8257149: Improve G1 Service thread task scheduling to guarantee task …

…delay

Reviewed-by: sjohanss, iwalulya

src/hotspot/share/gc/g1/g1RemSet.cpp

@@ -558,12 +558,6 @@ class G1RemSetSamplingTask : public G1ServiceTask {
     update_vtime_accum(vtime.duration());
   }
 
-  // To avoid extensive rescheduling if the task is executed a bit early. The task is
-  // only rescheduled if the expected time is more than 1ms away.
-  bool should_reschedule() {
-    return reschedule_delay_ms() > 1;
-  }
-
   // There is no reason to do the sampling if a GC occurred recently. We use the
   // G1ConcRefinementServiceIntervalMillis as the metric for recently and calculate
   // the diff to the last GC. If the last GC occurred longer ago than the interval
@@ -580,9 +574,9 @@ class G1RemSetSamplingTask : public G1ServiceTask {
     SuspendibleThreadSetJoiner sts;
 
     // Reschedule if a GC happened too recently.
-    if (should_reschedule()) {
-      // Calculate the delay given the last GC and the interval.
-      schedule(reschedule_delay_ms());
+    jlong delay_ms = reschedule_delay_ms();
+    if (delay_ms > 0) {
+      schedule(delay_ms);
       return;
     }
 

src/hotspot/share/gc/g1/g1ServiceThread.cpp

@@ -111,7 +111,7 @@ G1ServiceThread::~G1ServiceThread() {
   delete _periodic_gc_task;
 }
 
-void G1ServiceThread::register_task(G1ServiceTask* task, jlong delay) {
+void G1ServiceThread::register_task(G1ServiceTask* task, jlong delay_ms) {
   guarantee(!task->is_registered(), "Task already registered");
   guarantee(task->next() == NULL, "Task already in queue");
 
@@ -130,7 +130,7 @@ void G1ServiceThread::register_task(G1ServiceTask* task, jlong delay) {
 
   // Schedule the task to run after the given delay. The service will be
   // notified to check if this task is first in the queue.
-  schedule_task(task, delay);
+  schedule_task(task, delay_ms);
 }
 
 void G1ServiceThread::schedule(G1ServiceTask* task, jlong delay_ms) {
@@ -163,8 +163,9 @@ int64_t G1ServiceThread::time_to_next_task_ms() {
     return 0;
   }
 
-  // Return sleep time in milliseconds.
-  return (int64_t) TimeHelper::counter_to_millis(time_diff);
+  // Return sleep time in milliseconds. Using ceil to make sure we never
+  // schedule a task too early.
+  return (int64_t) ceil(TimeHelper::counter_to_millis(time_diff));
 }
 
 void G1ServiceThread::notify() {

src/hotspot/share/gc/g1/g1ServiceThread.hpp

@@ -131,13 +131,14 @@ class G1ServiceThread: public ConcurrentGCThread {
   G1ServiceThread();
   ~G1ServiceThread();
 
-  // Register a task with the service thread and schedule it. If
-  // no delay is specified the task is scheduled to run directly.
-  void register_task(G1ServiceTask* task, jlong delay = 0);
-
-  // Schedule the task and notify the service thread that a new
-  // task might be ready to run.
-  void schedule_task(G1ServiceTask* task, jlong delay);
+  // Register a task with the service thread. The task is guaranteed not to run
+  // until at least `delay_ms` has passed. If no delay is specified or the
+  // delay is 0, the task will run in the earliest time possible.
+  void register_task(G1ServiceTask* task, jlong delay_ms = 0);
+
+  // Schedule an already-registered task to run in at least `delay_ms` time,
+  // and notify the service thread.
+  void schedule_task(G1ServiceTask* task, jlong delay_ms);
 };
 
 #endif // SHARE_GC_G1_G1SERVICETHREAD_HPP