AHeise commented on a change in pull request #14573:
URL: https://github.com/apache/flink/pull/14573#discussion_r553252820



##########
File path: 
flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/tasks/mailbox/MailboxDefaultAction.java
##########
@@ -59,6 +60,16 @@
          * Calling this method signals that the mailbox-thread should 
(temporarily) stop invoking
          * the default action, e.g. because there is currently no input 
available. This method must
          * be invoked from the mailbox-thread only!
+         *
+         * @param suspensionIdleTimer started (ticking) {@link TimerGauge} 
that measures how long
+         *     the default action was suspended/idling. If mailbox loop is 
busy processing mails,
+         *     this timer should be paused for the time required to process 
the mails.
+         */
+        Suspension suspendDefaultAction(TimerGauge suspensionIdleTimer);
+
+        /**
+         * Same as {@link #suspendDefaultAction(TimerGauge)} but without any 
associated timer
+         * measuring the idle time.
          */
         Suspension suspendDefaultAction();

Review comment:
       Okay makes sense, we could still try to deprecate 
`suspendDefaultAction`. I don't see it being used outside of legacy sources and 
I kinda consider it a hack there.

##########
File path: 
flink-streaming-java/src/test/java/org/apache/flink/streaming/runtime/tasks/StreamTaskTest.java
##########
@@ -1241,9 +1241,60 @@ public void testProcessWithAvailableOutput() throws 
Exception {
         }
     }
 
+    /**
+     * In this weird construct, we are:
+     *
+     * <ul>
+     *   <li>1. We start a thread, which will...
+     *   <li>2. ... sleep for X ms, and enqueue another mail, that will...
+     *   <li>3. ... sleep for Y ms, and make the output available again
+     * </ul>
+     *
+     * <p>2nd step is to check that back pressure or idle counter is at least 
X. In the last 3rd
+     * step, we test whether this counter was paused for the duration of 
processing mails.
+     */
+    private static class WaitingThread extends Thread {
+        private final MailboxExecutor executor;
+        private final RunnableWithException resumeTask;
+        private final long sleepTimeInsideMail;
+        private final long sleepTimeOutsideMail;
+
+        public WaitingThread(
+                MailboxExecutor executor,
+                RunnableWithException resumeTask,
+                long sleepTimeInsideMail,
+                long sleepTimeOutsideMail) {
+            this.executor = executor;
+            this.resumeTask = resumeTask;
+            this.sleepTimeInsideMail = sleepTimeInsideMail;
+            this.sleepTimeOutsideMail = sleepTimeOutsideMail;
+        }
+
+        @Override
+        public void run() {
+            try {
+                Thread.sleep(sleepTimeOutsideMail);
+            } catch (InterruptedException e) {
+                e.printStackTrace();

Review comment:
       Wouldn't the test fail on a spurious wake-up? Might be hard to debug if 
you swallow the exception here. Would it be better to LOG?
   
   Same inside the execute.

##########
File path: 
flink-streaming-java/src/test/java/org/apache/flink/streaming/runtime/tasks/StreamTaskTest.java
##########
@@ -1256,49 +1307,82 @@ public void testProcessWithUnAvailableOutput() throws 
Exception {
 
             final RunnableWithException completeFutureTask =
                     () -> {
-                        Thread.sleep(sleepTime + 1);
                         assertEquals(1, inputProcessor.currentNumProcessCalls);
                         
assertTrue(task.mailboxProcessor.isDefaultActionUnavailable());
                         
environment.getWriter(1).getAvailableFuture().complete(null);
                     };
 
+            // Make sure WaitingThread is started after Task starts processing.
             executor.submit(
-                    () -> {
-                        executor.submit(
-                                completeFutureTask,
-                                "This task will complete the future to resume 
process input action.");
-                    },
+                    () ->
+                            new WaitingThread(
+                                            executor,
+                                            completeFutureTask,
+                                            sleepTimeInsideMail,
+                                            sleepTimeOutsideMail)
+                                    .start(),
                     "This task will submit another task to execute after 
processing input once.");
 
