MEDIUM: task: perform a single tree lookup per run queue batch

The run queue is designed to perform a single tree lookup and to
use multiple passes to eb32sc_next(). The scheduler rework took a
conservative approach first but this is not needed anymore and it
increases the processing cost of process_runnable_tasks() and even
the time during which the RQ lock is held if the global queue is
heavily loaded. Let's simply move the initial lookup to the entry
of the loop like the previous scheduler used to do. This has reduced
by a factor of 5.5 the number of calls to eb32sc_lookup_get() there.

diff --git a/src/task.c b/src/task.c
index ce5b4f907e2c1442d3c416064d71b64167b659e7..3f193f2d092cdd5ce802a3041d15179da5deb0e7 100644 (file)
--- a/src/task.c
+++ b/src/task.c
@@ -290,9 +290,8 @@ void process_runnable_tasks()
                 * much elements from the global list as to have a bigger local queue
                 * than the average.
                 */
+               rq_next = eb32sc_lookup_ge(&rqueue, rqueue_ticks - TIMER_LOOK_BACK, tid_bit);
                while ((task_list_size[tid] + rqueue_size[tid]) * global.nbthread <= tasks_run_queue) {
-                       /* we have to restart looking up after every batch */
-                       rq_next = eb32sc_lookup_ge(&rqueue, rqueue_ticks - TIMER_LOOK_BACK, tid_bit);
                        if (unlikely(!rq_next)) {
                                /* either we just started or we reached the end
                                 * of the tree, typically because <rqueue_ticks>
@@ -327,14 +326,12 @@ void process_runnable_tasks()
         * get too much in the task list, but put a bit more than
         * the max that will be run, to give a bit more fairness
         */
+       rq_next = eb32sc_lookup_ge(&rqueue_local[tid], rqueue_ticks - TIMER_LOOK_BACK, tid_bit);
        while (max_processed + (max_processed / 10) > task_list_size[tid]) {
                /* Note: this loop is one of the fastest code path in
                 * the whole program. It should not be re-arranged
                 * without a good reason.
                 */
-
-               /* we have to restart looking up after every batch */
-               rq_next = eb32sc_lookup_ge(&rqueue_local[tid], rqueue_ticks - TIMER_LOOK_BACK, tid_bit);
                if (unlikely(!rq_next)) {
                        /* either we just started or we reached the end
                         * of the tree, typically because <rqueue_ticks>