Re: [PATCH] mm: make VM_MAX_READAHEAD configurable

[Wu Fengguang <fengguang.wu@intel.com>]

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