Re: [RFC -v7 PATCH 0/7] directed yield for Pause Loop Exiting

Avi Kivity
2011-01-27T13:21:11Z

On 01/26/2011 11:56 PM, Rik van Riel wrote:
> When running SMP virtual machines, it is possible for one VCPU to be
> spinning on a spinlock, while the VCPU that holds the spinlock is not
> currently running, because the host scheduler preempted it to run
> something else.
>
> Both Intel and AMD CPUs have a feature that detects when a virtual
> CPU is spinning on a lock and will trap to the host.
>
> The current KVM code sleeps for a bit whenever that happens, which
> results in eg. a 64 VCPU Windows guest taking forever and a bit to
> boot up. This is because the VCPU holding the lock is actually
> running and not sleeping, so the pause is counter-productive.
>
> In other workloads a pause can also be counter-productive, with
> spinlock detection resulting in one guest giving up its CPU time
> to the others. Instead of spinning, it ends up simply not running
> much at all.
>
> This patch series aims to fix that, by having a VCPU that spins
> give the remainder of its timeslice to another VCPU in the same
> guest before yielding the CPU - one that is runnable but got
> preempted, hopefully the lock holder.
>
> v7:
> - move the vcpu to pid mapping to inside the vcpu->mutex
> - rename ->yield to ->skip
> - merge patch 5 into patch 4
> v6:
> - implement yield_task_fair in a way that works with task groups,
> this allows me to actually get a performance improvement!
> - fix another race Avi pointed out, the code should be good now
> v5:
> - fix the race condition Avi pointed out, by tracking vcpu->pid
> - also allows us to yield to vcpu tasks that got preempted while in qemu
> userspace
> v4:
> - change to newer version of Mike Galbraith's yield_to implementation
> - chainsaw out some code from Mike that looked like a great idea, but
> turned out to give weird interactions in practice
> v3:
> - more cleanups
> - change to Mike Galbraith's yield_to implementation
> - yield to spinning VCPUs, this seems to work better in some
> situations and has little downside potential
> v2:
> - make lots of cleanups and improvements suggested
> - do not implement timeslice scheduling or fairness stuff
> yet, since it is not entirely clear how to do that right
> (suggestions welcome)
>
>
> Benchmark results:
>
> Two 4-CPU KVM guests are pinned to the same 4 physical CPUs.
>
> One guest runs the AMQP performance test, the other guest runs
> 0, 2 or 4 infinite loops, for CPU overcommit factors of 0, 1.5
> and 4.
>
> The AMQP perftest is run 30 times, with message payloads of 8 and 16 bytes.
>
> size8 no overcommit 1.5x overcommit 2x overcommit
>
> no PLE 223801 135137 104951
> PLE 224135 141105 118744
>
> size16 no overcommit 1.5x overcommit 2x overcommit
>
> no PLE 222424 126175 105299
> PLE 222534 138082 132945
>
> Note: this is with the KVM guests NOT running inside cgroups. There
> seems to be a CPU load balancing issue with cgroup fair group scheduling,
> which often results in one guest getting only 80% CPU time and the other
> guest 320%. That will have to be fixed to get meaningful results with
> cgroups.
>
> CPU time division between the AMQP guest and the infinite loop guest
> were not exactly fair, but the guests got close to the same amount
> of CPU time in each test run.
>
> There is a substantial amount of randomness in CPU time division between
> guests, but the performance improvement is consistent between multiple
> runs.

The kvm bits look fine; I'll be happy to apply them after the scheduler
changes are accepted into tip.git.

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