Re: High vm-exit latencies during kvm boot-up/shutdown

Jan Kiszka
2007-10-24T16:22:27Z

Avi Kivity wrote:
> Jan Kiszka wrote:
>> Avi Kivity wrote:
>>
>>> Jan Kiszka wrote:
>>>
>>>> I ran
>>>>
>>>> user/kvmctl user/test/bootstrap user/test/smp.flat
>>>>
>>>> with the busy loop hacked into bootstrap, but I got no latency spots
>>>> this time. And what should I look for in the output of kvm_stat?
>>>>
>>>>
>>>>
>>> The first numeric column is the total number of exits; the second is the
>>> rate of change (per second). For a spinning workload, both irq_exits
>>> and exits rate should equal the timer frequency.
>>>
>> It matches roughly, but this isn't surprising given the lack of latency
>> peaks in this scenario.
>>
>>
>
> Okay, that's expected. But I really can't think what causes the latency
> spike you see running the bios code. 300 microseconds is enough for 3000
> cache misses. No instruction can cause so many except rep movs and
> friends, and these are supposed to be preemptible.
>
> You might try isolating it by adding code to output markers to some I/O
> port and scattering it throughout the bios. A binary search should find
> the problem spot quickly.
>

Got it! It's wbinvd from smm_init in rombios32.c! Anyone any comments on
this?

Jan (who's leaving now...)

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