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