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Date: Mon, 3 May 2004 17:10:05 -0700
From: Andrew Morton <akpm@osdl.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: peter@mysql.com, linuxram@us.ibm.com, alexeyk@mysql.com,
       linux-kernel@vger.kernel.org, axboe@suse.de
Subject: Re: Random file I/O regressions in 2.6
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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.