Subject: Re: Drastic Slowdown of 'fseek()' Calls From 2.4 to 2.6 -- VMM
	Change?
From: Arjan van de Ven <arjan@infradead.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Ingo Oeser <ioe-lkml@rameria.de>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@osdl.org>, Marr <marr@flex.com>,
       reiserfs-dev@namesys.com
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	 <200602261407.33262.ioe-lkml@rameria.de>  <4401B233.7050308@yahoo.com.au>
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Date: Sun, 26 Feb 2006 15:11:20 +0100
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On Mon, 2006-02-27 at 00:50 +1100, Nick Piggin wrote:
> 
> Not really. The app is not silly if it does an fseek() then a _write_.
> Writing page sized and aligned chunks should not require previously
> uptodate pagecache, so doing a pre-read like this is a complete waste.
> 
> Actually glibc tries to turn this pre-read off if the seek is to a page
> aligned offset, presumably to handle this case. However a big write
> would only have to RMW the first and last partial pages, so pre-reading
> 128KB in this case is wrong.
> 
> And I would also say a 4K read is wrong as well, because a big read will
> be less efficient due to the extra syscall and small IO.


I can very much see the point of issuing a sys_readahead instead.....


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