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

From: Rik van Riel <riel@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>, Waiman Long <Waiman.Long@hp.com>
Cc: 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>
Subject: Re: [PATCH v15 16/16] unfair qspinlock: a queue based unfair lock
Date: Thu, 09 Apr 2015 09:16:24 -0400	[thread overview]
Message-ID: <55267BA8.9060009__38061.5035458051$1428585471$gmane$org@redhat.com> (raw)
In-Reply-To: <20150409070146.GL27490@worktop.programming.kicks-ass.net>

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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