Author: Christian Werner <[email protected]> State: Draft Type: Project Created: 06-August-2021 Post-History: Keywords: Tcl Tcl-Version: 8.7 Tcl-Branch: tip-609
This TIP proposes to add a new qualifier to the Tcl_ThreadQueueEvent() function in order to allow implicit thread wakeup on empty queue condition.
Most code using Tcl_ThreadQueueEvent() is immediately followed by a call to Tcl_ThreadAlert(). This involves more than one pass through a critical section locking the same process wide resource. By considering the queue state (the empty condition) on causing the underlying Tcl_AlertNotifier() which is the work horse of Tcl_ThreadAlert(), the Tcl_ThreadAlert() can be completely left out, and the critical section must be locked/unlocked only once instead of twice, reducing potential spurious wakeups.
The new qualifier TCL_QUEUE_ALERT_IF_EMPTY is added for Tcl_ThreadQueueEvent() eliminating the need to call Tcl_ThreadAlert() afterwards. This qualifier checks for the state of the event queue of the target thread and perform an implicit equivalent of Tcl_ThreadAlert() if the queue is empty at the begin of Tcl_ThreadQueueEvent(). It can be used in combination ('|') with the already existing qualifiers.
By using conditional compilation and preprocessor symbols it should be in most cases possible to allow for backward compatibility, e.g.
#if (TCL_MAJOR_VERSION > 8) || (TCL_MINOR_VERSION > 6)
Tcl_ThreadQueueEvent(threadId, &event->header, TCL_QUEUE_TAIL|TCL_QUEUE_ALERT_IF_EMPTY);
Tcl_ThreadQueueEvent(threadId, &event->header, TCL_QUEUE_TAIL);
This document has been placed in the public domain.