Subject: Re: Increase dirty_ratio and dirty_background_ratio?
From: Chris Mason <chris.mason@oracle.com>
To: Jan Kara <jack@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       David Miller <davem@davemloft.net>, akpm@linux-foundation.org,
       peterz@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       npiggin@suse.de
In-Reply-To: <20090109180241.GA15023@duck.suse.cz>
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Date: Fri, 09 Jan 2009 14:07:22 -0500
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On Fri, 2009-01-09 at 19:02 +0100, Jan Kara wrote:

>   What we observe in the seekwatcher graphs is, that there are three
> processes writing back the single database file in parallel (2 pdflush
> threads because the machine has 2 CPUs, and the database process itself
> because of dirty throttling). Each of the processes is writing back the
> file at a different offset and so they together create even more random IO
> (I'm attaching the graph and can provide blocktrace data if someone is
> interested). If there was just one process doing the writeback, we'd be
> writing back those data considerably faster...

I spent some time trying similar things for btrfs, and went as far as
making my own writeback thread and changing pdflush and throttled writes
to wait on it.  It was a great hack, but in the end I found the real
problem was the way write_cache_pages is advancing the page_index.

You probably remember the related ext4 discussion, and you could try my
simple patch in this workload to see if it helps ext3.

http://lkml.org/lkml/2008/10/1/278

Ext3 may need similar tricks.

-chris


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