linux-kernel - Re: [patch 21/21] slab defrag: Obsolete SLAB

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Date:	Fri, 16 May 2008 13:16:35 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Mel Gorman <mel@...net.ie>, mpm@...enic.com,
	Matthew Wilcox <matthew@....cx>
Subject: Re: [patch 21/21] slab defrag: Obsolete SLAB


On Thu, 2008-05-15 at 10:05 -0700, Christoph Lameter wrote:
> On Thu, 15 May 2008, Zhang, Yanmin wrote:
> 
> > > It can thrash cachelines if objects from the same slab page are freed 
> > > simultaneously on multiple processors. That occurred in the hackbench 
> > > regression that we addressed with the dynamic configuration of slab sizes.
> > hackbench regression is because of slow allocation instead of slow freeing.
> > With ÿÿdynamic configuration of slab sizes, fast allocation becomes 97% (the bad
> > one is 68%), but fast free is always 8~9% with/without the patch.
> 
> Thanks for using the slab statistics. I wish I had these numbers for the 
> TPC benchmark. That would allow us to understand what is going on while it 
> is running.
> 
> The frees in the hackbench were slow because partial list updates occurred 
> to frequently. The first fix was to let slab sit longer on the partial 
> list. 
I forgot that. 2.6.24 merged the patch.

> The other was the increase of the slab sizes which also increases 
> the per cpu slab size and therefore the objects allocatable without a 
> round trip to the page allocator.
That is what I am talking. 2.6.26-rc merged the patch.

>  Freeing to a per cpu slab never requires 
> partial list updates. So the frees also benefitted from the larger slab 
> sizes. But the effect shows up in the count of partial list updates not in 
> the fast/free collumn.
I agree. It might be better if SLUB could be optimized again to have more consideration
when the slow free percentage is high, because the page lock might ping-pong
among processors if multi-processors access the same slab at the same time.

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