Hi,
We found a performance problem:
kernel version: 5.10
disk: ssd
scheduler: bfq
arch: arm64 / x86_64
test param: direct=1, ioengine=psync, bs=4k, rw=randread, numjobs=32
We are using 32 threads here, test results showed that iops is equal
to single thread.
After digging into the problem, I found root cause of the problem is
strange:
bfq_add_request
bfq_bfqq_handle_idle_busy_switch
bfq_add_bfqq_busy
bfq_activate_bfq
bfq_activate_requeue_entity
__bfq_activate_requeue_entity
__bfq_activate_entity
if (!bfq_entity_to_bfqq(entity))
if (!entity->in_groups_with_pending_reqs)
entity->in_groups_with_pending_reqs = true;
bfqd->num_groups_with_pending_reqs++
If test process is not in root cgroup, num_groups_with_pending_reqs will
be increased after request was instered to bfq.
bfq_select_queue
bfq_better_to_idle
idling_needed_for_service_guarantees
bfq_asymmetric_scenario
return varied_queue_weights || multiple_classes_busy ||
bfqd->num_groups_with_pending_reqs > 0
After issuing IO to driver, num_groups_with_pending_reqs is ensured to
be nonzero, thus bfq won't expire the queue. This is the root cause of
degradating to single-process performance.
One the other hand, if I set slice_idle to zero, bfq_better_to_idle will
return false early, and the problem will disapear. However, relative
control will be inactive.
My question is that, is this a known flaw for bfq? If not, as cfq don't
have such problem, is there a suitable solution?
Thanks!
Yu Kuai
such problem,
