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Message-ID: <20150409141348.GX5029@twins.programming.kicks-ass.net>
Date: Thu, 9 Apr 2015 16:13:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Rik van Riel <riel@...hat.com>
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 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.
> Having said that, only KVM and Xen seem to support very
> large guests, and PV spinlock is available there.
>
> I believe both VMware and Hyperv have a 32 VCPU limit, anyway.
Don't we have Hyperv paravirt drivers? They could add support for
paravirt spinlocks too.
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