From: Andrew Morton <akpm@linux-foundation.org>
To: Dave Hansen <dave@sr71.net>
Cc: Christoph Lameter <cl@linux.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Pravin B Shelar <pshelar@nicira.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Andi Kleen <ak@linux.intel.com>,
Pekka Enberg <penberg@kernel.org>
Subject: Re: [RFC][PATCH 0/7] re-shrink 'struct page' when SLUB is on.
Date: Wed, 18 Dec 2013 16:41:09 -0800 [thread overview]
Message-ID: <20131218164109.5e169e258378fac44ec5212d@linux-foundation.org> (raw)
In-Reply-To: <52B23CAF.809@sr71.net>
On Wed, 18 Dec 2013 16:24:15 -0800 Dave Hansen <dave@sr71.net> wrote:
> On 12/17/2013 07:17 AM, Christoph Lameter wrote:
> > On Mon, 16 Dec 2013, Dave Hansen wrote:
> >
> >> I'll do some testing and see if I can coax out any delta from the
> >> optimization myself. Christoph went to a lot of trouble to put this
> >> together, so I assumed that he had a really good reason, although the
> >> changelogs don't really mention any.
> >
> > The cmpxchg on the struct page avoids disabling interrupts etc and
> > therefore simplifies the code significantly.
> >
> >> I honestly can't imagine that a cmpxchg16 is going to be *THAT* much
> >> cheaper than a per-page spinlock. The contended case of the cmpxchg is
> >> way more expensive than spinlock contention for sure.
> >
> > Make sure slub does not set __CMPXCHG_DOUBLE in the kmem_cache flags
> > and it will fall back to spinlocks if you want to do a comparison. Most
> > non x86 arches will use that fallback code.
>
>
> I did four tests. The first workload allocs a bunch of stuff, then
> frees it all with both the cmpxchg-enabled 64-byte struct page and the
> 48-byte one that is supposed to use a spinlock. I confirmed the 'struct
> page' size in both cases by looking at dmesg.
>
> Essentially, I see no worthwhile benefit from using the double-cmpxchg
> over the spinlock. In fact, the increased cache footprint makes it
> *substantially* worse when doing a tight loop.
>
> Unless somebody can find some holes in this, I think we have no choice
> but to unset the HAVE_ALIGNED_STRUCT_PAGE config option and revert using
> the cmpxchg, at least for now.
>
So your scary patch series which shrinks struct page while retaining
the cmpxchg_double() might reclaim most of this loss?
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next prev parent reply other threads:[~2013-12-19 0:41 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-13 23:59 Dave Hansen
2013-12-13 23:59 ` [RFC][PATCH 1/7] mm: print more details for bad_page() Dave Hansen
2013-12-16 16:52 ` Christoph Lameter
2013-12-16 17:20 ` Andi Kleen
2013-12-13 23:59 ` [RFC][PATCH 2/7] mm: page->pfmemalloc only used by slab/skb Dave Hansen
2013-12-13 23:59 ` [RFC][PATCH 3/7] mm: slabs: reset page at free Dave Hansen
2013-12-13 23:59 ` [RFC][PATCH 4/7] mm: rearrange struct page Dave Hansen
2013-12-13 23:59 ` [RFC][PATCH 5/7] mm: slub: rearrange 'struct page' fields Dave Hansen
2013-12-13 23:59 ` [RFC][PATCH 6/7] mm: slub: remove 'struct page' alignment restrictions Dave Hansen
2013-12-14 3:13 ` Andi Kleen
2013-12-13 23:59 ` [RFC][PATCH 7/7] mm: slub: cleanups after code churn Dave Hansen
2013-12-17 0:01 ` [RFC][PATCH 0/7] re-shrink 'struct page' when SLUB is on Andrew Morton
2013-12-17 0:45 ` Dave Hansen
2013-12-17 15:17 ` Christoph Lameter
2013-12-19 0:24 ` Dave Hansen
2013-12-19 0:41 ` Andrew Morton [this message]
2013-12-19 0:48 ` Dave Hansen
2013-12-19 15:21 ` Christoph Lameter
2013-12-19 19:14 ` Dave Hansen
2013-12-18 8:51 ` Pekka Enberg
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