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

From: Rik van Riel
Date: Thu Apr 09 2015 - 09:17:40 EST

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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