On Wed, Oct 9, 2013 at 7:48 PM, Marc Zyngier <marc.zyng...@arm.com> wrote:
> On 09/10/13 14:26, Gleb Natapov wrote:
>> On Wed, Oct 09, 2013 at 03:09:54PM +0200, Alexander Graf wrote:
>>>
>>> On 07.10.2013, at 18:53, Gleb Natapov <g...@redhat.com> wrote:
>>>
>>>> On Mon, Oct 07, 2013 at 06:30:04PM +0200, Alexander Graf wrote:
>>>>>
>>>>> On 07.10.2013, at 18:16, Marc Zyngier <marc.zyng...@arm.com> wrote:
>>>>>
>>>>>> On 07/10/13 17:04, Alexander Graf wrote:
>>>>>>>
>>>>>>> On 07.10.2013, at 17:40, Marc Zyngier <marc.zyng...@arm.com> wrote:
>>>>>>>
>>>>>>>> On an (even slightly) oversubscribed system, spinlocks are quickly
>>>>>>>> becoming a bottleneck, as some vcpus are spinning, waiting for a
>>>>>>>> lock to be released, while the vcpu holding the lock may not be
>>>>>>>> running at all.
>>>>>>>>
>>>>>>>> This creates contention, and the observed slowdown is 40x for
>>>>>>>> hackbench. No, this isn't a typo.
>>>>>>>>
>>>>>>>> The solution is to trap blocking WFEs and tell KVM that we're now
>>>>>>>> spinning. This ensures that other vpus will get a scheduling boost,
>>>>>>>> allowing the lock to be released more quickly.
>>>>>>>>
>>>>>>>>> From a performance point of view: hackbench 1 process 1000
>>>>>>>>
>>>>>>>> 2xA15 host (baseline): 1.843s
>>>>>>>>
>>>>>>>> 2xA15 guest w/o patch: 2.083s 4xA15 guest w/o patch: 80.212s
>>>>>>>>
>>>>>>>> 2xA15 guest w/ patch: 2.072s 4xA15 guest w/ patch: 3.202s
>>>>>>>
>>>>>>> I'm confused. You got from 2.083s when not exiting on spin locks to
>>>>>>> 2.072 when exiting on _every_ spin lock that didn't immediately
>>>>>>> succeed. I would've expected to second number to be worse rather than
>>>>>>> better. I assume it's within jitter, I'm still puzzled why you don't
>>>>>>> see any significant drop in performance.
>>>>>>
>>>>>> The key is in the ARM ARM:
>>>>>>
>>>>>> B1.14.9: "When HCR.TWE is set to 1, and the processor is in a Non-secure
>>>>>> mode other than Hyp mode, execution of a WFE instruction generates a Hyp
>>>>>> Trap exception if, ignoring the value of the HCR.TWE bit, conditions
>>>>>> permit the processor to suspend execution."
>>>>>>
>>>>>> So, on a non-overcommitted system, you rarely hit a blocking spinlock,
>>>>>> hence not trapping. Otherwise, performance would go down the drain very
>>>>>> quickly.
>>>>>
>>>>> Well, it's the same as pause/loop exiting on x86, but there we have
>>>>> special hardware features to only ever exit after n number of
>>>>> turnarounds. I wonder why we have those when we could just as easily exit
>>>>> on every blocking path.
>>>>>
>>>> It will hurt performance if vcpu that holds the lock is running.
>>>
>>> Apparently not so on ARM. At least that's what Marc's numbers are showing.
>>> I'm not sure what exactly that means. Basically his logic is "if we spin,
>>> the holder must have been preempted". And it seems to work out surprisingly
>>> well.
>
> Yes. I basically assume that contention should be rare, and that ending
> up in a *blocking* WFE is a sign that we're in thrashing mode already
> (no event is pending).
>
>>>
>> For not contended locks it make sense. We need to recheck if x86
>> assumption is still true there, but x86 lock is ticketing which
>> has not only lock holder preemption, but also lock waiter
>> preemption problem which make overcommit problem even worse.
>
> Locks are ticketing on ARM as well. But there is one key difference here
> with x86 (or at least what I understand of it, which is very close to
> none): We only trap if we would have blocked anyway. In our case, it is
> almost always better to give up the CPU to someone else rather than
> waiting for some event to take the CPU out of sleep.
Benefits of "Yield CPU when vcpu executes a WFE" seems to depend on:
1. How spin lock is implemented in Guest OS?
we cannot assume
that underlying Guest OS is always Linux.
2. How bad/good is spin
It will be good if we can enable/disable "Yield CPU when vcpu executes a WFE
>
> M.
> --
> Jazz is not dead. It just smells funny...
>
>
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