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From: Paul Brook <paul@codesourcery.com>
Subject: Re: [Qemu-devel] High CPU use of -usbdevice tablet (was Re: KVM
	usability)
Date: Sun, 4 Apr 2010 22:53:17 +0100
References: <4B87A6BF.3090301@redhat.com>
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To: qemu-devel@nongnu.org
Cc: Chris Webb <chris@arachsys.com>, Jernej@gnu.org, Avi Kivity <avi@redhat.com>, kvm@vger.kernel.org, =?utf-8?q?Simon=C4=8Di=C4=8D?= <jernej's-kvm@eternallybored.org>

> > My guess is that the overhead you're seeing is entirely from the USB host
> > adapter having to wake up and check the transport descriptor lists. This
> > will only result in the guest being woken if a device actually responds
> > (as mentioned above it should not).
> 
> A quick profile on the host side doesn't show this.  Instead, I see a
> lot of select() overhead.

This actually confirms my hypothesis. After fixing the UHCI bug the guest is 
completely idle, but the host still needs to wake up at 1ms intervals to do 
UHCI emulation. I can believe that the most visible part of this is the 
select() syscall.

> 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.

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.

Paul