On Thu, Nov 09, 2017 at 02:19:24PM -0800, Tejun Heo wrote: > Currently, the latency= parameter specifies the extra latency on top > of the estimated baseline latency. This doesn't seem ideal for the > following two reasons. > > 1. Sometimes we want to use absolute target latencies, especially as > the baseline latency estimation isn't always reliable. > > 2. If we want to set a latency target relative to the estimated > baseline latency, it makes a lot more sense to express the target > as a percentage of the baseline (say, 120% of the expected latency) > rather than the baseline latency + an absolute offset. > > This patch makes the existing latency= parameter to be interpreted as > an absolute latency target instead of an offset to the baseline. The > next patch will add support for relative latency target expressed in > terms of percentage. > > While this is a userland visible change, io.low support is still > evoling and highly experimental. This isn't expected to break any > actual usages.
The percentage latency makes sense, but the absolute latency doesn't to me. A 4k IO latency could be much smaller than 1M IO latency. If we don't add baseline latency, we can't specify a latency target which works for both 4k and 1M IO. Thanks, Shaohua > Signed-off-by: Tejun Heo <t...@kernel.org> > Cc: Shaohua Li <s...@kernel.org> > --- > block/blk-throttle.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > --- a/block/blk-throttle.c > +++ b/block/blk-throttle.c > @@ -2299,8 +2299,7 @@ void blk_throtl_bio_endio(struct bio *bi > > bucket = request_bucket_index( > blk_stat_size(&bio->bi_issue_stat)); > - threshold = tg->td->avg_buckets[bucket].latency + > - tg->latency_target; > + threshold = tg->latency_target; > if (lat > threshold) > tg->bad_bio_cnt++; > /*
