Re: Revert "dm: always call blk_queue_split() in dm_process_bio()"

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On Mon, Sep 14 2020 at  9:33pm -0400,
Mike Snitzer <snitzer@xxxxxxxxxx> wrote:

> On Thu, Sep 10 2020 at  3:29pm -0400,
> Vijayendra Suman <vijayendra.suman@xxxxxxxxxx> wrote:
> 
> > Hello Mike,
> > 
> > I checked with upstream, performance measurement is similar and
> > shows performance improvement when
> > 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74 is reverted.
> > 
> > On 9/10/2020 7:54 PM, 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 ?
> >
> > Yes
> >
> > >>    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?
> >
> > Let me know if you want to check some patch.
> 
> Hi,
> 
> Could you please test this branch?:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-5.10
> (or apply at least the first 4 patches, commit 63f85d97be69^..b6a80963621fa)
> 
> So far I've done various DM regression testing.  But I haven't tested
> with pgbench or with the misaaligned IO scenario documented in the
> header for commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74.  But I'll
> test that scenario tomorrow.

Training DM core to set chunk_sectors and always use blk_queue_split
resolves the inefficient splitting documented in the header for
commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74.

xfs_io -d -c 'pread -b 2m 224s 4072s' /dev/mapper/stripe_dev

before, so with commit 120c9257f5f19e5d1e87efcbb5531b7cd81b7d74:

253,2    5        1     0.000000000  4382  Q   R 224 + 2064 [xfs_io]
253,2    5        2     0.000003414  4382  X   R 224 / 256 [xfs_io]
253,2    5        3     0.000017838  4382  X   R 256 / 512 [xfs_io]
253,2    5        4     0.000019852  4382  X   R 512 / 768 [xfs_io]
253,2    5        5     0.000031316  4382  X   R 768 / 1024 [xfs_io]
253,2    5        6     0.000034333  4382  X   R 1024 / 1280 [xfs_io]
253,2    5        7     0.000037684  4382  X   R 1280 / 1536 [xfs_io]
253,2    5        8     0.000041011  4382  X   R 1536 / 1792 [xfs_io]
253,2    5        9     0.000043962  4382  X   R 1792 / 2048 [xfs_io]
253,2    5       10     0.000074765  4382  Q   R 2288 + 2008 [xfs_io]
253,2    5       11     0.000075020  4382  X   R 2288 / 2304 [xfs_io]
253,2    5       12     0.000077009  4382  X   R 2304 / 2560 [xfs_io]
253,2    5       13     0.000080509  4382  X   R 2560 / 2816 [xfs_io]
253,2    5       14     0.000084182  4382  X   R 2816 / 3072 [xfs_io]
253,2    5       15     0.000087274  4382  X   R 3072 / 3328 [xfs_io]
253,2    5       16     0.000090342  4382  X   R 3328 / 3584 [xfs_io]
253,2    5       17     0.000095348  4382  X   R 3584 / 3840 [xfs_io]
253,2    5       18     0.000097776  4382  X   R 3840 / 4096 [xfs_io]

after, so with 'dm-5.10' branch refernced above, meaning dm_process_bio
w/ unconditional blk_queue_split (w/ chunk_sectors):

253,2   17        1     0.000000000  2176  Q   R 224 + 2280 [xfs_io]
253,2   17        2     0.000001978  2176  X   R 224 / 256 [xfs_io]
253,2   17        3     0.000017882  2176  X   R 256 / 512 [xfs_io]
253,2   17        4     0.000020406  2176  X   R 512 / 768 [xfs_io]
253,2   17        5     0.000031298  2176  X   R 768 / 1024 [xfs_io]
253,2   17        6     0.000034654  2176  X   R 1024 / 1280 [xfs_io]
253,2   17        7     0.000038474  2176  X   R 1280 / 1536 [xfs_io]
253,2   17        8     0.000042299  2176  X   R 1536 / 1792 [xfs_io]
253,2   17        9     0.000054088  2176  X   R 1792 / 2048 [xfs_io]
253,2   17       10     0.000057884  2176  X   R 2048 / 2304 [xfs_io]
253,2   17       11     0.000081358  2176  Q   R 2504 + 1792 [xfs_io]
253,2   17       12     0.000081778  2176  X   R 2504 / 2560 [xfs_io]
253,2   17       13     0.000083496  2176  X   R 2560 / 2816 [xfs_io]
253,2   17       14     0.000085301  2176  X   R 2816 / 3072 [xfs_io]
253,2   17       15     0.000092374  2176  X   R 3072 / 3328 [xfs_io]
253,2   17       16     0.000094774  2176  X   R 3328 / 3584 [xfs_io]
253,2   17       17     0.000097977  2176  X   R 3584 / 3840 [xfs_io]
253,2   17       18     0.000100094  2176  X   R 3840 / 4096 [xfs_io]

> Any chance you could provide some hints on how you're running pgbench
> just so I can try to test/reproduce/verify locally?

I'm going to defer to you on pgbench testing.

What is your underlying storage?

Could it be that DM using unconditional blk_queue_split() is helping
your pgbench workload because it splits IO more (so smaller IO, lower
latency per IO)?

Do you have comparison blktrace data?

Mike

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