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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