On Thu, Sep 10, 2020 at 10:24:39AM -0400, Mike Snitzer wrote:
> [cc'ing dm-devel and linux-block because this is upstream concern too]
>
> On Wed, Sep 09 2020 at 1:00pm -0400,
> Vijayendra Suman <vijayendra.suman@xxxxxxxxxx> wrote:
>
> > Hello Mike,
> >
> > While Running pgbench tool with 5.4.17 kernel build
> >
> > Following performance degrade is found out
> >
> > buffer read/write metric : -17.2%
> > cache read/write metric : -18.7%
> > disk read/write metric : -19%
> >
> > buffer
> > number of transactions actually processed: 840972
> > latency average = 24.013 ms
> > tps = 4664.153934 (including connections establishing)
> > tps = 4664.421492 (excluding connections establishing)
> >
> > cache
> > number of transactions actually processed: 551345
> > latency average = 36.949 ms
> > tps = 3031.223905 (including connections establishing)
> > tps = 3031.402581 (excluding connections establishing)
> >
> > After revert of Commit
> > 2892100bc85ae446088cebe0c00ba9b194c0ac9d ( Revert "dm: always call
> > blk_queue_split() in dm_process_bio()")
>
> I assume 2892100bc85ae446088cebe0c00ba9b194c0ac9d is 5.4-stable's
> backport of upstream commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74 ?
>
> > Performance is Counter measurement
> >
> > buffer ->
> > number of transactions actually processed: 1135735
> > latency average = 17.799 ms
> > tps = 6292.586749 (including connections establishing)
> > tps = 6292.875089 (excluding connections establishing)
> >
> > cache ->
> > number of transactions actually processed: 648177
> > latency average = 31.217 ms
> > tps = 3587.755975 (including connections establishing)
> > tps = 3587.966359 (excluding connections establishing)
> >
> > Following is your commit
> >
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index cf71a2277d60..1e6e0c970e19 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -1760,8 +1760,9 @@ static blk_qc_t dm_process_bio(struct mapped_device
> > *md,
> > * won't be imposed.
> > */
> > if (current->bio_list) {
> > - blk_queue_split(md->queue, &bio);
> > - if (!is_abnormal_io(bio))
> > + if (is_abnormal_io(bio))
> > + blk_queue_split(md->queue, &bio);
> > + else
> > dm_queue_split(md, ti, &bio);
> > }
> >
> > Could you have a look if it is safe to revert this commit.
>
> No, it really isn't a good idea given what was documented in the commit
> header for commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74 -- the
> excessive splitting is not conducive to performance either.
>
> So I think we need to identify _why_ reverting this commit is causing
> such a performance improvement. Why is calling blk_queue_split() before
> dm_queue_split() benefiting your pgbench workload?
blk_queue_split() takes every queue's limit into account, and dm_queue_split()
only splits bio according to max len(offset, chunk size), so the
splitted bio may not be optimal one from device viewpoint.
Maybe DM can switch to blk_queue_split() if 'chunk_sectors' limit is power-2
aligned.
Thanks,
Ming
