Date: Thu, 8 Jan 2009 08:48:23 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Miller <davem@davemloft.net>
cc: akpm@linux-foundation.org, peterz@infradead.org, jack@suse.cz,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org, npiggin@suse.de
Subject: Re: Increase dirty_ratio and dirty_background_ratio?
In-Reply-To: <20090108.082413.156881254.davem@davemloft.net>
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On Thu, 8 Jan 2009, David Miller wrote:

> From: Andrew Morton <akpm@linux-foundation.org>
> Date: Thu, 8 Jan 2009 03:02:45 -0800
> 
> > The kernel can't get this right - it doesn't know the usage
> > patterns/workloads, etc.
> 
> I don't agree with that.

We can certainly try to tune it better. 

And I do agree that we did a very drastic reduction in the dirty limits, 
and we can probably look at raising it up a bit. I definitely do not want 
to go back to the old 40% dirty model, but I could imagine 10/20% for 
async/sync (it's 5/10 now, isn't it?)

But I do not want to be guided by benchmarks per se, unless they are 
latency-sensitive. And one of the reasons for the drastic reduction was 
that there was actually a real deadlock situation with the old limits, 
although we solved that one twice - first by reducing the limits 
drastically, and then by making them be relative to the non-highmem memory 
(rather than all of it).

So in effect, we actually reduced the limits more than originally 
intended, although that particular effect should be noticeable mainly just 
on 32-bit x86.

I'm certainly open to tuning. As long as "tuning" doesn't involve 
something insane like dbench numbers.

			Linus
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