WQ_HIGHPRI increases throughput and decreases disk latency when using
dm-verity. This is important in Android for camera startup speed.
The following tests were run by doing 60 seconds of random reads using a
dm-verity device backed by two ramdisks.
With WQ_HIGHPRI:
lat (usec): min=13, max=7854, avg=31.15, stdev=30.42
READ: bw=116MiB/s (121MB/s), 116MiB/s-116MiB/s (121MB/s-121MB/s)
Without WQ_HIGHPRI
lat (usec): min=13, max=3947, avg=69.53, stdev=50.55
READ: bw=51.1MiB/s (53.6MB/s), 51.1MiB/s-51.1MiB/s (53.6MB/s-53.6MB/s)
Further testing was done by measuring how long it takes to open a camera
on an Android device.
With WQ_HIGHPRI:
Total verity work queue wait times (ms):
528.824, 439.191, 433.300
Without WQ_HIGHPRI
Total verity work queue wait times (ms):
880.960, 789.517, 898.852
The time it takes to open the camera is reduced by 350ms on average.
Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx>
---
drivers/md/dm-verity-target.c | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index 981821f18a18..341fc58053ce 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -1390,14 +1390,11 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
/* WQ_UNBOUND greatly improves performance when running on ramdisk */
wq_flags = WQ_MEM_RECLAIM | WQ_UNBOUND;
- if (v->use_tasklet) {
- /*
- * Allow verify_wq to preempt softirq since verification in
- * tasklet will fall-back to using it for error handling
- * (or if the bufio cache doesn't have required hashes).
- */
- wq_flags |= WQ_HIGHPRI;
- }
+ /*
+ * WQ_HIGHPRI reduces waiting times when reading from a dm-verity
+ * device. This improves throughput and completion latency.
+ */
+ wq_flags |= WQ_HIGHPRI;
v->verify_wq = alloc_workqueue("kverityd", wq_flags, num_online_cpus());
if (!v->verify_wq) {
ti->error = "Cannot allocate workqueue";
--
2.37.2.672.g94769d06f0-goog
--
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