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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: Sun, 04 Apr 2010 19:58:02 +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 04/04/2010 05:25 PM, Paul Brook wrote:
Looks like the tablet is set to 100 Hz polling rate.  We may be able
to get away with 30 Hz or even less (ep_bInterval, in ms, in
Changing the USB tablet polling interval from 10ms to 100ms in both
hw/usb-wacom.c and hw/usb-hid.c made no difference except the an increase
  in bInterval shown in lsusb -v in the guest and the hint of jerky mouse
  movement I expected from setting this value so high. A similar change to
  the polling interval for the keyboard and mouse also made no difference to
  their performance impact.
The USB HID devices implement the SET_IDLE command, so the polling interval
will have no real effect on performance.

On a Linux guest (F12), I see 125 USB interrupts per second with no mouse movement, so something is broken (on the guest or host).

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. Surprising as there are ~10 descriptors being polled, so ~1200 polls per second. Maybe epoll will help here.

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

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