM_MAX_READAHEAD configurable |
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On Mon, Oct 12, 2009 at 05:29:48PM +0800, Christian Ehrhardt wrote: > Wu Fengguang wrote: > > [SNIP] > >>> May I ask for more details about your performance regression and why > >>> it is related to readahead size? (we didn't change VM_MAX_READAHEAD..) > >>> > >>> > >> Sure, the performance regression appeared when comparing Novell SLES10 > >> vs. SLES11. > >> While you are right Wu that the upstream default never changed so far, > >> SLES10 had a > >> patch applied that set 512. > >> > > > > I see. I'm curious why SLES11 removed that patch. Did it experienced > > some regressions with the larger readahead size? > > > > > > Only the obvious expected one with very low free/cacheable > memory and a lot of parallel processes that do sequential I/O. > The RA size scales up for all of them but 64xMaxRA then > doesn't fit. > > For example iozone with 64 threads (each on one disk for its own), > sequential access pattern read with I guess 10 M free for cache > suffered by ~15% due to trashing.
FYI, I just finished with a patch for dealing with readahead thrashing. Will do some tests and post the result :)
Thanks, Fengguang
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