Re: RFC: Ideal Adaptive Spinning Conditions

[Rik van Riel <riel@redhat.com>]

On 03/31/2010 10:25 PM, Steven Rostedt wrote:
> On Wed, 2010-03-31 at 19:13 -0700, Darren Hart wrote:
>> Steven Rostedt wrote:
>>> On Wed, 2010-03-31 at 16:21 -0700, Darren Hart wrote:
>>>
>>>> o What type of lock hold times do we expect to benefit?
>>>
>>> 0 (that's a zero) :-p
>>>
>>> I haven't seen your patches but you are not doing a heuristic approach,
>>> are you? That is, do not "spin" hoping the lock will suddenly become
>>> free. I was against that for -rt and I would be against that for futex
>>> too.
>>
>> I'm not sure what you're getting at here. Adaptive spinning is indeed
>> hoping the lock will become free while you are spinning and checking
>> it's owner...
>
> I'm talking about the original idea people had of "lets spin for 50us
> and hope it is unlocked before then", which I thought was not a good
> idea.

Maybe not a good idea when running on bare metal, but it
could be a big help when running virtualized.

A lock with a short hold time can, occasionally, have a
very long hold time, when the VCPU holding the lock is
preempted by the host/hypervisor.

An adaptive lock would spin-and-acquire if the lock holder
is running, while turning into a sleep lock when the lock
holder has been preempted.