Re: [PATCH] kvm-vmx: add module parameter to avoid trapping HLT instructions (v2)

Anthony Liguori
2010-12-03T23:31:42Z

On 12/02/2010 02:12 PM, Marcelo Tosatti wrote:
>>> It should be possible to achieve determinism with
>>> a scheduler policy?
>>>
>> If the desire is the ultimate desire is to have the guests be
>> scheduled in a non-work conserving fashion, I can't see a more
>> direct approach that to simply not have the guests yield (which is
>> ultimately what hlt trapping does).
>>
>> Anything the scheduler would do is after the fact and probably based
>> on inference about why the yield.
>>
> Another issue is you ignore the hosts idea of the best way to sleep
> (ACPI, or whatever).
>

Non-work conserving schedulers kill polar bears. There's simply no way
around it.

The best strategy for power savings is to complete you work as quickly
as you can and then spend as much time in the deepest sleep mode you
can. If you're using a non-work conserving scheduler, you're going to
take more time to complete a workload spending needless cycles in
shallow sleep states.

But that's the price we pay for determinism. Maybe we can plant some
trees at the next KVM Forum to offset CPU limits? :-)

> And handling inactive HLT state (which was never enabled) can be painful.
>

Sorry, I'm not sure what you mean by this.

Regards,

Anthony Liguori