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Taskq locking optimizations

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Testing has shown that tq->tq_lock can be highly contended when a
large number of small work items are dispatched.  The lock hold time
is reduced by the following changes:

1) Use exclusive threads in the work_waitq

When a single work item is dispatched we only need to wake a single
thread to service it.  The current implementation uses non-exclusive
threads so all threads are woken when the dispatcher calls wake_up().
If a large number of threads are in the queue this overhead can become
non-negligible.

2) Conditionally add/remove threads from work waitq

Taskq threads need only add themselves to the work wait queue if
there are no pending work items.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #32
This commit is contained in:
Ned Bass 2012-01-19 10:33:19 -08:00 committed by Brian Behlendorf
parent 0bb43ca282
commit 3c6ed5410b
1 changed files with 2 additions and 2 deletions
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4
module/spl/spl-taskq.c
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@ -454,17 +454,17 @@ taskq_thread(void *args)
while (!kthread_should_stop()) {
add_wait_queue(&tq->tq_work_waitq, &wait);
if (list_empty(&tq->tq_pend_list) &&
list_empty(&tq->tq_prio_list)) {
add_wait_queue_exclusive(&tq->tq_work_waitq, &wait);
spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
schedule();
spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
remove_wait_queue(&tq->tq_work_waitq, &wait);
} else {
__set_current_state(TASK_RUNNING);
}
remove_wait_queue(&tq->tq_work_waitq, &wait);
if (!list_empty(&tq->tq_prio_list))
pend_list = &tq->tq_prio_list;