8261034: improve jcmd GC.class_histogram to support parallel

Reviewed-by: cjplummer, sspitsyn

src/hotspot/share/services/diagnosticCommand.cpp

@@ -496,13 +496,30 @@ void HeapDumpDCmd::execute(DCmdSource source, TRAPS) {
 ClassHistogramDCmd::ClassHistogramDCmd(outputStream* output, bool heap) :
                                        DCmdWithParser(output, heap),
   _all("-all", "Inspect all objects, including unreachable objects",
-       "BOOLEAN", false, "false") {
+       "BOOLEAN", false, "false"),
+  _parallel_thread_num("-parallel",
+       "Number of parallel threads to use for heap inspection. "
+       "0 (the default) means let the VM determine the number of threads to use. "
+       "1 means use one thread (disable parallelism). "
+       "For any other value the VM will try to use the specified number of "
+       "threads, but might use fewer.",
+       "INT", false, "0") {
   _dcmdparser.add_dcmd_option(&_all);
+  _dcmdparser.add_dcmd_option(&_parallel_thread_num);
 }
 
 void ClassHistogramDCmd::execute(DCmdSource source, TRAPS) {
+  jlong num = _parallel_thread_num.value();
+  if (num < 0) {
+    output()->print_cr("Parallel thread number out of range (>=0): " JLONG_FORMAT, num);
+    return;
+  }
+  uint parallel_thread_num = num == 0
+      ? MAX2<uint>(1, (uint)os::initial_active_processor_count() * 3 / 8)
+      : num;
   VM_GC_HeapInspection heapop(output(),
-                              !_all.value() /* request full gc if false */);
+                              !_all.value(), /* request full gc if false */
+                              parallel_thread_num);
   VMThread::execute(&heapop);
 }
 

src/hotspot/share/services/diagnosticCommand.hpp

@@ -339,6 +339,7 @@ class HeapDumpDCmd : public DCmdWithParser {
 class ClassHistogramDCmd : public DCmdWithParser {
 protected:
   DCmdArgument<bool> _all;
+  DCmdArgument<jlong> _parallel_thread_num;
 public:
   ClassHistogramDCmd(outputStream* output, bool heap);
   static const char* name() {

test/hotspot/jtreg/serviceability/dcmd/gc/ClassHistogramTest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2021, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -21,6 +21,7 @@
  * questions.
  */
 
+import org.testng.annotations.DataProvider;
 import org.testng.annotations.Test;
 
 import java.util.regex.Pattern;
@@ -42,16 +43,19 @@
 public class ClassHistogramTest {
     public static class TestClass {}
     public static TestClass[] instances = new TestClass[1024];
-    protected String classHistogramArgs = "";
 
     static {
         for (int i = 0; i < instances.length; ++i) {
             instances[i] = new TestClass();
         }
     }
 
-    public void run(CommandExecutor executor) {
+    public void run(CommandExecutor executor, String classHistogramArgs, String expactedErrMsg) {
         OutputAnalyzer output = executor.execute("GC.class_histogram " + classHistogramArgs);
+        if (!expactedErrMsg.isEmpty()) {
+            output.shouldMatch(expactedErrMsg);
+            return;
+        }
 
         /*
          * example output:
@@ -87,8 +91,34 @@ public void run(CommandExecutor executor) {
             Pattern.quote(TestClass.class.getName()) + "\\s*$");
     }
 
-    @Test
-    public void jmx() {
-        run(new JMXExecutor());
+    @DataProvider(name="ArgsProvider")
+    private Object[][] getArgs() {
+        String parallelErr = "Parallel thread number out of range";
+        return new Object[][] {
+                // valid args
+                {"", ""},
+                {"-parallel=0", ""},
+                {"-parallel=1", ""},
+                {"-parallel=2", ""},
+                {"-parallel="+Long.MAX_VALUE, ""},
+                {"-all=false -parallel=0", ""},
+                {"-all=false -parallel=1", ""},
+                {"-all=false -parallel=2", ""},
+                {"-all=true", ""},
+                {"-all=true -parallel=0", ""},
+                {"-all=true -parallel=1", ""},
+                {"-all=true -parallel=2", ""},
+                {"-parallel=2 -all=true", ""},
+                // invalid args
+                {"-parallel=-1", parallelErr},
+                {"-parallel="+Long.MIN_VALUE, parallelErr},
+                {"-all=false -parallel=-10", parallelErr},
+                {"-all=true -parallel=-100", parallelErr},
+        };
+    }
+
+    @Test(dataProvider="ArgsProvider")
+    public void jmx(String args, String expactedErrMsg) {
+        run(new JMXExecutor(), args, expactedErrMsg);
     }
 }