Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 12 Apr 2010 09:40:59 +0300 | From | Avi Kivity <> | Subject | Re: VM performance issue in KVM guests. |
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On 04/12/2010 05:04 AM, Zhang, Xiantao wrote: > >> What was the performance hit? What was your I/O setup (image format, >> using aio?) >> > The issue only happens when vcpu number is over-committed(e.g. vcpu/pcpu>2) and physical cpus are saturated. For example, when run webbench in windows OS in this case, its performance drops by 80%. In our experiment, we are using image file through virtio, and I think aio should be used by default also. >
Is this on a machine that does pause-loop exits? The current handing of PLE is very suboptimal. With proper directed yield we should be much better there.
Without PLE, we need paravirtualized spinlocks, no way around it.
>>> After analysis about Linux scheduler, we found it is indeed caused >>> by the known features of Linux schduler, such as AFFINE_WAKEUPS, >>> SYNC_WAKEUPS etc. With these features on, linux schduler often tries >>> to schedule the vcpu threads of one guests to one same logical >>> processor when vcpus are over-committed and logical processors are >>> saturated. Once the vcpu threads of one VM are scheduled to the same >>> LP, system performance drops dramatically with some workloads(like >>> webbench running in windows OS). >>> >>> >> Were the affine wakeups due to the kernel (emulated guest IPIs) or >> qemu? >> > We have basic guesses about the reasone, one is wakeup affinity between vcpu threads due to IPI, and the other is wakeup affinity between io theads and vcpu threads. >
It would be good to find out.
>> Most likely it also hits non-virtualized loads as well. If the >> scheduler pulls two long-running threads to the same cpu, performance >> will take a hit. >> > Since the hit only happens when physical cpus are saturated, and sheduling non-virtualized multiple threads of one process to same processor can benefit the performance due to cache share or other affinities, but you know it hurts performance a lot once schedule two vcpu theads to a same processor due to mutual spin-lock in guests. >
Spin loops need to be addressed first, they are known to kill performance in overcommit situations.
-- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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