Messages in this thread | | | Date | Mon, 29 Aug 2011 09:09:16 -0400 | From | Vivek Goyal <> | Subject | Re: [PATCH] blk-throttle: Do not trim slices if there is remainder after division |
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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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