linux-kernel - Re: Increase dirty_ratio and dirty_background

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Message-Id: <20090107.125133.214628094.davem@davemloft.net>
Date:	Wed, 07 Jan 2009 12:51:33 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...ux-foundation.org
Cc:	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?

From: Linus Torvalds <torvalds@...ux-foundation.org>
Date: Wed, 7 Jan 2009 08:39:01 -0800 (PST)

> On Wed, 7 Jan 2009, Peter Zijlstra wrote:
> > 
> > >   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.
> 
> Not just a bit. If you have 4GB of RAM (not at all unusual for even just a 
> regular desktop, never mind a "real" workstation), it's simply crazy to 
> allow 1.5GB of dirty memory. Not unless you have a really wicked RAID 
> system with great write performance that can push it out to disk (with 
> seeking) in just a few seconds.
> 
> And few people have that.
> 
> For a server, where throughput matters but latency generally does not, go 
> ahead and raise it. But please don't raise it for anything sane. The only 
> time it makes sense upping that percentage is for some odd special-case 
> benchmark that otherwise can fit the dirty data set in memory, and never 
> syncs it (ie it deletes all the files after generating them).
> 
> In other words, yes, 40% dirty can make a big difference to benchmarks, 
> but is almost never actually a good idea any more.

I have to say that my workstation is still helped by reverting this
change and all I do is play around in GIT trees and read email.

It's a slow UltraSPARC-IIIi 1.5GHz machine with a very slow IDE disk
and 2GB of ram.

With the dirty ratio changeset there, I'm waiting for disk I/O
seemingly all the time.  Without it, I only feel the machine seize up
in disk I/O when I really punish it.

Maybe all the dirty I/O is from my not using 'noatime', and if that's
how I should "fix" this then we can ask why isn't it the default? :)

I did mention this when the original changeset went into the tree.

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