Re: [Qemu-devel] [PATCH 0/3] throttle: use AioContext for dataplane support

["Benoît Canet" <benoit.canet@irqsave.net>]

The Wednesday 14 May 2014 à 16:22:44 (+0200), Stefan Hajnoczi wrote :
> This series applies on top of my "dataplane: use QEMU block layer" series.
> 
> Now that the dataplane code path is using the QEMU block layer we should make
> I/O throttling limits safe to use.  When the block_set_io_throttle monitor
> command is executed, the BlockDriverState's AioContext must be acquired in
> order to prevent race conditions with the IOThread that is processing requests
> from the guest.
> 
> The new block layer AioContext detach/attach mechanism needs to be extended to
> move the throttling timer to a new AioContext.  This makes throttling work
> across bdrv_set_aio_context() calls.
> 
> The result of this series is that I/O throttling works with dataplane and
> limits may be changed at runtime using the monitor.
> 
> Stefan Hajnoczi (3):
>   throttle: add throttle_detach/attach_aio_context()
>   throttle: add detach/attach test case
>   blockdev: acquire AioContext in block_set_io_throttle
> 
>  block.c                 |  7 +++++++
>  blockdev.c              |  6 ++++++
>  include/qemu/throttle.h | 10 ++++++++++
>  tests/test-throttle.c   | 49 ++++++++++++++++++++++++++++++++++++++++++++-----
>  util/throttle.c         | 27 +++++++++++++++++++++++----
>  5 files changed, 90 insertions(+), 9 deletions(-)
> 
> -- 
> 1.9.0
> 

As a side note the throttling code does eat some CPU time.
So if people starts using throttling with dataplane by setting 50k iops as a limit
the timers callback will start to kill half a core.

The solution could be to optionaly make the throttling resolution coarser.

I have future plan to do this for SSD usage

Best regards

Benoît