[Qemu-devel] [RFC 0/4] block: fix I/O throttling oscillations

[Stefan Hajnoczi <stefanha@redhat.com>]

Benoît Canet <benoit@irqsave.net> reported that QEMU I/O throttling can
oscillate under continuous I/O.  The test case runs 50 threads performing
random writes and a -drive iops=150 limit is used.

Since QEMU I/O throttling is implemented using 100 millisecond time slices,
we'd expect 150 +/- 15 IOPS.  Anything outside that range indicates a problem
with the I/O throttling algorithm.

It turned out that even a single thread performing sequential I/O continuously
is throttled outside the 150 +/- 15 IOPS range.  The continous stream of I/O
slows down as time goes on but resets to 150 IOPS again when interrupted.  This
can be tested with:

  $ iostat -d 1 -x /dev/vdb &
  $ dd if=/dev/vdb of=/dev/null bs=4096 iflag=direct

This patches addresses these problems as follows:

1. Account for I/O requests when they are submitted instead of completed.  This
   ensures that we do not exceed the budget for this slice.  Exceeding the
   budget leads to fluctuations since we have to make up for this later.

2. Use constant 100 millisecond slice time.  Adjusting the slice time at
   run-time led to oscillations.  Since the reason for adjusting slice time is
   not clear, drop this behavior.

I have also included two code clean-up patches.

Benoît: Please let me know if this solves the problems you're seeing.

Stefan Hajnoczi (4):
  block: fix I/O throttling accounting blind spot
  block: keep I/O throttling slice time constant
  block: drop duplicated slice extension code
  block: clean up I/O throttling wait_time code

 block.c                   | 47 ++++++++++++++++++++---------------------------
 blockdev.c                |  1 -
 include/block/block_int.h |  3 +--
 3 files changed, 21 insertions(+), 30 deletions(-)

-- 
1.8.1.4