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Date:	Mon, 07 Apr 2014 12:38:13 -0400
From:	Waiman Long <waiman.long@...com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v8 00/10] qspinlock: a 4-byte queue spinlock with PV support

On 04/07/2014 02:14 AM, Raghavendra K T wrote:
>
>
> I tested the v7,v8 of qspinlock with unfair config on kvm guest.
> I was curious about unfair locks performance in undercommit cases. 
> (overcommit case is expected to perform well)
>
> But I am seeing hang in overcommit cases. Gdb showed that many vcpus
> are halted and there was no progress. Suspecting the problem /race with
> halting, I removed the halt() part of kvm_hibernate().  I am yet to 
> take a closer look at the code on halt() related changes.

It seems like there may still be race conditions where the current code 
is not handling correctly. I will look into that to see where the 
problem is. BTW, what test do you use to produce the hang condition?

>
> Patch series with that change gave around 20% improvement for dbench 
> 2x and 30% improvement for ebizzy 2x cases. (1x has no significant 
> loss/gain).
>
>

What is the baseline for the performance improvement? Is it without the 
unfair lock and PV qspinlock?

-Longman

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