rednaxelafx

GC notifications are sent to the ServiceThread, as shown in the Groovy shell example in this gist. The ServiceThread is a JavaThread, so it can execute Java code.

To see GC notification in action, refer to a conversation in hotspot-gc-dev. Another thread of interest is the discussions on 7110173: GCNotifier::pushNotification publishes stale data.

In the HotSpot VM, GC notifications are pushed with GCNotifier::pushNotification(), in GCMemoryManager::gc_end(), which is in turn called by MemoryService::gc_end().

hotspot/src/share/vm/runtime/serviceThread.cpp

rednaxelafx

$ sudo $JAVA_HOME/bin/java -cp $JAVA_HOME/lib/sa-jdi.jar sun.jvm.hotspot.CLHSDB
Password:
hsdb> attach 15168
Attaching to process 15168, please wait...
hsdb> class scala.Predef$
scala/Predef$ @0x00000007c003eb60 
hsdb> print 0x00000007c003eb60

public final class scala.Predef$ @0x00000007c003eb60

rednaxelafx

[Global flags]
uintx AdaptivePermSizeWeight               = 20               {product}
uintx AdaptiveSizeDecrementScaleFactor     = 4                {product}
uintx AdaptiveSizeMajorGCDecayTimeScale    = 10               {product}
uintx AdaptiveSizePausePolicy              = 0                {product}
uintx AdaptiveSizePolicyCollectionCostMargin  = 50               {product}
uintx AdaptiveSizePolicyInitializingSteps  = 20               {product}
uintx AdaptiveSizePolicyOutputInterval     = 0                {product}
uintx AdaptiveSizePolicyWeight             = 10               {product}
uintx AdaptiveSizeThroughPutPolicy         = 0                {product}

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{method} 
 - klass: {other class}
 - this oop:          0x00000000bc925718
 - method holder:     'StopThread$1'
 - constants:         0x00000000bc925430 constant pool [29] for 'StopThread$1' cache=0x00000000bc925a90
 - access:            0xc1000001  public 
 - name:              'run'
 - signature:         '()V'
 - max stack:         1
 - max locals:        2

rednaxelafx

D:\temp>ruby petype.rb petype.rb
Not an executable

D:\temp>ruby petype.rb C:\Windows\system32\notepad.exe
amd64

D:\temp>ruby petype.rb C:\Windows\system32\jscript9.dll
amd64

D:\temp>ruby petype.rb C:\Windows\syswow64\jscript9.dll

rednaxelafx

import java.util.Arrays;

public class TestSIMD {

  public static void doCompute(int[] a, int[] x, int[] y, int[] z) {
    for (int i = 0; i < a.length; i++) {
      a[i] = a[i] + x[i] * y[i] * z[i];
    }
  }

rednaxelafx

$ bin/spark-shell
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.apache.spark.unsafe.Platform (file:/home/krismok/code/work/oss-spark-readonly/assembly/target/scala-2.12/jars/spark-unsafe_2.12-3.0.0-SNAPSHOT.jar) to constructor java.nio.DirectByteBuffer(long,int)
WARNING: Please consider reporting this to the maintainers of org.apache.spark.unsafe.Platform
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
19/07/30 06:03:19 WARN NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel). For SparkR, use setLogLevel(newLevel).

rednaxelafx

$ clang++ -S -emit-llvm yy.cc

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using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Runtime.CompilerServices;
using System.Threading;
internal class Test
{
	[CompilerGenerated]
	private sealed class <GetNaturals>d__0 : IEnumerable<int>, IEnumerable, IEnumerator<int>, IEnumerator, IDisposable

rednaxelafx

The "size" field shows total reserved GC heap size in MB, which includes all generations (including PermGen).
"32-bits Oops" means the shift is zero; otherwise it'd be non-zero.

If UseCompressedOops is false, or if the heap is too big to use compressed oops, then PrintCompressedOopsMode won't print anything. The -Xmx32g example below show that ergonomics didn't set UseCompressedOops to true.

If the heap is too big, HotSpot would print a warning message and set UseCompressedOops to false.