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

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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.

Thanks,
Mike




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