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

Avi Kivity
2007-10-24T09:33:33Z

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.

--
error compiling committee.c: too many arguments to function