linux-kernel - Re: bdi_threshold slow to reach steady state

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Date:	Wed, 14 Oct 2009 14:55:20 +0100
From:	Richard Kennedy <richard@....demon.co.uk>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Martin Bligh <mbligh@...gle.com>
Subject: Re: bdi_threshold slow to reach steady state

On Wed, 2009-10-14 at 13:37 +0200, Peter Zijlstra wrote:
> On Wed, 2009-10-14 at 12:09 +0100, Richard Kennedy wrote:
> > Hi Peter,

> 
> Right, so we measure time in page writeback completions, and the measure
> I used was the round up power of two of the dirty_thresh. We adjust in
> the same time it takes to write out a full dirty_thresh amount of data.
> 
> The idea was that people would scale their dirty thesh according to
> their writeout capacity, etc..
> 
> Martin J Bligh complained about this very same issue and I told them to
> experiment with that same scale function. But I guess the result of that
> got lost in the google filter (stuff goes in, nothing ever comes back
> out).
> 
> Anyway, the dirty_thresh relation seems sensible still, but the exact
> parameters could be poked at. I have no objection to reducing the period
> with a factor of 16 like you did, except that we need some more
> feedback, preferably from people with more than a few spindles.

Sure, hopefully big fast machines have large amounts of memory so it
should be a good fit.

Yes, it would be good if someone with a big box tested this ;)
Here's a patch just in case anyone does feel like giving it a spin.

> (The initial ramp will be roughly twice as slow, since the steady state
> of this approximation is half-full).
> 
> > I know that my machine is getting a bit old now, it's AMDX2 & only has
> > sata 150 drives, so I'm not suggesting that this change is going to be
> > correct for all machines but maybe we can set a better default? or take
> > more factors in to account other than just memory size. 
> > 
> > BTW why is it ilog2(dirty_total -1) -- what does the -1 do?
> 
> http://lkml.org/lkml/2007/1/26/143
> 
thanks for that
regards
Richard

(patch against 2.6.32-rc4)

commit 11735a2336ba08cf21aebf79a706c86aca5e44b2
Author: Richard Kennedy <richard@....demon.co.uk>
Date:   Wed Oct 14 14:46:21 2009 +0100

    mm: speed up per bdi dirty threshold calculations
    
    
    Signed-off-by: Richard Kennedy <richard@....demon.co.uk>

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index a3b1409..018024e 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -144,7 +144,7 @@ static int calc_period_shift(void)
 	else
 		dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
 				100;
-	return 2 + ilog2(dirty_total - 1);
+	return ilog2(dirty_total - 1) - 2;
 }
 
 /*


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