linux-kernel - Re: Change in default vm_dirty

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Date:	Tue, 19 Jun 2007 12:04:33 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	John Stoffel <john@...ffel.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: Change in default vm_dirty_ratio

On Tue, 19 Jun 2007, John Stoffel wrote:
> 
> Shouldn't the vm_dirty_ratio be based on the speed of the device, and
> not the size of memory?

Yes. It should depend on:
 - speed of the device(s) in question
 - seekiness of the workload
 - wishes of the user as per the latency of other operations.

However, nobody has ever found the required algorithm.

So "at most 10% of memory dirty" is a simple (and fairly _good_) 
heuristic. Nobody has actually ever ended up complaining about the change 
from 40% -> 10%, and as far as I know this was the first report (and it's 
not so much because the change was bad, but because it showed up on a 
benchmark - and I don't think that actually says anythign about anything 
else then the behaviour of the benchmark itself)

So are there better algorithms in theory? Probably lots of them.

		Linus
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