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SubjectRe: [PATCH] blk-throttle: Do not trim slices if there is remainder after division
On Thu, Aug 25, 2011 at 04:32:25PM -0400, Vivek Goyal wrote:
> Throttling code divides a second into smaller slices of 100ms each and
> then decides how many IOPs are allowed in that 100ms. Once IO has been
> dispatched, these slices are reaped off from effective slice.
>
> Division by a factor of 10, can lead to error if input IOPS rate is
> not a mulitple of 10. As remainder is discarded. So if input rate is
> 69 IOPS, then we effectively get only 60 IOPS.
>
> Hence do not trim slice at every 100ms period if input rate is not
> a multiple of 10. Instead wait for full second to complete and
> then trim 10 slices at one go.
>
> Reported-by: Ryan Harper <ryanh@us.ibm.com>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Hi Jens,

Do you have any concerns with this patch. If not, can you please cosider
applying it.

Thanks
Vivek

> ---
> block/blk-throttle.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> Index: linux-2.6/block/blk-throttle.c
> ===================================================================
> --- linux-2.6.orig/block/blk-throttle.c 2011-08-25 16:27:55.869757580 -0400
> +++ linux-2.6/block/blk-throttle.c 2011-08-25 16:28:34.866572646 -0400
> @@ -534,7 +534,7 @@ throtl_slice_used(struct throtl_data *td
> static inline void
> throtl_trim_slice(struct throtl_data *td, struct throtl_grp *tg, bool rw)
> {
> - unsigned long nr_slices, time_elapsed, io_trim;
> + unsigned long nr_slices, time_elapsed, io_trim, io_remainder;
> u64 bytes_trim, tmp;
>
> BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]));
> @@ -569,6 +569,19 @@ throtl_trim_slice(struct throtl_data *td
>
> io_trim = (tg->iops[rw] * throtl_slice * nr_slices)/HZ;
>
> + /*
> + * Due to division operation of input rate, we can lose some accuracy
> + * as raminder is discarded. For example with iops=69, we will dispatch
> + * 6 ios but then trim slice after 100ms and never extend slice enough
> + * so that over a period of 1 second 69 IOs are dispatched. Instead we
> + * dispatch 6 IO at each slice interval and trim the slice. So go
> + * ahead with trim operation only when there is no remainder. That
> + */
> + io_remainder = (tg->iops[rw] * throtl_slice * nr_slices)%HZ;
> +
> + if (io_remainder)
> + return;
> +
> if (!bytes_trim && !io_trim)
> return;
>


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