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H A Ddm-snap.cdiff 3f1637f2 Sun Mar 17 06:22:58 MDT 2019 Nikos Tsironis <ntsironis@arrikto.com> dm snapshot: Use fine-grained locking scheme

Substitute the global locking scheme with a fine grained one, employing
the read-write semaphore and the scalable exception tables with
per-bucket locks introduced by the previous two commits.

Summarizing, we now use a read-write semaphore to protect the mostly
read fields of the snapshot structure, e.g., valid, active, etc., and
per-bucket bit spinlocks to protect accesses to the complete and pending
exception tables.

Finally, we use an extra spinlock (pe_allocation_lock) to serialize the
allocation of new exceptions by the exception store. This allocation is
really fast, so the extra spinlock doesn't hurt the performance.

This scheme allows dm-snapshot to scale better, resulting in increased
IOPS and reduced latency.

Following are some benchmark results using the null_blk device:

modprobe null_blk gb=1024 bs=512 submit_queues=8 hw_queue_depth=4096 \
queue_mode=2 irqmode=1 completion_nsec=1 nr_devices=1

* Benchmark fio_origin_randwrite_throughput_N, from the device mapper
test suite [1] (direct IO, random 4K writes to origin device, IO
engine libaio):

+--------------+-------------+------------+
| # of workers | IOPS Before | IOPS After |
+--------------+-------------+------------+
| 1 | 57708 | 66421 |
| 2 | 63415 | 77589 |
| 4 | 67276 | 98839 |
| 8 | 60564 | 109258 |
+--------------+-------------+------------+

* Benchmark fio_origin_randwrite_latency_N, from the device mapper test
suite [1] (direct IO, random 4K writes to origin device, IO engine
psync):

+--------------+-----------------------+----------------------+
| # of workers | Latency (usec) Before | Latency (usec) After |
+--------------+-----------------------+----------------------+
| 1 | 16.25 | 13.27 |
| 2 | 31.65 | 25.08 |
| 4 | 55.28 | 41.08 |
| 8 | 121.47 | 74.44 |
+--------------+-----------------------+----------------------+

* Benchmark fio_snapshot_randwrite_throughput_N, from the device mapper
test suite [1] (direct IO, random 4K writes to snapshot device, IO
engine libaio):

+--------------+-------------+------------+
| # of workers | IOPS Before | IOPS After |
+--------------+-------------+------------+
| 1 | 72593 | 84938 |
| 2 | 97379 | 134973 |
| 4 | 90610 | 143077 |
| 8 | 90537 | 180085 |
+--------------+-------------+------------+

* Benchmark fio_snapshot_randwrite_latency_N, from the device mapper
test suite [1] (direct IO, random 4K writes to snapshot device, IO
engine psync):

+--------------+-----------------------+----------------------+
| # of workers | Latency (usec) Before | Latency (usec) After |
+--------------+-----------------------+----------------------+
| 1 | 12.53 | 10.6 |
| 2 | 19.78 | 14.89 |
| 4 | 40.37 | 23.47 |
| 8 | 89.32 | 48.48 |
+--------------+-----------------------+----------------------+

[1] https://github.com/jthornber/device-mapper-test-suite

Co-developed-by: Ilias Tsitsimpis <iliastsi@arrikto.com>
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Completed in 290 milliseconds