linux-kernel - Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression

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Date:	Wed, 09 Sep 2015 22:09:20 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Chinner <david@...morbit.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [4.2, Regression] Queued spinlocks cause major XFS performance
 regression

On 09/06/2015 07:47 PM, Dave Chinner wrote:
> On Fri, Sep 04, 2015 at 01:32:33PM +0200, Peter Zijlstra wrote:
>> On Fri, Sep 04, 2015 at 06:12:34PM +1000, Dave Chinner wrote:
>>> You probably don't even need a VM to reproduce it - that would
>>> certainly be an interesting counterpoint if it didn't....
>> Even though you managed to restore your DEBUG_SPINLOCK performance by
>> changing virt_queued_spin_lock() to use __delay(1), I ran the thing on
>> actual hardware just to test.
>>
>> [ Note: In any case, I would recommend you use (or at least try)
>>    PARAVIRT_SPINLOCKS if you use VMs, as that is where we were looking for
>>    performance, the test-and-set fallback really wasn't meant as a
>>    performance option (although it clearly sucks worse than expected).
> FSUse%        Count         Size    Files/sec     App Overhead
>       0      1600000            0     319431.5         10116018
>       0      3200000            0     307824.5         10054299
>       0      4800000            0     296971.5         10770197
>       0      6400000            0     281653.6         11748423
> ....
>
> PARAVIRT_SPINLOCKS seems to work OK these days, too. I'll leave that
> set so I'll end up testing whatever comes along down that pipe...
>
> Cheers,
>
> Dave.

I am working on patches to improve PV qspinlock performance and will run 
your fstest to verify that there will be no regression.

Cheers,
Longman
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