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SubjectRe: [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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