linux-kernel - Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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Message-ID: <51DD4A90.7040507@linux.vnet.ibm.com>
Date:	Wed, 10 Jul 2013 17:20:40 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Andrew Jones <drjones@...hat.com>, mingo@...hat.com,
	ouyang@...pitt.edu, habanero@...ux.vnet.ibm.com, jeremy@...p.org,
	x86@...nel.org, konrad.wilk@...cle.com, hpa@...or.com,
	pbonzini@...hat.com, linux-doc@...r.kernel.org,
	xen-devel@...ts.xensource.com, peterz@...radead.org,
	mtosatti@...hat.com, stefano.stabellini@...citrix.com,
	andi@...stfloor.org, attilio.rao@...rix.com, gregkh@...e.de,
	agraf@...e.de, chegu_vinod@...com, torvalds@...ux-foundation.org,
	avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

On 07/10/2013 05:11 PM, Gleb Natapov wrote:
> On Wed, Jul 10, 2013 at 04:54:12PM +0530, Raghavendra K T wrote:
>>>> Ingo, Gleb,
>>>>
>>>>  From the results perspective, Andrew Theurer, Vinod's test results are
>>>> pro-pvspinlock.
>>>> Could you please help me to know what will make it a mergeable
>>>> candidate?.
>>>>
>>> I need to spend more time reviewing it :) The problem with PV interfaces
>>> is that they are easy to add but hard to get rid of if better solution
>>> (HW or otherwise) appears.
>>
>> Infact Avi had acked the whole V8 series, but delayed for seeing how
>> PLE improvement would affect it.
>>
> I see that Ingo was happy with it too.
>
>> The only addition from that series has been
>> 1. tuning the SPIN_THRESHOLD to 32k (from 2k)
>> and
>> 2. the halt handler now calls vcpu_on_spin to take the advantage of
>> PLE improvements. (this can also go as an independent patch into
>> kvm)
>>
>> The rationale for making SPIN_THERSHOLD 32k needs big explanation.
>> Before PLE improvements, as you know,
>> kvm undercommit scenario was very worse in ple enabled cases.
>> (compared to ple disabled cases).
>> pvspinlock patches behaved equally bad in undercommit. Both had
>> similar reason so at the end there was no degradation w.r.t base.
>>
>> The reason for bad performance in PLE case was unneeded vcpu
>> iteration in ple handler resulting in high yield_to calls and double
>> run queue locks.
>> With pvspinlock applied, same villain role was played by excessive
>> halt exits.
>>
>> But after ple handler improved, we needed to throttle unnecessary halts
>> in undercommit for pvspinlock to be on par with 1x result.
>>
> Make sense. I will review it ASAP. BTW the latest version is V10 right?
>

Yes. Thank you.

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