Re: [Qemu-devel] Windows slow boot: contractor wanted

[Avi Kivity]

On 08/21/2012 06:21 PM, Richard Davies wrote:
> Avi Kivity wrote:
>> Richard Davies wrote:
>> > We're running host kernel 3.5.1 and qemu-kvm 1.1.1.
>> >
>> > I hadn't though about it, but I agree this is related to cpu overcommit. 
>> > The
>> > slow boots are intermittent (and infrequent) with cpu overcommit whereas I
>> > don't think it occurs without cpu overcommit.
>> >
>> > In addition, if there is a slow boot ongoing, and you kill some other VMs 
>> > to
>> > reduce cpu overcommit then this will sometimes speed it up.
>> >
>> > I guess the question is why even with overcommit most boots are fine, but
>> > some small fraction then go slow?
>>
>> Could be a bug.  The scheduler and the spin-loop handling code fight
>> each other instead of working well.
>>
>> Please provide snapshots of 'perf top' while a slow boot is in progress.
> 
> Below are two 'perf top' snapshots during a slow boot, which appear to me to
> support your idea of a spin-lock problem.
> 
> There are a lot more "unprocessable samples recorded" messages at the end of
> each snapshot which I haven't included. I think these may be from the guest
> OS - the kernel is listed, and qemu-kvm itself is listed on some other
> traces which I did, although not these.
> 
> Richard.
> 
> 
> 
>    PerfTop:   62249 irqs/sec  kernel:96.9%  exact:  0.0% [4000Hz cycles],  
> (all, 16 CPUs)
> --------------------------------------------------------------------------------------------------------------------------------
> 
>     35.80%  [kernel]          [k] _raw_spin_lock_irqsave
>     21.64%  [kernel]          [k] isolate_freepages_block

Please disable ksm, and if this function persists in the profile, reduce
some memory from the guests.

>      5.91%  [kernel]          [k] yield_to
>      4.95%  [kernel]          [k] _raw_spin_lock
>      3.37%  [kernel]          [k] kvm_vcpu_on_spin

Except for isolate_freepages_block, all functions up to here have to do
with dealing with cpu overcommit.  But let's deal with them after we see
a profile with isolate_freepages_block removed.



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