定时线程分配统计事件：jdk.ThreadAllocationStatistics

引入版本：Java 11

相关 ISSUES：

1. Test jdk/jfr/event/runtime/TestThreadAllocationEvent.java fails with null thread：在某些情况下，jdk.ThreadAllocationStatistics 没有采集到 JFR 相关线程，导致空指针。这个 Bug 和 jdk/jfr/event/compiler/TestCompilerCompile.java failed due to "RuntimeException: No thread in event" 重复.该 BUG 对于使用没有大影响，于 Java 16 修复。
2. Per thread IO statistics in JFR，这是一个新特性，期望和 ThreadAllocationStatistics 事件类似，提供每个线程的 IO 数据统计，这个特性还没有通过讨论。

各版本配置：

从 Java 11 引入之后没有改变过：

默认配置（default.jfc of Java 11，default.jfc of Java 12，default.jfc of Java 13，default.jfc of Java 14，default.jfc of Java 15，default.jfc of Java 16，default.jfc of Java 17）:

采样配置（profile.jfc of Java 11，profile.jfc of Java 12，profile.jfc of Java 13，profile.jfc of Java 14，profile.jfc of Java 15，profile.jfc of Java 16，profile.jfc of Java 17）:

为何需要这个事件？

Java 中业务线程都会分配对象，对于以下关键的业务，对象的分配可能更加频繁。有时候我们可能会遇到以下两个情况：

1. 线上应用频繁 GC，可能是先开始 Young GC（New GC，Light GC，不同 GC 叫法不一样），之后伴随着 Old GC 或者 Full GC，可以观察到线上应用占用 CPU 高的线程也是 GC 线程。这个很可能是某个业务 BUG 拉取了大量的数据，例如查询数据库没加条件导致查询整个表，三方接口没限制 limit 返回了超级多的数据。想要直观快速的定位这个问题，可以通过观察那些线程突然分配了很多内存，然后查看这些线程的堆栈进一步确认。jdk.ThreadAllocationStatistics 就是用来查看这个的。
2. 线上 GC 变多，想要减少 GC，光看 jmap 对象统计太多抽象，并不直观知道是那些代码创建的这些对象。堆 dump 太重，对于大内存进程分析成本也很大。可以通过 jdk.ThreadAllocationStatistics 查看是那些线程分配对象比较多，采集这些线程的堆栈可以定位相关代码。

事件包含属性

使用代码测试这个事件

package com.github.hashjang.jfr.test;

import com.sun.management.ThreadMXBean;
import jdk.jfr.Recording;
import jdk.jfr.consumer.RecordedEvent;
import jdk.jfr.consumer.RecordedThread;
import jdk.jfr.consumer.RecordingFile;
import sun.hotspot.WhiteBox;

import java.io.File;
import java.io.IOException;
import java.lang.management.ManagementFactory;
import java.nio.file.Path;
import java.time.Duration;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.stream.Collectors;

public class TestThreadAllocationStatistics {
private static String EVENT_NAME = "jdk.ThreadAllocationStatistics";
//50ms采集一次
private static long eventPeriodMillis = 50;
//对于字节数组对象头占用16字节
private static final int BYTE_ARRAY_OVERHEAD = 16;
//我们要测试的对象大小是1kb
private static final int OBJECT_SIZE = 1024;

public static void main(String[] args) throws IOException {

Recording recording = new Recording();
recording.enable(EVENT_NAME).withPeriod(Duration.ofMillis(eventPeriodMillis));
recording.start();
//使用 WhiteBox 执行 FullGC，清除干扰
WhiteBox whiteBox = WhiteBox.getWhiteBox();
whiteBox.fullGC();

