Re: [PATCH] fix readahead breakage for sequential after random reads

[Andrew Morton <akpm@osdl.org>]

Ram Pai <linuxram@us.ibm.com> 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@szeredi.hu> 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@us.ibm.com> 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?