linux-kernel - Re: [RFC][PATCH 0/7] re-shrink 'struct page' when SLUB is on.

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Date:	Wed, 18 Dec 2013 16:41:09 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Hansen <dave@...1.net>
Cc:	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Pravin B Shelar <pshelar@...ira.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [RFC][PATCH 0/7] re-shrink 'struct page' when SLUB is on.

On Wed, 18 Dec 2013 16:24:15 -0800 Dave Hansen <dave@...1.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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