Allocator allocators[] = new Allocator[4];
CountDownLatch allocationsDoneLatch = new CountDownLatch(allocators.length);
for (int i = 0; i < allocators.length; i++) {
allocators[i] = new Allocator(allocationsDoneLatch, OBJECT_SIZE * (i + 1) - BYTE_ARRAY_OVERHEAD);
allocators[i].setDaemon(true);
allocators[i].start();
}
Map<Long, Allocator> map = Arrays.stream(allocators).collect(Collectors.toMap(Thread::getId, v -> v));
Map<Long, Long> threadAllocatedMap = new HashMap<>();
try {
allocationsDoneLatch.await();
//再等待一段时间让定时采集 jdk.ThreadAllocationStatistics 采集到最新的
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}

recording.stop();
Path path = new File(new File(".").getAbsolutePath(), "recording-" + recording.getId() + "-pid" + ProcessHandle.current().pid() + ".jfr").toPath();
recording.dump(path);
for (RecordedEvent event : RecordingFile.readAllEvents(path)) {
if (!EVENT_NAME.equals(event.getEventType().getName())) {
continue;
}
long javaThreadId = ((RecordedThread) event.getValue("thread")).getJavaThreadId();
if (map.containsKey(javaThreadId)) {
//事件是时间有序的（采集时间比较短），所以放入的最后一个就是采集到的线程最终的分配大小
threadAllocatedMap.put(javaThreadId, event.getLong("allocated"));
System.out.println(event);
}
}

map.forEach((id, thread) -> {
System.out.println("Thread " + id + " allocated(from JMX): " + thread.totalAllocated + "; allocated(from jdk.ThreadAllocationStatistics): " + threadAllocatedMap.get(id));
});
}

public static class Allocator extends Thread {
private volatile long totalAllocated = -1;
private final int allocationSize;
public byte[] buffer;
private final CountDownLatch allocationsDoneLatch;

public Allocator(CountDownLatch allocationsDoneLatch, int allocationSize) {
this.allocationsDoneLatch = allocationsDoneLatch;
this.allocationSize = allocationSize;
}

@Override
public void run() {
for (int batches = 0; batches < 100; batches++) {
for (int i = 0; i < 1024; i++) {
buffer = new byte[allocationSize];
}
try {
//期望每个采集周期之间的分配次数为 5
Thread.sleep(eventPeriodMillis / 5);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
//获取当前线程分配的对象大小
totalAllocated = getThreadAllocatedBytes();
allocationsDoneLatch.countDown();
//设置线程为守护线程，等待主线程结束之后会自动结束
//这里进入死循环是因为防止定时采集 jdk.ThreadAllocationStatistics 事件的时候采集不到
while (true) {
Thread.yield();
}
}

private long getThreadAllocatedBytes() {
ThreadMXBean bean = (ThreadMXBean) ManagementFactory.getThreadMXBean();
return bean.getThreadAllocatedBytes(Thread.currentThread().getId());
}
}
}

使用以下 JVM 参数启动：

-Xbootclasspath/a:D:\github\jfr-spring-all\jdk-white-box\target\jdk-white-box-17.0-SNAPSHOT.jar  -Xms512m -Xmx512m  -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -Xlog:gc

