Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression
From: Dave Chinner
Date: Sun Sep 06 2015 - 19:47:41 EST
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...
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