On 09/14/2010 03:15 PM, Michael Tokarev wrote:
> >> - instrument calls to qemu_mod_timer() in hw/usb-*hci.c. Looks like >> these are all 1kHz, but something else is clearly happening. Ok. There's nothing interesting going on there either, apparently. It is using hw/usb-uhci.c. I added a few prints() in there, but they're firing at the defined 1KHz frequency. Just for test, I lowered the frequency (FRAME_TIMER_FREQ) from 1000 to 500, and the load dropped to half, from 19% to 9..10%. Looking at what hw/usb-uhci.c:uhci_frame_timer() routine does, it is quite expected to have that many writes and reads and that many gettimers(). It is polling for events every 1/1000th of a second, instead of using some form of select().
IIUC that's mandated by USB hardware. The guest may place data in memory, and USB polls it to see if it needs to tell send some message on the bus.
Please post an strace again, this time with -e trace=select. Looks like each timer callback results in >50 syscalls, 4 of which are select()s).
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