输出结果：

[0.016s][info][gc] Using G1
[0.679s][info][gc] GC(0) Pause Full (WhiteBox Initiated Full GC) 19M->2M(512M) 8.804ms
[0.714s][info][gc] GC(1) Pause Young (Normal) (G1 Evacuation Pause) 27M->2M(512M) 1.309ms
[1.099s][info][gc] GC(2) Pause Young (Normal) (G1 Evacuation Pause) 308M->2M(512M) 0.943ms
[1.445s][info][gc] GC(3) Pause Young (Normal) (G1 Evacuation Pause) 308M->2M(512M) 1.141ms
[1.788s][info][gc] GC(4) Pause Young (Normal) (G1 Evacuation Pause) 308M->2M(512M) 1.117ms
jdk.ThreadAllocationStatistics {
startTime = 12:41:29.915
allocated = 1.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.915
allocated = 2.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.915
allocated = 3.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.915
allocated = 4.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.965
allocated = 6.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.965
allocated = 12.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.965
allocated = 15.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:29.965
allocated = 20.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.015
allocated = 10.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.015
allocated = 20.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.015
allocated = 27.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.015
allocated = 36.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.066
allocated = 15.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.066
allocated = 28.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.066
allocated = 39.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.066
allocated = 50.3 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.116
allocated = 19.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.116
allocated = 36.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.116
allocated = 51.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.116
allocated = 64.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.167
allocated = 24.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.167
allocated = 44.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.167
allocated = 63.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.167
allocated = 80.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.217
allocated = 28.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.217
allocated = 52.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.217
allocated = 75.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.217
allocated = 96.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.268
allocated = 32.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.268
allocated = 61.3 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.268
allocated = 87.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.268
allocated = 109.5 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.319
allocated = 37.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.319
allocated = 68.3 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.319
allocated = 99.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.319
allocated = 124.3 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.370
allocated = 41.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.370
allocated = 78.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.370
allocated = 111.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.370
allocated = 144.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.420
allocated = 45.4 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.420
allocated = 86.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.420
allocated = 126.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.420
allocated = 160.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.470
allocated = 50.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.470
allocated = 96.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.470
allocated = 138.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.470
allocated = 180.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.520
allocated = 54.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.520
allocated = 104.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.520
allocated = 150.4 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.520
allocated = 196.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.571
allocated = 58.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.571
allocated = 112.3 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.571
allocated = 165.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.571
allocated = 212.5 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.621
allocated = 63.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.621
allocated = 122.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.621
allocated = 180.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.621
allocated = 232.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.671
allocated = 67.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.671
allocated = 130.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.671
allocated = 192.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.671
allocated = 248.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.722
allocated = 72.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.722
allocated = 140.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.722
allocated = 207.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.722
allocated = 268.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.772
allocated = 76.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.772
allocated = 148.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.772
allocated = 219.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.772
allocated = 284.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.581
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.581
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.581
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.581
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.631
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.631
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.631
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.631
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.682
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.682
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.682
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.682
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.732
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.732
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.732
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.732
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.823
allocated = 81.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.823
allocated = 158.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.823
allocated = 234.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.823
allocated = 304.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.874
allocated = 85.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.874
allocated = 166.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.874
allocated = 246.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.874
allocated = 320.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.925
allocated = 90.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.925
allocated = 176.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.925
allocated = 258.5 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.925
allocated = 340.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.975
allocated = 94.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.975
allocated = 184.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.975
allocated = 273.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:30.975
allocated = 356.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.024
allocated = 99.0 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.024
allocated = 194.0 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.024
allocated = 285.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.024
allocated = 372.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.075
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.075
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.075
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.075
allocated = 392.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.126
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.126
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.126
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.126
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.177
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.177
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.177
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.177
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.228
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.228
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.228
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.228
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.278
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.278
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.278
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.278
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.329
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.329
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.329
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.329
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.379
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.379
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.379
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.379
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.429
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.429
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.429
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.429
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.479
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.479
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.479
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.479
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.530
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.530
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.530
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.530
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.791
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.791
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.791
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.791
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.842
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.842
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.842
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.842
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.892
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.892
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.892
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.892
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.943
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.943
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.943
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.943
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.993
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.993
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.993
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:31.993
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.043
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.043
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.043
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.043
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.093
allocated = 100.3 MB
thread = "Thread-0" (javaThreadId = 27)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.093
allocated = 200.1 MB
thread = "Thread-1" (javaThreadId = 28)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.093
allocated = 300.0 MB
thread = "Thread-2" (javaThreadId = 29)
}

jdk.ThreadAllocationStatistics {
startTime = 12:41:32.093
allocated = 400.0 MB
thread = "Thread-3" (javaThreadId = 30)
}

Thread 27 allocated(from JMX): 105179160; allocated(from jdk.ThreadAllocationStatistics): 105179160
Thread 28 allocated(from JMX): 209780040; allocated(from jdk.ThreadAllocationStatistics): 209780040
Thread 29 allocated(from JMX): 314573616; allocated(from jdk.ThreadAllocationStatistics): 314573616
Thread 30 allocated(from JMX): 419431216; allocated(from jdk.ThreadAllocationStatistics): 419431216

底层原理以及相关 JVM 源码

每隔配置的间隔，就会调用

requestThreadAllocationStatistics(void)

jfrPeriodic.cpp

#define TRACE_REQUEST_FUNC(id)    void JfrPeriodicEventSet::request##id(void)

