[PATCH 1/2] ARM: KVM: Yield CPU when vcpu executes a WFE
agraf at suse.de
Mon Oct 7 12:30:04 EDT 2013
On 07.10.2013, at 18:16, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 07/10/13 17:04, Alexander Graf wrote:
>> On 07.10.2013, at 17:40, Marc Zyngier <marc.zyngier at 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
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.
I assume you simply don't contend and spin locks yet. Once you have more guest cores things would look differently. So once you have a system with more cores available, it might make sense to measure it again.
Until then, the numbers are impressive.
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