linux-kernel - Re: Increase dirty_ratio and dirty_background

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Message-ID: <alpine.LFD.2.00.0901080858500.3283@localhost.localdomain>
Date:	Thu, 8 Jan 2009 09:05:01 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Chris Mason <chris.mason@...cle.com>
cc:	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	peterz@...radead.org, jack@...e.cz, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, npiggin@...e.de
Subject: Re: Increase dirty_ratio and dirty_background_ratio?

On Thu, 8 Jan 2009, Chris Mason wrote:
> 
> Does it make sense to hook into kupdate?  If kupdate finds it can't meet
> the no-data-older-than 30 seconds target, it lowers the sync/async combo
> down to some reasonable bottom.  
> 
> If it finds it is going to sleep without missing the target, raise the
> combo up to some reasonable top.

I like autotuning, so that sounds like an intriguing approach. It's worked 
for us before (ie VM).

That said, 30 seconds sounds like a _loong_ time for something like this. 
I'd use the normal 5-second dirty_writeback_interval for this: if we can't 
clean the whole queue in that normal background writeback interval, then 
we try to lower the tagets. We already have that "congestion_wait()" thing 
there, that would be a logical place, methinks.

I'm not sure how to raise them, though. We don't want to raise any limits 
just because the user suddenly went idle. I think the raising should 
happen if we hit the sync/async ratio, and we haven't lowered in the last 
30 seconds or something like that.

		Linus
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