Re: high load with win7 & usb tablet

[Michael Tokarev <mjt@tls.msk.ru>]

14.09.2010 12:26, Brad Campbell wrote:
[]
> Just a data point. An idle XP guest for me without -usb sees the host
> running at about 8,000 context switches a second. With the guest using
> -usb -usbdevice tablet, the host runs at between 15,000 - 18,000 context
> switches a second.

Yes, I observed the same as well.

> The descriptors show the device is requesting a polling frequency of
> 100Hz (0x0A ms), but don't forget QEMU emulating a controller with a
> 1000hz microframe rate at worst.

Yes, that's 10ms, which is way too frequent IMHO.  WinXP performs
polling at 130Hz, win7 can do higher.

> There would probably be a lower load if the controller emulated was OHCI
> rather than UHCI just on a reduction of required work by the Guest.

What's the diff. between the two?

> If you were prepared to tolerate a bit less responsiveness, you could
> always tweak the endpoint descriptor to turn the polling frequency down.
> 
>         0x0a,       /*  u8  ep_bInterval; (255ms -- usb 2.0 spec) */
> 
> I'm unclear as to where that comment came from. The usb 2.0 spec says a
> LOW_SPEED device can request between 10->255 ms polling intervals. If
> the device is reporting as a HIGH_SPEED device, then all bets are off.

I already found this place, and already questioned where that
comment come from (no one answered).

But the thing is that changing this value to, say, 0xff (255ms)
does not actually change the load level for me -- not for linux
guest not for w7 guest.  The descriptor is indeed changes, lsusb
correctly reports bInterval=255, but exactly nothing changes.

/mjt