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Date:	Mon, 07 Apr 2014 12:38:13 -0400
From:	Waiman Long <>
To:	Raghavendra K T <>
CC:	Thomas Gleixner <>,
	Ingo Molnar <>,
	"H. Peter Anvin" <>,
	Peter Zijlstra <>,,,,,,,
	Paolo Bonzini <>,
	Konrad Rzeszutek Wilk <>,
	"Paul E. McKenney" <>,
	Rik van Riel <>,
	Linus Torvalds <>,
	David Vrabel <>,
	Oleg Nesterov <>,
	Gleb Natapov <>,
	Aswin Chandramouleeswaran <>,
	Scott J Norton <>,
	Chegu Vinod <>
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?


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