On Fri, Sep 11 2020 at 8:20am -0400,
Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> 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.
Yes, but the issue is blk_queue_split() is doing the wrong thing for the
case described in the header for commit
120c9257f5f19e5d1e87efcbb5531b7cd81b7d74
> Maybe DM can switch to blk_queue_split() if 'chunk_sectors' limit is power-2
> aligned.
Not seeing relation to chunk_sectors being power of 2 -- other than that
is all block core supports. But chunk_sectors isn't set for DM, you
added chunk_sectors for MD or something and DM was caught out, so
blk_queue_split() falls back to splitting on max_sectors.
You're saying DM should set 'chunk_sectors' IFF it'd be a power of 2?
While I could do that, it seems like just continuing a sequence of
hacks around earlier imposed chunk_sectors infrastructure that was a
half-measure to begin with. Think chunk_sectors logic in block core
needs to be enhanced -- but I'll take a closer look.
Mike
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