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.
On Sat, 2010-04-03 at 21:50 -0400, Rik van Riel wrote:
> 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.
>
Which is precisely what the RT variant does. Each iteration of the
'spin' loop verifies that the lock owner is on CPU. If the owner is
not, all other tasks stop spinning and sleep.
So how does a timeout help in the VCPU case?
Best,
-PWM
Rik van Riel wrote:
> 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.
Right, Steven was referring to a braindead 50us spin, regardless of the
state of the owner. This is the kind of spinning userspace spinlocks
currently implement because they have no information regarding the state
of the owner. By adding adaptive spinning to the kernel, these userspace
locks can make more intelligent spinning decisions.
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team