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

From: Avi Kivity
Subject: Re: [Qemu-devel] High CPU use of -usbdevice tablet (was Re: KVM usability)
Date: Mon, 05 Apr 2010 11:22:32 +0300
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

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 

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