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

[Rik van Riel <riel@xxxxxxxxxx>]

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.

Fair enough.


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