Configure thread pool executor to run in LIFO order to minimize blocked task count (#4701)

Fixes #4611

We basically reverse the execution order of tasks submitted to the
`mill.exec.ExecutionContexts.ThreadPool` executor to be LIFO order
instead of FIFO order. Since child tasks are always spawned after parent
tasks, this prioritizes scheduling child tasks (which can actually run
and do work) over parent tasks (which are sometimes blocked waiting for
child tasks to be scheduled, run, and complete).

There's no `LinkedBlockingStack` equivalent of `LinkedBlockingQueue`,
but `LinkedBlockingDeque` can serve the same purpose by forwarding
`poll` and `take` to `pollLast` and `takeLast`. `ThreadPoolExecutor`
only seems to call `size`, `isEmpty`, `remove`, `poll`, `take`, `offer`
on it's `workQueue` reference, so we don't need to worry about
forwarding other methods.

This LIFO order is similar to the approach that `ForkJoinPool` and other
schedulers that prioritize child tasks take.

Ideally we would use a `PriorityBlockingQueue` to properly prioritize
all child tasks to run before all parent tasks, but because of our usage
of Scala `Future`s tasks end up being wrapped and it gets hard to
distinguish exactly where the `Runnable`s submitted to the queue
actually come from. Using a LIFO is a crude approximation of the
prioritization we actually want, but has the benefit of working without
any knowledge of the individual tasks that are submitted, with the
downside that later-scheduled parent tasks can take priority over
earlier-scheduled child tasks spawned from an even-earlier parent.

Tested locally via `./mill-assembly.jar -i integration.__.local.server`.
Without this patch, we see large numbers of tasks buffering up in the
prompt. with this PR, it seems to be limited to 5. Also tweaked some of
the `test-grouping-parallel` tests

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: lihaoyi

core/exec/src/mill/exec/ExecutionContexts.scala

@@ -5,7 +5,7 @@ import os.Path
 
 import scala.concurrent.{Await, Future}
 import scala.concurrent.duration.Duration
-import java.util.concurrent.{ExecutorService, LinkedBlockingQueue, ThreadPoolExecutor, TimeUnit}
+import java.util.concurrent.{ExecutorService, LinkedBlockingDeque, ThreadPoolExecutor, TimeUnit}
 import mill.api.Logger
 
 private object ExecutionContexts {
@@ -38,7 +38,15 @@ private object ExecutionContexts {
       threadCount0,
       0,
       TimeUnit.SECONDS,
-      new LinkedBlockingQueue[Runnable]()
+      // Use a `Deque` rather than a normal `Queue`, with the various `poll`/`take`
+      // operations reversed, providing elements in a LIFO order. This ensures that
+      // child `fork.async` tasks always take priority over parent tasks, avoiding
+      // large numbers of blocked parent tasks from piling up
+      new LinkedBlockingDeque[Runnable]() {
+        override def poll(): Runnable = super.pollLast()
+        override def poll(timeout: Long, unit: TimeUnit): Runnable = super.pollLast(timeout, unit)
+        override def take(): Runnable = super.takeLast()
+      }
     )
 
     val threadPool: ExecutorService = executor

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsA.java

@@ -5,7 +5,7 @@
 public class RandomTestsA extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Storm", 38);
+    testGreeting("Storm", 380);
   }
   // Removed other tests to simplify
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsB.java

@@ -5,6 +5,6 @@
 public class RandomTestsB extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Dakota", 18);
+    testGreeting("Dakota", 180);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsC.java

@@ -5,6 +5,6 @@
 public class RandomTestsC extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Jordan", 95);
+    testGreeting("Jordan", 950);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsD.java

@@ -5,6 +5,6 @@
 public class RandomTestsD extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Kai", 14);
+    testGreeting("Kai", 140);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsE.java

@@ -5,6 +5,6 @@
 public class RandomTestsE extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Sage", 28);
+    testGreeting("Sage", 280);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsF.java

@@ -5,6 +5,6 @@
 public class GroupX1 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Aether", 55);
+    testGreeting("Aether", 550);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsG.java

@@ -5,6 +5,6 @@
 public class GroupX2 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Chronos", 35);
+    testGreeting("Chronos", 350);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsH.java

@@ -5,6 +5,6 @@
 public class GroupX3 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Fortuna", 25);
+    testGreeting("Fortuna", 250);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsI.java

@@ -5,6 +5,6 @@
 public class GroupX4 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Janus", 21);
+    testGreeting("Janus", 210);
   }
 }

example/javalib/testing/4-test-parallel/foo/test/src/foo/RandomTestsJ.java

@@ -5,6 +5,6 @@
 public class GroupX5 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Orion", 95);
+    testGreeting("Orion", 950);
   }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX1.java

@@ -5,6 +5,6 @@
 public class GroupY1 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Hades", 15);
+    testGreeting("Hades", 150);
   }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX10.java

@@ -5,6 +5,6 @@
 public class GroupY2 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Odin", 34);
+    testGreeting("Odin", 340);
   }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX2.java

@@ -5,6 +5,6 @@
 public class GroupY3 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Ra", 21);
+    testGreeting("Ra", 210);
   }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX3.java

@@ -5,6 +5,6 @@
 public class GroupY4 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Wotan", 95);
+    testGreeting("Wotan", 950);
   }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX4.java

@@ -5,6 +5,6 @@
 public class GroupY5 extends RandomTestsUtils {
   @Test
   public void test1() throws Exception {
-    testGreeting("Xiuhtecuhtli", 26);
+    testGreeting("Xiuhtecuhtli", 260);
   }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX5.java

@@ -6,6 +6,6 @@ class RandomTestsA : RandomTestsUtils() {
     @Test
     @Throws(Exception::class)
     fun test1() {
-        testGreeting("Storm", 38)
+        testGreeting("Storm", 380)
     }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX6.java

@@ -6,6 +6,6 @@ class RandomTestsB : RandomTestsUtils() {
     @Test
     @Throws(Exception::class)
     fun test1() {
-        testGreeting("Dakota", 18)
+        testGreeting("Dakota", 180)
     }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX7.java

@@ -6,6 +6,6 @@ class RandomTestsC : RandomTestsUtils() {
     @Test
     @Throws(Exception::class)
     fun test1() {
-        testGreeting("Jordan", 95)
+        testGreeting("Jordan", 950)
     }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX8.java

@@ -6,6 +6,6 @@ class RandomTestsD : RandomTestsUtils() {
     @Test
     @Throws(Exception::class)
     fun test1() {
-        testGreeting("Kai", 14)
+        testGreeting("Kai", 140)
     }
 }

example/javalib/testing/6-test-group-parallel/foo/test/src/foo/GroupX9.java

@@ -6,6 +6,6 @@ class RandomTestsE : RandomTestsUtils() {
     @Test
     @Throws(Exception::class)
     fun test1() {
-        testGreeting("Sage", 28)
+        testGreeting("Sage", 280)
     }
 }