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Date: Mon, 07 Apr 2014 12:38:13 -0400
From: Waiman Long <waiman.long@hp.com>
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To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
CC: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
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        xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
        Paolo Bonzini <paolo.bonzini@gmail.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
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Subject: Re: [PATCH v8 00/10] qspinlock: a 4-byte queue spinlock with PV support
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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