Re: Random file I/O regressions in 2.6

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> 
>>>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);
>>
> 
> 
> That would cause the kernel to perform lots of pointless pagecache lookups
> when the file is already 100% cached.
> 


That's pretty sad. You need a "preread" or something which
sends the pages back... or uses the actor itself. readahead
would then have to be reworked to only run off the end of
the read window, but that is what it should be doing anyway.