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

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Date:	Mon, 7 Sep 2015 09:32:26 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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 Fri, Sep 04, 2015 at 05:25:23PM +0200, Peter Zijlstra wrote:
> Something like so...
> 
> ---
> Subject: locking: Fix virt test-and-set lock implementation
> 
> Dave ran into horrible performance on a VM without PARAVIRT_SPINLOCKS
> set and Linus noted that the test-and-set implementation was retarded.
> 
> One should spin on the variable with a load, not a rmw.
> 
> While there, remove the queued from the name, as the lock isn't queued
> at all, but a simple test-and-set.
> 
> Reported-by: Dave Chinner <david@...morbit.com>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

FSUse%        Count         Size    Files/sec     App Overhead
     0      1600000            0     319511.8          9398733
     0      3200000            0     301517.9          9516230
     0      4800000            0     300357.6          9676651
     0      6400000            0     282167.8         11513255

Looks OK.

Tested-by: Dave Chinner <david@...morbit.com>

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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