Fix: don't wait after completion of job batch if work queue is empty

On completion of a batch of jobs from the work queue, a wait of 10
ms (using poll()) is performed if there is no work present in the
work queue before waiting on its futex.

The work queue thread's structure is inspired by the call-rcu thread.
In the context of the call-rcu thread, my understanding is that the
intention is to ensure that the thread does not continuously wake-up
to process a single queued item. This is fine as an application should
not wait for a call-rcu job to be executed (or at least I don't see a
use-case for that).

In the context of the work queue, waiting for more work to be available
artificially slows down the execution of work on which an application
may wait.

I have observed a case where LTTng's session daemon's shutdown is
takes around 4 seconds as a large number of cds_lfht objects are
destroyed. Removing the wait reduces the duration of this phase of the
shut-down to almost ~10ms.

If a workqueue user even need the explicit delay for batching (e.g. if
a call-rcu implementation would ever use the workqueue worker thread),
it can add it within the worker_before_wait_fct callback received as
argument from workqueue creation.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/src/workqueue.c b/src/workqueue.c
index 6707ffe3b45e2c2226d99c3b163d4e1c820028aa..0b1a9eae41f91ba5bad8dc7e43db2731707d8854 100644 (file)
--- a/src/workqueue.c
+++ b/src/workqueue.c
@@ -238,7 +238,6 @@ static void *workqueue_thread(void *arg)
                        if (cds_wfcq_empty(&workqueue->cbs_head,
                                        &workqueue->cbs_tail)) {
                                futex_wait(&workqueue->futex);
                        if (cds_wfcq_empty(&workqueue->cbs_head,
                                        &workqueue->cbs_tail)) {
                                futex_wait(&workqueue->futex);

-                               (void) poll(NULL, 0, 10);

                                uatomic_dec(&workqueue->futex);
                                /*
                                 * Decrement futex before reading
                                uatomic_dec(&workqueue->futex);
                                /*
                                 * Decrement futex before reading