[Qemu-devel] Confusing I/O throttling behavior with burst_length > 1 and max < avg

[Stefan Hajnoczi]

Hi Berto,
The following throttling behavior was observed with bps=512000
bps_max=51200 throttling.bps-total-max-length=2:

  READ: io=512KB, aggrb=50KB/s, minb=50KB/s, maxb=50KB/s, mint=10061msec, 
maxt=10061msec

If bps-total-max-length=1 then the result is different:

  READ: io=5056KB, aggrb=505KB/s, minb=505KB/s, maxb=505KB/s, mint=10006msec, 
maxt=10006msec

The burst limits guest bps instead of adding on top of it when
burst_length > 1.

I'm not sure what behavior you intended?  Replacing the following should
allow the guest to get its burst on top of the bps:

  return throttle_do_compute_wait(bkt->max, extra);

with:

  return throttle_do_compute_wait(MAX(bkt->avg, bkt->max), extra);

What do you think?

For more details, see Nini Gu's bug report:
https://bugzilla.redhat.com/show_bug.cgi?id=1355665

Stefan

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