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

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Date:	Tue, 17 Dec 2013 15:17:37 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	Dave Hansen <dave@...1.net>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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 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.


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