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

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Message-ID: <Pine.LNX.4.64.0711031904470.14407@blonde.wat.veritas.com>
Date:	Sat, 3 Nov 2007 19:09:57 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	Christoph Lameter <clameter@....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, Christoph Lameter wrote:
> On Sat, 3 Nov 2007, Hugh Dickins wrote:
> 
> > The count of active Objects shown by Slub's slabinfo is too approximate,
> > because each cpu slab is counted as all in use, even when lots are free.
> > That makes tracing leaks harder than it need be.
> 
> True but that is the way it is for performance reasons.

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!)

> > Add a free count into kmem_cache_cpu (which doesn't enlarge it on 64-bit),
> > to keep that count in the hot and dirty per-cpu cacheline.
> 
> Adds to much overhead to the fast paths

You've come to that conclusion very quickly!
Any numbers to back it up?

> and will make the current optimizations in mm impossible.

I'll have to wait and see what those are: you move too fast for me.

Hugh
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