Re: [PATCH] fix readahead breakage for sequential after randomreads

[Ram Pai]

On Mon, 2004-07-26 at 17:08, Andrew Morton wrote:
> Ram Pai <linuxram@xxxxxxxxxx> wrote:
> >
> > Andrew,
> > 	Yes the patch fixes a valid bug.
> > 
> 
> Please don't top-post :(
> > RP
> > 
> > On Mon, 2004-07-26 at 16:29, Andrew Morton wrote:
> > > Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> > > >
> > > > Current readahead logic is broken when a random read pattern is
> > > >  followed by a long sequential read.  The cause is that on a window
> > > >  miss ra->next_size is set to ra->average, but ra->average is only
> > > >  updated at the end of a sequence, so window size will remain 1 until
> > > >  the end of the sequential read.
> > > > 
> > > >  This patch fixes this by taking the current sequence length into
> > > >  account (code taken from towards end of page_cache_readahead()), and
> > > >  also setting ra->average to a decent value in handle_ra_miss() when
> > > >  sequential access is detected.
> > > 
> > > Thanks.   Do you have any performance testing results from this patch?
> > > 
> > Ram Pai <linuxram@xxxxxxxxxx> wrote:
> >
> > Andrew,
> > 	Yes the patch fixes a valid bug.
> 
> Fine, but the readahead code is performance-sensitive, and it takes quite
> some time for any regressions to be discovered.  So I'm going to need to
> either sit on this patch for a very long time, or extensively test it
> myself, or await convincing test results from someone else.
> 
> Can you help with that?

yes I will run all my standard testsuites before we take this patch.
(DSS workload, iozone, sysbench). I will get back with some results
sooon. Probably by the end of this week.

Also I think the bug that Miklos, found is really hard to reproduce. Did
he find this bug by code inspection? Its really really hard to get into
a state where the current window is of size 1 page with zero pages in
the readahead window, and then the sequential read pattern to just right
then. 

RP

> 

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