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sch: don't receive front signal all the time #42

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Merged
Gerold103 merged 1 commit into from
Apr 28, 2025
Merged

sch: don't receive front signal all the time #42

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5 changes: 4 additions & 1 deletion src/mg/sch/TaskScheduler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -130,10 +130,13 @@ namespace sch {
// -------------------------------------------------------
// Handle front tasks.

mySignalFront.Receive();
// Popping the front queue takes linear time due to how the multi-producer queue
// is implemented. It is not batched so far, but even for millions of tasks it is
// a few milliseconds tops.
// Note, the front signal is not received, because under a high load the queue is
// rarely expected to be empty. So receiving this signal would be pointless. At
// the same time, if the queue does become empty for a while, the front signal is
// received when the scheduler has nothing to do and goes to sleep on that signal.
t = myQueueFront.PopAll(tail);
myQueuePending.Append(t, tail);
batch = 0;
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11 changes: 3 additions & 8 deletions tla/TaskScheduler.tla
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Expand Up @@ -516,15 +516,10 @@ SchedCheckFront(wid) ==
\* ---
/\ LET tid == FrontQueue[1] t == Tasks[tid] IN
/\ IF ArrLen(FrontQueue) = 1 THEN
\* The entire front queue is consumed. Consume the signal too. This
\* should be done before the queue becomes empty. Otherwise a new task
\* might arrive after the last task was popped and just before the front
\* signal is consumed. Then the queue would be non-empty but without
\* this signal and that would be a deadlock.
/\ IsFrontSignaled' = FALSE
\* The entire front queue is consumed. Switch to the next state.
/\ WorkerThreads' = ArrSetState(wid, "sched_wait_front", WorkerThreads)
ELSE
/\ UNCHANGED<<IsFrontSignaled, WorkerThreads>>
/\ UNCHANGED<<WorkerThreads>>
/\ FrontQueue' = ArrPopHead(FrontQueue)
\* Status check + change can be done via atomic compare-exchange.
\* The 'wait' flag does not need to be atomic. It is never accessed by more
Expand All @@ -545,7 +540,7 @@ SchedCheckFront(wid) ==
/\ UNCHANGED<<Tasks>>
/\ WaitingQueue' = WaitingQueue \ {tid}
/\ ReadyQueue' = ArrAppend(tid, ReadyQueue)
/\ UNCHANGED<<IsReadySignaled, IsSchedulerTaken>>
/\ UNCHANGED<<IsFrontSignaled, IsReadySignaled, IsSchedulerTaken>>

\* If has no ready tasks then wait until anything comes to the front or the
\* closest task-deadline expires. Keep the scheduler role during that. Other
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