linux-kernel - Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

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Message-Id: <006322ad-2e72-c8fe-361f-233f1821063a@linux.vnet.ibm.com>
Date:   Thu, 5 Jan 2017 16:16:38 +0800
From:   Pan Xinhui <xinhui@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v2] locking/pvqspinlock: Relax cmpxchg's to improve
 performance on some archs



在 2017/1/4 17:41, Peter Zijlstra 写道:
> On Tue, Jan 03, 2017 at 05:07:54PM -0500, Waiman Long wrote:
>> On 01/03/2017 11:18 AM, Peter Zijlstra wrote:
>>> On Sun, Dec 25, 2016 at 03:26:01PM -0500, Waiman Long wrote:
>>>> A number of cmpxchg calls in qspinlock_paravirt.h were replaced by more
>>>> relaxed versions to improve performance on architectures that use LL/SC.
>>> Claim without numbers ;-)
>>
>> Well it is hard to produce actual numbers here as I don't have the setup
>> to gather data.
>
> Surely RHT has big PPC machines around? I know that getting to them is a
> wee bit of a bother, but they should be available somewhere.
>
hi,

I do some tests about cmpxchg and cmpxchg_acquire before on ppc.

loops in 15s of each cmpxchg is below.

cmpxchg_relaxed: 336663
cmpxchg_release: 369054
cmpxchg_acquire: 363364
cmpxchg:	 179435

so cmpxchg is really expensive than others.
but I also have doubt about the cmpxchg_relaxed, it should be the cheapest, but from the tests, release/acquire are faster than it.

thanks
xinhui