linux-kernel - Re: [PATCH v15 16/16] unfair qspinlock: a queue based unfair lock

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Message-ID: <55268CEA.70303@redhat.com>
Date:	Thu, 09 Apr 2015 10:30:02 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Waiman Long <Waiman.Long@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, 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>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v15 16/16] unfair qspinlock: a queue based unfair lock

On 04/09/2015 10:13 AM, Peter Zijlstra wrote:
> On Thu, Apr 09, 2015 at 09:16:24AM -0400, Rik van Riel wrote:
>> On 04/09/2015 03:01 AM, Peter Zijlstra wrote:
>>> On Wed, Apr 08, 2015 at 02:32:19PM -0400, Waiman Long wrote:
>>>> For a virtual guest with the qspinlock patch, a simple unfair byte lock
>>>> will be used if PV spinlock is not configured in or the hypervisor
>>>> isn't either KVM or Xen. The byte lock works fine with small guest
>>>> of just a few vCPUs. On a much larger guest, however, byte lock can
>>>> have serious performance problem.
>>>
>>> Who cares?
>>
>> There are some people out there running guests with dozens
>> of vCPUs. If the code exists to make those setups run better,
>> is there a good reason not to use it?
> 
> Well use paravirt, !paravirt stuff sucks performance wise anyhow.
> 
> The question really is: is the added complexity worth the maintenance
> burden. And I'm just not convinced !paravirt virt is a performance
> critical target.

Fair enough.

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