Re: [PATCH] blk-throttle: ignore discard request size

[Shaohua Li <shli@kernel.org>]

On Fri, Aug 18, 2017 at 01:06:46PM -0600, Jens Axboe wrote:
> On 08/18/2017 10:28 AM, Shaohua Li wrote:
> > On Fri, Aug 18, 2017 at 09:35:01AM -0600, Jens Axboe wrote:
> >> On 08/18/2017 09:13 AM, Shaohua Li wrote:
> >>> discard request usually is very big and easily use all bandwidth budget
> >>> of a cgroup. discard request size doesn't really mean the size of data
> >>> written, so it doesn't make sense to account it into bandwidth budget.
> >>> This patch ignores discard requests size. It makes sense to account
> >>> discard request into iops budget though.
> >>
> >> Some (most) devices to touch media for a discard operation, but the
> >> cost tends to be fairly constant and independent of discard size.
> >> Would it make sense to just treat it as a constant cost? Zero
> >> cost seems wrong.
> > 
> > that would be hard to find the cost. Would this make sense?
> > 
> > min_t(unsigned int, bio->bi_iter.bi_size, queue_max_sectors(q) << 9)
> 
> It's all going to be approximations, for sure, unfortunately it isn't
> an exact science. Why not just use a constant small value? If we assume
> that a 4k and 8M discard end up writing roughly the same to media, then
> it would follow that just using a smaller constant value (regardless of
> actual discard command size) would be useful.

Sounds good. what number do you suggest? queue_max_sectors or a
random number?

Thanks,
Shaohua