+            long startTs = System.currentTimeMillis();
             TaskIOMetricGroup ioMetricGroup =
                     task.getEnvironment().getMetricGroup().getIOMetricGroup();
             task.invoke();
+            long totalDuration = System.currentTimeMillis() - startTs;
             assertThat(
                     ioMetricGroup.getBackPressuredTimePerSecond().getCount(),
-                    Matchers.greaterThanOrEqualTo(sleepTime));
+                    Matchers.greaterThanOrEqualTo(sleepTimeOutsideMail));
+            assertThat(
+                    ioMetricGroup.getBackPressuredTimePerSecond().getCount(),
+                    Matchers.lessThanOrEqualTo(totalDuration - 
sleepTimeInsideMail));

Review comment:
       Okay, I just wanted to double check (after recent test instabilities).

##########
File path: 
flink-streaming-java/src/test/java/org/apache/flink/streaming/runtime/tasks/StreamTaskTest.java
##########
@@ -1256,49 +1307,82 @@ public void testProcessWithUnAvailableOutput() throws 
Exception {
 
             final RunnableWithException completeFutureTask =
                     () -> {
-                        Thread.sleep(sleepTime + 1);
                         assertEquals(1, inputProcessor.currentNumProcessCalls);
                         
assertTrue(task.mailboxProcessor.isDefaultActionUnavailable());
                         
environment.getWriter(1).getAvailableFuture().complete(null);
                     };
 
+            // Make sure WaitingThread is started after Task starts processing.
             executor.submit(
-                    () -> {
-                        executor.submit(
-                                completeFutureTask,
-                                "This task will complete the future to resume 
process input action.");
-                    },
+                    () ->
+                            new WaitingThread(
+                                            executor,
+                                            completeFutureTask,
+                                            sleepTimeInsideMail,
+                                            sleepTimeOutsideMail)
+                                    .start(),
                     "This task will submit another task to execute after 
processing input once.");
 
+            long startTs = System.currentTimeMillis();
             TaskIOMetricGroup ioMetricGroup =
                     task.getEnvironment().getMetricGroup().getIOMetricGroup();
             task.invoke();
+            long totalDuration = System.currentTimeMillis() - startTs;
             assertThat(
                     ioMetricGroup.getBackPressuredTimePerSecond().getCount(),
-                    Matchers.greaterThanOrEqualTo(sleepTime));
+                    Matchers.greaterThanOrEqualTo(sleepTimeOutsideMail));
+            assertThat(
+                    ioMetricGroup.getBackPressuredTimePerSecond().getCount(),
+                    Matchers.lessThanOrEqualTo(totalDuration - 
sleepTimeInsideMail));
             assertThat(ioMetricGroup.getIdleTimeMsPerSecond().getCount(), 
is(0L));
             assertEquals(numberOfProcessCalls, 
inputProcessor.currentNumProcessCalls);
         }
     }
 
     @Test
     public void testProcessWithUnAvailableInput() throws Exception {
-        final long unAvailableTime = 42;
+        final long sleepTimeOutsideMail = 42;
+        final long sleepTimeInsideMail = 44;
         try (final MockEnvironment environment = setupEnvironment(new 
boolean[] {true, true})) {
             final UnAvailableTestInputProcessor inputProcessor =
-                    new UnAvailableTestInputProcessor(unAvailableTime);
+                    new UnAvailableTestInputProcessor();
             final StreamTask task =
                     new MockStreamTaskBuilder(environment)
                             .setStreamInputProcessor(inputProcessor)
                             .build();
 
+            final MailboxExecutor executor = 
task.mailboxProcessor.getMainMailboxExecutor();
+            final RunnableWithException completeFutureTask =
+                    () -> {
+                        inputProcessor
+                                .availabilityProvider
+                                .getUnavailableToResetAvailable()
+                                .complete(null);
+                    };
+
+            // Make sure WaitingThread is started after Task starts processing.
+            executor.submit(
+                    () ->
+                            new WaitingThread(

Review comment:
       I was thinking if it's safer to also join the `WaitingThread` after 
`task.invoke();` but I guess that's more or less done through the 
`completeFutureTask` (should that be called in a `finally`?).




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