Currently, kcopyd has a sub-job size of 64KiB and a maximum number of 8
sub-jobs. As a result, for any kcopyd job, we have a maximum of 512KiB
of I/O in flight.
This upper limit to the amount of in-flight I/O under-utilizes fast
devices and results in decreased throughput, e.g., when writing to a
snapshotted thin LV with I/O size less than the pool's block size (so
COW is performed using kcopyd).
Increase kcopyd's sub-job size to 512KiB, so we have a maximum of 4MiB
of I/O in flight for each kcopyd job. This results in an up to 96%
improvement of bandwidth when writing to a snapshotted thin LV, with I/O
sizes less than the pool's block size.
We evaluate the performance impact of the change by running the
snap_breaking_throughput benchmark, from the device mapper test suite
[1].
The benchmark:
1. Creates a 1G thin LV
2. Provisions the thin LV
3. Takes a snapshot of the thin LV
4. Writes to the thin LV with:
dd if=/dev/zero of=/dev/vg/thin_lv oflag=direct bs=<I/O size>
Running this benchmark with various thin pool block sizes and dd I/O
sizes (all combinations triggering the use of kcopyd) we get the
following results:
+-----------------+-------------+------------------+-----------------+
| Pool block size | dd I/O size | BW before (MB/s) | BW after (MB/s) |
+-----------------+-------------+------------------+-----------------+
| 1 MB | 256 KB | 242 | 280 |
| 1 MB | 512 KB | 238 | 295 |
| | | | |
| 2 MB | 256 KB | 238 | 354 |
| 2 MB | 512 KB | 241 | 380 |
| 2 MB | 1 MB | 245 | 394 |
| | | | |
| 4 MB | 256 KB | 248 | 412 |
| 4 MB | 512 KB | 234 | 432 |
| 4 MB | 1 MB | 251 | 474 |
| 4 MB | 2 MB | 257 | 504 |
| | | | |
| 8 MB | 256 KB | 239 | 420 |
| 8 MB | 512 KB | 256 | 431 |
| 8 MB | 1 MB | 264 | 467 |
| 8 MB | 2 MB | 264 | 502 |
| 8 MB | 4 MB | 281 | 537 |
+-----------------+-------------+------------------+-----------------+
[1] https://github.com/jthornber/device-mapper-test-suite
Signed-off-by: Nikos Tsironis <ntsironis@xxxxxxxxxxx>
---
drivers/md/dm-kcopyd.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/md/dm-kcopyd.c b/drivers/md/dm-kcopyd.c
index 671c24332802..db0a7d1e33b7 100644
--- a/drivers/md/dm-kcopyd.c
+++ b/drivers/md/dm-kcopyd.c
@@ -28,7 +28,7 @@
#include "dm-core.h"
-#define SUB_JOB_SIZE 128
+#define SUB_JOB_SIZE 1024
#define SPLIT_COUNT 8
#define MIN_JOBS 8
#define RESERVE_PAGES (DIV_ROUND_UP(SUB_JOB_SIZE << SECTOR_SHIFT, PAGE_SIZE))
--
2.11.0
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