Re: Random file I/O regressions in 2.6

[Nick Piggin]

Andrew Morton wrote:
> Peter Zaitsev <peter@xxxxxxxxx> wrote:
>
> > On Mon, 2004-05-03 at 13:57, Andrew Morton wrote:
> >
> > > Ram Pai <linuxram@xxxxxxxxxx> wrote:
> > >
> > > > > The place which needs attention is handle_ra_miss().  But first I'd like to
> > > > > reacquaint myself with the intent behind the lazy-readahead patch.  Was
> > > > > never happy with the complexity and special-cases which that introduced.
> > > >
> > > > lazy-readahead has no role to play here.
> > Andrew,
> >
> > Could you please clarify how this things become to be dependent on
> > read-ahead at all.
>
>
> readahead is currently the only means by which we build up nice large
> multi-page BIOs.
>
>
> > At my understanding read-ahead it to catch sequential (or other) access
> > pattern and do some advance reading, so instead of 16K request we do
> > 128K request, or something similar.
>
>
> That's one of its usage patterns.  It's also supposed to detect the
> fixed-sized-reads-seeking-all-over-the-place situation.  In which case it's
> supposed to submit correctly-sized multi-page BIOs.  But it's not working
> right for this workload.
>
> A naive solution would be to add special-case code which always does the
> fixed-size readahead after a seek.  Basically that's
>
> 	if (ra->next_size == -1UL)
> 		force_page_cache_readahead(...)

I think a better solution to this case would be to ensure the
readahead window is always min(size of read, some large number);

The size of the read is basically a free and accurate "hint" to
the minimum size of the required readahead.

Either that or do a simple "preread" while you're still in the
read request window, and run readahead when that completes.
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