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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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