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

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Message-ID: <55267BA8.9060009@redhat.com>
Date:	Thu, 09 Apr 2015 09:16:24 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@...com>
CC:	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 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?

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.

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