On Thu, Feb 04 2016 at 8:54P -0500, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > On Thu, Feb 04 2016 at 1:54am -0500, > Hannes Reinecke <hare@xxxxxxx> wrote: > > > On 02/03/2016 07:24 PM, Mike Snitzer wrote: > > > 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. > > > > > Well, the most obvious one being that you're using 1 dm-mq queue vs > > 4 null_blk queues. > > So you will have have to do an additional context switch for 75% of > > the total I/Os submitted. > > Right, that case is certainly prone to more context switches. But I'm > initially most concerned about the case where both only have 1 queue. > > > Have you tested with 4 dm-mq hw queues? > > Yes, it makes performance worse. This is likely rooted in dm-mpath IO > path not being lockless. But I also have concern about whether the > clone, sent to the underlying path, is completing on a different cpu > than dm-mq's original request. > > I'll be using ftrace to try to dig into the various aspects of this > (perf, as I know how to use it, isn't giving me enough precision in its > reporting). > > > To avoid context switches we would have to align the dm-mq queues to > > the underlying blk-mq layout for the paths. > > Right, we need to take more care (how remains TBD). But for now I'm > just going to focus on the case where both dm-mq and null_blk have 1 for > nr_hw_queues. As you can see even in that config the number of context > switches goes from 1970 to 667784 (and there is a huge loss of system > cpu utilization) once dm-mq w/ 1 hw_queue is stacked ontop on the > null_blk device. > > Once we understand the source of all the additional context switching > for this more simplistic stacked configuration we can look closer at > scaling as we add more underlying paths. Following is RFC because it really speaks to dm-mq _needing_ a variant of blk_mq_complete_request() that supports partial completions. Not supporting partial completions really isn't an option for DM multipath. From: Mike Snitzer <snitzer@xxxxxxxxxx> Date: Fri, 5 Feb 2016 08:49:01 -0500 Subject: [RFC PATCH] dm: fix excessive dm-mq context switching Request-based DM's blk-mq support (dm-mq) was reported to be 50% slower than if an underlying null_blk device were used directly. This biggest reason for this drop in performance is that blk_insert_clone_request() was calling blk_mq_insert_request() with @async=true. This forced the use of kblockd_schedule_delayed_work_on() to run the queues which ushered in ping-ponging between process context (fio in this case) and kblockd's kworker to submit the cloned request. The ftrace function_graph tracer showed: kworker-2013 => fio-12190 fio-12190 => kworker-2013 ... kworker-2013 => fio-12190 fio-12190 => kworker-2013 ... Fixing blk_mq_insert_request() to _not_ use kblockd to submit the cloned requests isn't enough to fix eliminated the oberved context switches. In addition to this dm-mq specific blk-core fix, there were 2 DM core fixes to dm-mq that (when paired with the blk-core fix) completely eliminate the observed context switching: 1) don't blk_mq_run_hw_queues in blk-mq request completion Motivated by desire to reduce overhead of dm-mq, punting to kblockd just increases context switches. In my testing against a really fast null_blk device there was no benefit to running blk_mq_run_hw_queues() on completion (and no other blk-mq driver does this). So hopefully this change doesn't induce the need for yet another revert like commit 621739b00e16ca2d ! 2) use blk_mq_complete_request() in dm_complete_request() blk_complete_request() doesn't offer the traditional q->mq_ops vs .request_fn branching pattern that other historic block interfaces do (e.g. blk_get_request). Using blk_mq_complete_request() for blk-mq requests is important for performance but it doesn't handle partial completions -- which is a pretty big problem given the potential for partial completions with DM multipath due to path failure(s). As such this makes this entire patch only RFC-worthy. dm-mq "fix" #2 is _much_ more important than #1 for eliminating the excessive context switches. Before: cpu : usr=15.10%, sys=59.39%, ctx=7905181, majf=0, minf=475 After: cpu : usr=20.60%, sys=79.35%, ctx=2008, majf=0, minf=472 With these changes the multithreaded async read IOPs improved from ~950K to ~1350K for this dm-mq stacked on null_blk test-case. The raw read IOPs of the underlying null_blk device for the same workload is ~1950K. Reported-by: Sagi Grimberg <sagig@xxxxxxxxxxxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> --- block/blk-core.c | 2 +- drivers/md/dm.c | 13 ++++++------- 2 files changed, 7 insertions(+), 8 deletions(-) diff --git a/block/blk-core.c b/block/blk-core.c index ab51685..c60e233 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -2198,7 +2198,7 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq) if (q->mq_ops) { if (blk_queue_io_stat(q)) blk_account_io_start(rq, true); - blk_mq_insert_request(rq, false, true, true); + blk_mq_insert_request(rq, false, true, false); return 0; } diff --git a/drivers/md/dm.c b/drivers/md/dm.c index c683f6d..a618477 100644 --- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -1119,12 +1119,8 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue) * back into ->request_fn() could deadlock attempting to grab the * queue lock again. */ - if (run_queue) { - if (md->queue->mq_ops) - blk_mq_run_hw_queues(md->queue, true); - else - blk_run_queue_async(md->queue); - } + if (!md->queue->mq_ops && run_queue) + blk_mq_run_hw_queues(md->queue, true); /* * dm_put() must be at the end of this function. See the comment above @@ -1344,7 +1340,10 @@ static void dm_complete_request(struct request *rq, int error) struct dm_rq_target_io *tio = tio_from_request(rq); tio->error = error; - blk_complete_request(rq); + if (!rq->q->mq_ops) + blk_complete_request(rq); + else + blk_mq_complete_request(rq, rq->errors); } /* -- 2.5.4 (Apple Git-61) -- 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
