linux-kernel - Re: [PATCH 2/2] slub: fix Objects count

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Message-ID: <Pine.LNX.4.64.0711031226131.5695@schroedinger.engr.sgi.com>
Date:	Sat, 3 Nov 2007 12:33:01 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Hugh Dickins <hugh@...itas.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] slub: fix Objects count

On Sat, 3 Nov 2007, Hugh Dickins wrote:

> I was afraid you might say something like that.
> Perhaps it'll be a patch I need to use in my own builds.
> Though I'd have thought others would want that accuracy too.
> Didn't SLAB give it?  (The "r*gr*ss**n" word!)

Slab also only counts objects that are not in the queues. See free_block() 
f.e.

We could improve the situation by flushing all cpu slabs before counts are 
determined.

Which can be done manually. Run

	slabinfo -s

and then look at the numbers.

> > Adds to much overhead to the fast paths
> 
> You've come to that conclusion very quickly!

I have just spend a few weeks optimizing the fast and slow paths and there 
is some additional overhead that I am still trying to eliminate.

> Any numbers to back it up?

The performance in the fast paths depends on updating only a single word
for an allocation. Adding another counter makes that impossible.

See the recent post on SLUB regression on SMP.
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