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
