Commit d256d796 authored Nov 18, 2019 by Jeffle Xu Committed by Mike Snitzer Nov 18, 2019

dm thin: wakeup worker only when deferred bios exist



Single thread fio test (read, bs=4k, ioengine=libaio, iodepth=128,
numjobs=1) over dm-thin device has poor performance versus bare nvme
device.

Further investigation with perf indicates that queue_work_on() consumes
over 20% CPU time when doing IO over dm-thin device. The call stack is
as follows.

- 40.57% thin_map
    + 22.07% queue_work_on
    + 9.95% dm_thin_find_block
    + 2.80% cell_defer_no_holder
      1.91% inc_all_io_entry.isra.33.part.34
    + 1.78% bio_detain.isra.35

In cell_defer_no_holder(), wakeup_worker() is always called, no matter
whether the tc->deferred_bio_list list is empty or not. In single thread
IO model, this list is most likely empty. So skip waking up worker thread
if tc->deferred_bio_list list is empty.

Single thread IO performance improves from 448 MiB/s to 646 MiB/s (+44%)
once the needless wake_worker() calls are properly skipped.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

parent d537858a

drivers/md/dm-thin.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -882,11 +882,14 @@ static void cell_defer_no_holder(struct thin_c tc, struct dm_bio_prison_cell c
		{
		struct pool *pool = tc->pool;
		unsigned long flags;
		int has_work;

		spin_lock_irqsave(&tc->lock, flags);
		cell_release_no_holder(pool, cell, &tc->deferred_bio_list);
		has_work = !bio_list_empty(&tc->deferred_bio_list);
		spin_unlock_irqrestore(&tc->lock, flags);

		if (has_work)
		wake_worker(pool);
		}

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