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Re: [Qemu-devel] Performance issues with -usb

From: Leonardo E. Reiter
Subject: Re: [Qemu-devel] Performance issues with -usb
Date: Sat, 29 Jul 2006 14:22:43 -0400
User-agent: Thunderbird (X11/20060516)


I think the 0x20 (32ms) value might be fine for something like VNC
display, but on a local display the mouse response is noticeably slow
(for me at least.)  I tried using 0x0a (10), same as the plain USB
mouse, but the idle CPU utilization was still around 10%.  What is
really odd is that this was not a problem in older versions of QEMU,
unless I'm reading data incorrectly.  Do you know what else might have
changed?  I will look as well and see if I can apply some logic to this.
 I suspect it's probably something with the new clock mechanism, but it
still doesn't make a lot of sense to me.

I've looked at both Windows 2000 and XP guests and seen similar results
so far with the latest QEMU/KQEMU combination.  The good news at least
is that the 0x0a interval seems very smooth even on a local display, but
certainly is much friendlier than the CVS version on the CPU.  Still,
10% idle usage is quite high.  If it can't logically be fixed to provide
both a) low idle CPU and b) smooth local display response, then I might
just post a runtime option patch allowing the user to configure the
interval on the command line.  I'm not sure how correct that is, but it
would be useful to me at least.

- Leo Reiter

Brad Campbell wrote:
> Lonnie Mendez wrote:
>>    Perhaps tweaking the value of ep_bInterval for the tablet's status
>> change endpoint would help?  The endpoint descriptor for the tablet
>> currently has this at 3 milliseconds.  The hid mouse reports a 10
>> millisecond polling interval.
> Indeed. I'm not quite sure how or why I did that in the 1st place as the
> tablet started life as a copy of the mouse in any case.
> I've had good drag through the specs and all the data sheets for mouse
> chips I could find out there and most of them seem to recommend a value
> no faster than 8ms.
> This drops the cpu utilisation of a Windows guest while idle about 75%
> when using -usbdevice tablet
> I've not noticed any change in usability or mouse responsiveness.
> (I played with values up to 0xFF but after about 0x20 there seemed to be
> immeasurable/no difference)

Leonardo E. Reiter
Vice President of Product Development, CTO

Win4Lin, Inc.
Virtual Computing that means Business
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Fax: +1 512 532 6501

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