linux-kernel - Re: Increase dirty_ratio and dirty_background

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Date:	Wed, 07 Jan 2009 17:25:46 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: Increase dirty_ratio and dirty_background_ratio?

On Wed, 2009-01-07 at 16:45 +0100, Jan Kara wrote:
> Hi,
> 
>   I'm writing mainly to gather opinions of clever people here ;). In commit
> 07db59bd6b0f279c31044cba6787344f63be87ea (in April 2007) Linus has
> decreased default /proc/sys/vm/dirty_ratio from 40 to 10 and
> /proc/sys/vm/dirty_background_ratio from 10 to 5. 

> While tracking
> performance regressions in SLES11 wrt SLES10 we noted that this has severely
> affected perfomance of some workloads using Berkeley DB (basically because
> what the database does is that it creates a file almost as big as available
> memory, mmaps it and randomly scribbles all over it and with lower limits
> it gets much earlier throttled / pdflush is more aggressive writing back
> stuff which is counterproductive in this particular case).

>   So the question is: What kind of workloads are lower limits supposed to
> help? Desktop? Has anybody reported that they actually help? I'm asking
> because we are probably going to increase limits to the old values for
> SLES11 if we don't see serious negative impact on other workloads...

Adding some CCs.

The idea was that 40% of the memory is a _lot_ these days, and writeback
times will be huge for those hitting sync or similar. By lowering these
you'd smooth that out a bit.


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