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

[Benoît Canet]

Hi Stefan,

This solve the bug seen by the customer.
Whole patchset tested by on behalf of the user.

Tested-By: Benoit Canet <address@hidden>

Best regards

Benoît

> Le Thursday 21 Mar 2013 à 15:49:55 (+0100), Stefan Hajnoczi a écrit :
> 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
>