linux-kernel - Re: RCU related performance regression in 3.3

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Message-ID: <4FA44CF9.7050604@wanadoo.fr>
Date:	Fri, 04 May 2012 23:41:13 +0200
From:	Pascal Chapperon <pascal.chapperon@...adoo.fr>
To:	paulmck@...ux.vnet.ibm.com
CC:	Josh Boyer <jwboyer@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-team@...oraproject.org
Subject: Re: RCU related performance regression in 3.3

Le 04/05/2012 17:04, Paul E. McKenney a écrit :
> On Fri, May 04, 2012 at 04:42:54PM +0200, Pascal Chapperon wrote:
>> Le 01/05/2012 17:45, Paul E. McKenney a écrit :
>>
>>> Here is my RCU_FAST_NO_HZ patch stack on top of v3.4-rc4.
>>>
>>> Or you can pull branch fnh.2012.05.01a from:
>>>
>>> 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
>>>
>>> 							Thanx, Paul
>>>
>> I applied your global patch on top of v3.4-rc4. But the slowdown is
>> worse than before : boot sequence took 80s instead 20-30s (12s for
>> initramfs instead of 2s).
>>
>> I'll send you rcu tracing log in a second mail.
>
> Hmmm...  Well, I guess I am glad that I finally did something that
> had an effect, but I sure wish that the effect had been in the other
> direction!
>
> Just to make sure I understand: the difference between the 20-30s and
> the 80s is exactly the patch I sent you?
>
> 							Thanx, Paul
>
>
Yes. Exactly same kernel config as in previous results, I applied your 
patch against v3.4-rc4, and sorry, the result is exactly what I said;
I saw that your global patch was quite huge, and addresses things which
are not directly related with the initial patch (commit 
7cb92499000e3c86dae653077b1465458a039ef6); maybe a side effect?

However, I'm ready to try this patch on my smaller laptop which
supports well CONFIG_FAST_NO_HZ=y and systemd, if you think it can
help ?

Another thought: this issue as nothing to do with i7 Hyper-threading
capacities ? (as I test core2duo, Pentium ulv in same conditions and I
don't encountered any slowdown ?)

Pascal

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