Re: Random file I/O regressions in 2.6

Nick Piggin
2004-05-03T23:58:09Z

Andrew Morton wrote:
> Peter Zaitsev <peter@mysql.com> wrote:
>
>>On Mon, 2004-05-03 at 13:57, Andrew Morton wrote:
>>
>>>Ram Pai <linuxram@us.ibm.com> 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.