linux-kernel - Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

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Date:	Mon, 14 May 2012 00:38:33 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Avi Kivity <avi@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Attilio Rao <attilio.rao@...rix.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	linux-doc@...r.kernel.org, KVM <kvm@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>
Subject: Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

On 05/13/2012 11:45 AM, Raghavendra K T wrote:
> On 05/07/2012 08:22 PM, Avi Kivity wrote:
>
> I could not come with pv-flush results (also Nikunj had clarified that
> the result was on NOn PLE
>
>> I'd like to see those numbers, then.
>>
>> Ingo, please hold on the kvm-specific patches, meanwhile.
>>
>
> 3 guests 8GB RAM, 1 used for kernbench
> (kernbench -f -H -M -o 20) other for cpuhog (shell script with  while
> true do hackbench)
>
> 1x: no hogs
> 2x: 8hogs in one guest
> 3x: 8hogs each in two guest
>
> kernbench on PLE:
> Machine : IBM xSeries with Intel(R) Xeon(R)  X7560 2.27GHz CPU with 32
> core, with 8 online cpus and 4*64GB RAM.
>
> The average is taken over 4 iterations with 3 run each (4*3=12). and
> stdev is calculated over mean reported in each run.
>
>
> A): 8 vcpu guest
>
>                  BASE                    BASE+patch %improvement w.r.t
>                  mean (sd)               mean (sd)             
> patched kernel time
> case 1*1x:    61.7075  (1.17872)    60.93     (1.475625)    1.27605
> case 1*2x:    107.2125 (1.3821349)    97.506675 (1.3461878)   9.95401
> case 1*3x:    144.3515 (1.8203927)    138.9525  (0.58309319)  3.8855
>
>
> B): 16 vcpu guest
>                  BASE                    BASE+patch %improvement w.r.t
>                  mean (sd)               mean (sd)             
> patched kernel time
> case 2*1x:    70.524   (1.5941395)    69.68866  (1.9392529)   1.19867
> case 2*2x:    133.0738 (1.4558653)    124.8568  (1.4544986)   6.58114
> case 2*3x:    206.0094 (1.3437359)    181.4712  (2.9134116)   13.5218
>
> B): 32 vcpu guest
>                  BASE                    BASE+patch %improvementw.r.t
>                  mean (sd)               mean (sd)             
> patched kernel time
> case 4*1x:    100.61046 (2.7603485)     85.48734  (2.6035035)  17.6905

What does the "4*1x" notation mean? Do these workloads have overcommit
of the PCPU resources?

When I measured it, even quite small amounts of overcommit lead to large
performance drops with non-pv ticket locks (on the order of 10%
improvements when there were 5 busy VCPUs on a 4 cpu system).  I never
tested it on larger machines, but I guess that represents around 25%
overcommit, or 40 busy VCPUs on a 32-PCPU system.

    J
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