linux-kernel - Re: SLUB: Avoid atomic operation for slab

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Date:	Fri, 19 Oct 2007 12:12:00 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Lameter <clameter@....com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: SLUB: Avoid atomic operation for slab_unlock

On Friday 19 October 2007 12:01, Christoph Lameter wrote:
> On Fri, 19 Oct 2007, Nick Piggin wrote:
> > > Yes that is what I attempted to do with the write barrier. To my
> > > knowledge there are no reads that could bleed out and I wanted to avoid
> > > a full fence instruction there.
> >
> > Oh, OK. Bit risky ;) You might be right, but anyway I think it
> > should be just as fast with the optimised bit_unlock on most
> > architectures.
>
> How expensive is the fence? An store with release semantics would be safer
> and okay for IA64.

I'm not sure, I had an idea it was relatively expensive on ia64,
but I didn't really test with a good workload (a microbenchmark
probably isn't that good because it won't generate too much out
of order memory traffic that needs to be fenced).

> > Which reminds me, it would be interesting to test the ia64
> > implementation I did. For the non-atomic unlock, I'm actually
> > doing an atomic operation there so that it can use the release
> > barrier rather than the mf. Maybe it's faster the other way around
> > though? Will be useful to test with something that isn't a trivial
> > loop, so the slub case would be a good benchmark.
>
> Lets avoid mf (too expensive) and just use a store with release semantics.

OK, that's what I've done at the moment.

> Where can I find your patchset? I looked through lkml but did not see it.

Infrastructure in -mm, starting at bitops-introduce-lock-ops.patch.
bit_spin_lock-use-lock-bitops.patch and ia64-lock-bitops.patch are
ones to look at.

The rest of the patches I have queued here, apart from the SLUB patch,
I guess aren't so interesting to you (they don't do anything fancy
like convert to non-atomic unlocks, just switch things like page and
buffer locks to use new bitops).
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