Avoid "hot idle": A series of rapid select() calls with zero timeout.

Squid uses "infinite" precision when it comes to deciding whether the next
timed event is ready but uses millisecond (1e-3) precision when deciding how
long to wait before the next event will be ready. This inconsistency results
in the EventScheduler engine telling the main loop that it has 0 milliseconds
to poll pending I/O, but when asked again (after the I/O is quickly polled),
the EventScheduler engine often does not schedule the promised event and tells
the main loop to poll for another 0 milliseconds again. This cycling may
happen many times in a row (until enough time is wasted for the next event to
become ready using higher precision).

The fixed code adds a minimum 1ms delay for not-yet-ready events. It also
places both decisions into one method (EventScheduler::timeRemaining), and
tries to polish/document decision logic (which is more complex than it may
seem) because the code has to avoid both inconsistent decisions and hot idle
loops while maintaining the traditional "no event is fired before it is due"
guarantee.

TODO: Idle Squid still runs hotter than it should because the maximum waiting
time is artificially capped outside the event queue to EVENT_LOOP_TIMEOUT=1s.
This causes at most one extra loop iteration per second.

diff --git a/src/event.cc b/src/event.cc
index 12db66a9b82489df610d20bee85b0e214036b7c4..830299557a8500384b83fe50f167d3242234eea2 100644 (file)
--- a/src/event.cc
+++ b/src/event.cc
@@ -39,6 +39,10 @@
 #include "Store.h"
 #include "tools.h"
 
+#if HAVE_MATH_H
+#include <math.h>
+#endif
+
 /* The list of event processes */
 
 static OBJH eventDump;
@@ -219,39 +223,37 @@ EventScheduler::cancel(EVH * func, void *arg)
         debug_trap("eventDelete: event not found");
 }
 
+// The event API does not guarantee exact timing, but guarantees that no event
+// is fired before it is due. We may delay firing, but never fire too early.
 int
-EventScheduler::checkDelay()
+EventScheduler::timeRemaining() const
 {
     if (!tasks)
         return EVENT_IDLE;
 
-    int result = (int) ((tasks->when - current_dtime) * 1000);
-
-    if (result < 0)
-        return 0;
+    if (tasks->when <= current_dtime) // we are on time or late
+        return 0; // fire the event ASAP
 
-    return result;
+    const double diff = tasks->when - current_dtime; // microseconds
+    // Round UP: If we come back a nanosecond earlier, we will wait again!
+    const int timeLeft = static_cast<int>(ceil(1000*diff)); // milliseconds
+    // Avoid hot idle: A series of rapid select() calls with zero timeout.
+    const int minDelay = 1; // millisecond
+    return max(minDelay, timeLeft);
 }
 
 int
 EventScheduler::checkEvents(int timeout)
 {
-
-    ev_entry *event = NULL;
-
-    if (NULL == tasks)
-        return checkDelay();
-
-    if (tasks->when > current_dtime)
-        return checkDelay();
+    int result = timeRemaining();
+    if (result != 0)
+        return result;
 
     PROF_start(eventRun);
 
-    debugs(41, 5, HERE << "checkEvents");
-
-    while ((event = tasks)) {
-        if (event->when > current_dtime)
-            break;
+    do {
+        ev_entry *event = tasks;
+        assert(event);
 
         /* XXX assumes event->name is static memory! */
         AsyncCall::Pointer call = asyncCall(41,5, event->name,
@@ -265,14 +267,16 @@ EventScheduler::checkEvents(int timeout)
         tasks = event->next;
         delete event;
 
+        result = timeRemaining();
+
         // XXX: We may be called again during the same event loop iteration.
         // Is there a point in breaking now?
         if (heavy)
             break; // do not dequeue events following a heavy event
-    }
+    } while (result == 0);
 
     PROF_stop(eventRun);
-    return checkDelay();
+    return result;
 }
 
 void

diff --git a/src/event.h b/src/event.h
index ed66a9e0cf40e55c7c629cc541e14a77276eeb4a..41b7895891f94758168c35a08a476084e413e4ec 100644 (file)
--- a/src/event.h
+++ b/src/event.h
@@ -80,8 +80,8 @@ public:
     void cancel(EVH * func, void * arg);
     /* clean up the used memory in the scheduler */
     void clean();
-    /* how long until the next event ? */
-    int checkDelay();
+    /* either EVENT_IDLE or milliseconds remaining until the next event */
+    int timeRemaining() const;
     /* cache manager output for the event queue */
     void dump(StoreEntry *);
     /* find a scheduled event */