[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?
Thanks,
Mike
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