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

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Date:	Thu, 10 May 2012 10:40:05 +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 05/05/2012 01:14, Paul E. McKenney a écrit :
> On Fri, May 04, 2012 at 11:41:13PM +0200, Pascal Chapperon wrote:
>> 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 ?)
>
> Well, one possibility is that your setup starts the jiffies counter
> at some interesting value.  The attached patch (also against v3.4-rc4)
> applies a bit more paranoia to the initialization to handle this
> and other possibilities.
>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
I tried your new patch against v3.4-rc5 and saw no improvement :
still 75 s. for the boot sequence.

I'll send you the logs in a second mail.

Pascal


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