Re: high load with usb device

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

14.09.2010 20:00, Avi Kivity wrote:
>> As I mentioned in other emails in this thread:
>>
>> o yes, I do have CONFIG_EVENTFD set, and it is being used
>>    too (fd#12 in the above strace).
> 
> I thought that was the signalfd.

Uh. Yes, it was, i confused the two.  And yes, CONFIG_EVENTFD is
set and used too.

$ grep FD= config-host.mak
CONFIG_EVENTFD=y
CONFIG_SIGNALFD=y

>> o 0.13.0-rc1 behaves the same way (that is, it also shows
>>    high load when idle -- the same 18% of host CPU), but it
>>    has no pipe on fd#5.
> 
> I think it's host_alarm_handler()'s use of qemu_notify_event().  It's
> telling the main loop to rescan pending events, even though it's called
> from the main loop itself.  Please drop it and rerun.

Without qemu_notify_event() in host_alarm_handler():

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 98.96   48.184832       13747      3505           ioctl
  0.39    0.191613          25      7745        28 futex
  0.37    0.181032           1    173192           select
  0.09    0.045379           0    980369           clock_gettime
  0.05    0.024362           0    351024    173220 read
  0.05    0.023247           0    487766           gettimeofday
  0.04    0.017996           0    319428           timer_gettime
  0.03    0.013837           0    198267           timer_settime
  0.02    0.010036           0    177790           rt_sigaction
  0.00    0.000000           0         1           writev
  0.00    0.000000           0         2           poll
  0.00    0.000000           0         1           rt_sigpending
  0.00    0.000000           0         1         1 rt_sigtimedwait
------ ----------- ----------- --------- --------- ----------------
100.00   48.692334               2699091    173249 total

The picture is pretty similar to the one before :)
(And yes, I'm sure I've run the right binary).

But it looks like we're barking the wrong tree.  It's spending
99% time in ioctl.  Yes, there's quite large amount of reads
and gettimes, but they're taking very small time, even combined.

It is still spending much more time in the ioctl (apparently in
kvm_run).

For comparison, here's the same strace stats without -usbdevice:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 97.70    0.080237          22      3584           select
  1.09    0.000895           0      6018      3584 read
  0.33    0.000271           0      9670           clock_gettime
  0.31    0.000254           0      6086           gettimeofday
  0.26    0.000210           0      2432           rt_sigaction
  0.17    0.000137           0      3653           timer_gettime
  0.15    0.000122           0      2778           timer_settime
  0.00    0.000000           0         1           ioctl
  0.00    0.000000           0         1           rt_sigpending
  0.00    0.000000           0         1         1 rt_sigtimedwait
------ ----------- ----------- --------- --------- ----------------
100.00    0.082126                 34224      3585 total

Yes, it is still doing lots of unnecessary stuff, but the load
is <1%.

(This is without host_alarm_handler() in qemu_notify_event())

/mjt