On Wed, Feb 03 2016 at 1:04pm -0500, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > I'm still not clear on where the considerable performance loss is coming > from (on null_blk device I see ~1900K read IOPs but I'm still only > seeing ~1000K read IOPs when blk-mq DM-multipath is layered ontop). > What is very much apparent is: layering dm-mq multipath ontop of null_blk > results in a HUGE amount of additional context switches. I can only > infer that the request completion for this stacked device (blk-mq queue > ontop of blk-mq queue, with 2 completions: 1 for clone completing on > underlying device and 1 for original request completing) is the reason > for all the extra context switches. Starts to explain, certainly not the "reason"; that is still very much TBD... > Here are pictures of 'perf report' for perf datat collected using > 'perf record -ag -e cs'. > > Against null_blk: > http://people.redhat.com/msnitzer/perf-report-cs-null_blk.png if dm-mq nr_hw_queues=1 and null_blk nr_hw_queues=1 cpu : usr=25.53%, sys=74.40%, ctx=1970, majf=0, minf=474 if dm-mq nr_hw_queues=1 and null_blk nr_hw_queues=4 cpu : usr=26.79%, sys=73.15%, ctx=2067, majf=0, minf=479 > Against dm-mpath ontop of the same null_blk: > http://people.redhat.com/msnitzer/perf-report-cs-dm_mq.png if dm-mq nr_hw_queues=1 and null_blk nr_hw_queues=1 cpu : usr=11.07%, sys=33.90%, ctx=667784, majf=0, minf=466 if dm-mq nr_hw_queues=1 and null_blk nr_hw_queues=4 cpu : usr=15.22%, sys=48.44%, ctx=2314901, majf=0, minf=466 So yeah, the percentages reflected in these respective images didn't do the huge increase in context switches justice... we _must_ figure out why we're seeing so many context switches with dm-mq. The same fio job is ran to measure these context switches, e.g.: fio --cpus_allowed_policy=split --group_reporting --rw=randread --bs=4k --numjobs=12 --iodepth=32 --runtime=10 --time_based --loops=1 --ioengine=libaio --direct=1 --invalidate=1 --randrepeat=1 --norandommap --exitall --name task_nullb0 --filename=/dev/nullb0 fio --cpus_allowed_policy=split --group_reporting --rw=randread --bs=4k --numjobs=12 --iodepth=32 --runtime=10 --time_based --loops=1 --ioengine=libaio --direct=1 --invalidate=1 --randrepeat=1 --norandommap --exitall --name task_dm_mq --filename=/dev/mapper/dm_mq -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
