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

From: Waiman Long
Date: Thu Apr 09 2015 - 17:52:51 EST

On 04/09/2015 10:13 AM, Peter Zijlstra wrote:
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.

I am just thinking that the unfair qspinlock is better performing than the simple byte lock. However, my current priority is to get native and PV qspinlock upstream. The unfair qspinlock can certainly wait.

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