Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation

[Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>]

Reviewed-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>

On Wed, 2013-03-20 at 10:12 +0100, Benoît Canet wrote:
> This patch fix an I/O throttling behavior triggered by limiting at 150 iops
> and running a load of 50 threads doing random pwrites on a raw virtio device.
> 
> After a few moments the iops count start to oscillate steadily between 0 and a
> value upper than the limit.
> 
> As the load keep running the period and the amplitude of the oscillation
> increase until io bursts reaching the physical storage max iops count are
> done.
> 
> These bursts are followed by guest io starvation.
> 
> As the period of this oscillation cycle is increasing the cause must be a
> computation error leading to increase slowly the wait time.
> 
> This patch make the wait time a bit smaller and tests confirm that it solves
> the oscillating behavior.
> 
> Signed-off-by: Benoit Canet <benoit@irqsave.net>
> ---
>  block.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/block.c b/block.c
> index 0a062c9..455d8b0 100644
> --- a/block.c
> +++ b/block.c
> @@ -3739,7 +3739,7 @@ static bool bdrv_exceed_iops_limits(BlockDriverState *bs, bool is_write,
>      }
> 
>      /* Calc approx time to dispatch */
> -    wait_time = (ios_base + 1) / iops_limit;
> +    wait_time = ios_base / iops_limit;
>      if (wait_time > elapsed_time) {
>          wait_time = wait_time - elapsed_time;
>      } else {

-- 
Regards,

Zhi Yong Wu