From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH v15 16/16] unfair qspinlock: a queue based unfair lock
Date: Thu, 09 Apr 2015 10:30:02 -0400
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Waiman Long <Waiman.Long@hp.com>, linux-arch@vger.kernel.org, Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>, Oleg Nesterov <oleg@redhat.com>, kvm@vger.kernel.org, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Daniel J Blueman <daniel@numascale.com>, x86@kernel.org, Paolo Bonzini <paolo.bonzini@gmail.com>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Scott J Norton <scott.norton@hp.com>, Ingo Molnar <mingo@redhat.com>, David Vrabel <david.vrabel@citrix.com>, "H. Peter Anvin" <hpa@zytor.com>, xen-devel@lists.xenproject.org, Thomas Gleixner <tglx@linutronix.de>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Linus Torvalds <torvalds@linux-foundation.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Douglas Hatch <doug.hatch@hp.com>
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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.