linux-kernel - RE: [PATCH] lockref: remove cpu

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Message-ID: <3908561D78D1C84285E8C5FCA982C28F31CDC251@ORSMSX106.amr.corp.intel.com>
Date:	Thu, 5 Sep 2013 14:48:52 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] lockref: remove cpu_relax() again

> *If* however the cpu_relax() makes sense on other platforms maybe we could
> add something like we have already with "arch_mutex_cpu_relax()":

I'll do some more measurements on ia64.  During my first tests cpu_relax() seemed
to be a big win - but I only ran "./t" a couple of times.  Later (with the cpu_relax() in
place) I ran a bunch more iterations, and found that the variation from run to run
is much larger with lockref.  The mean score is 60% higher, but the standard deviation
is an order of magnitude bigger (enough that one run out of 20 with lockref scored
lower than the pre-lockref kernel).

I think this is expected ... cmpxchg is a free-for-all - and sometimes poor placement
across the four socket system might cause short term starvation to a thread while
threads on another socket monopolize the cache line.

-Tony
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