Re: [Qemu-devel] High CPU use of -usbdevice tablet (was Re: KVM usability)

[Avi Kivity]

On 04/05/2010 12:53 AM, Paul Brook wrote:
> > Surprising as there are ~10 descriptors being
> > polled, so ~1200 polls per second.  Maybe epoll will help here.
> >      
> I'm not sure where you get 1200 from.  select will be called once per host
> wakeup. i.e. if the USB controller is enabled then 1k times per second due to
> the frame tick.
>    

That was 125 interrupts/sec x 10 fds ('poll' here was the per-file 
callback in the kernel, not the select call).  With your numbers, it's 
more like 10k polls/sec which can be eliminated.

> Are you sure there are actually 10 descriptors being polled? Remember that the
> nfds argument is the value of the largest fd in the set (+1), not the number
> of descriptors in the set.
>    

I was estimating from the strace parsed output, the real number is 7.  
So the kernel calls 7k callbacks/sec only to return with a timeout.

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.