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

[Zhi Yong Wu]

They look good to me, but i have not done any test on my dev box,
thanks for your fix.

On Thu, Mar 21, 2013 at 10:49 PM, Stefan Hajnoczi <address@hidden> wrote:
> Benoît Canet <address@hidden> 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
>
>



-- 
Regards,

Zhi Yong Wu