Re: [RFC 0/4] KVM in-kernel PM Timer implementation

Anthony Liguori
2010-12-14T21:46:59Z

On 12/14/2010 01:54 PM, David S. Ahern wrote:
>
> On 12/14/10 12:49, Anthony Liguori wrote:
>
>> But that doesn't tell you what the impact is in real world workloads.
>> Before we start pushing all device emulation into the kernel, we need to
>> quantify how often gettimeofday() is really called in real workloads.
>>
> The workload that inspired that example program at its current max load
> calls gtod upwards of 1000 times per second. The overhead of
> gettimeofday was the biggest factor when comparing performance to bare
> metal and esx. That's why I wrote the test program --- boils a complex
> product/program to a single system call.
>

So the absolute performance impact was on the order of what?

The difference in CPU time of a light weight vs. heavy weight exit
should be something like 2-3us. That would mean 2-3ms of CPU time at a
rate of 1000 per second.

That should be pretty much in the noise.

There are possibly second order effects that might make a large impact
such as contention with the qemu_mutex. It's worth doing
experimentation to see if a non-mutex acquiring fast path in userspace
also resulted in a significant performance boost.

Regards,

Anthony Liguori

> David
>
>
>> Regards,
>>
>> Anthony Liguori
>>
>>
>>> What's the relative speed of the in-kernel pmtimer compared to the PIT?
>>>
>>> David
>>>
>>>
>>