//使用了宏定义函数，实际函数就是 requestThreadAllocationStatistics(void)
TRACE_REQUEST_FUNC(ThreadAllocationStatistics) {
ResourceMark rm;
//获取线程数量，虽然后面线程数量在采集过程中可能会改变，利用这个值初始化数组
int initial_size = Threads::number_of_threads();
//建立一个数组记录每个线程的分配大小
GrowableArray<jlong> allocated(initial_size);
//建立一个数组记录线程号，和上面那个数组一一对应
GrowableArray<traceid> thread_ids(initial_size);
//记录当前时间
JfrTicks time_stamp = JfrTicks::now();
//新建一个 JfrJavaThreadIterator 来遍历每个线程
JfrJavaThreadIterator iter;
while (iter.has_next()) {
JavaThread* const jt = iter.next();
assert(jt != NULL, "invariant");
//读取每个线程的 cooked_allocated_bytes()
allocated.append(jt->cooked_allocated_bytes());
//记录线程号（包括系统线程号以及 Java 线程号）
thread_ids.append(JFR_THREAD_ID(jt));
}

//遍历数组，生成 JFR 事件并采集
for(int i = 0; i < thread_ids.length(); i++) {
EventThreadAllocationStatistics event(UNTIMED);
//设置当前线程已分配大小
event.set_allocated(allocated.at(i));
//设置线程信息
event.set_thread(thread_ids.at(i));
//设置结束时间
event.set_endtime(time_stamp);
event.commit();
}
}

统计线程的分配内存的大小方法即

cooked_allocated_bytes()

thread.inline.hpp

inline jlong Thread::cooked_allocated_bytes() {
//原子读取 _allocated_bytes
jlong allocated_bytes = Atomic::load_acquire(&_allocated_bytes);
//如果开启了 TLAB（默认开启），则需要查看下当前线程的 TLAB 已经分配的对象大小
if (UseTLAB) {
//统计当前线程的 TLAB 已经分配的大小
size_t used_bytes = tlab().used_bytes();
//如果当前 TLAB 在初始化，或者全局 TLAB 在初始化，会发生 used_bytes > max_size 的情况，忽略这种情况。
if (used_bytes <= ThreadLocalAllocBuffer::max_size_in_bytes()) {
//这样统计其实有可能有点问题，即发生 TLAB retire 的时候，_allocated_bytes 会加上 used_bytes 之后申请一个新的 TLAB，这时候调用这个方法可能会把这个 used_bytes 加了两遍
return allocated_bytes + used_bytes;
}
}
return allocated_bytes;
}

那么哪里会修改

_allocated_bytes

_allocated_bytes

memAllocator.cpp

//如果是 TLAB 外分配，会调用这个方法
HeapWord* MemAllocator::allocate_outside_tlab(Allocation& allocation) const {
//标记当前分配为 TLAB 外分配
allocation._allocated_outside_tlab = true;
//执行 TLAB 外分配，不同 GC 不同
HeapWord* mem = Universe::heap()->mem_allocate(_word_size, &allocation._overhead_limit_exceeded);
if (mem == NULL) {
return mem;
}

NOT_PRODUCT(Universe::heap()->check_for_non_bad_heap_word_value(mem, _word_size));
//计算当前分配的字节大小
size_t size_in_bytes = _word_size * HeapWordSize;
//增加 _allocated_bytes 当前分配的字节大小
_thread->incr_allocated_bytes(size_in_bytes);

return mem;
}

TLAB retire 对应源码：
threadLocalAllocBuffer.cpp

void ThreadLocalAllocBuffer::retire(ThreadLocalAllocStats* stats) {
if (stats != NULL) {
accumulate_and_reset_statistics(stats);
}

if (end() != NULL) {
invariants();
//将当前 TLAB 使用的字节数加到 _allocated_bytes
//从这里可以看出，如果这个方法和 cooked_allocated_bytes() 同时被调用，可能 used_bytes 被加了两遍。
thread()->incr_allocated_bytes(used_bytes());
insert_filler();
initialize(NULL, NULL, NULL);
}
}

我们再来看看 JMX 中查看线程分配内存大小的具体实现。首先对应的 JMX 的代码是：

//我们示例中底层调用的实际就是这个方法获取线程分配的内存大小
protected long getThreadAllocatedBytes(long id) {
boolean verified = verifyThreadAllocatedMemory(id);

if (verified) {
//调用 getThreadAllocatedMemory0 获取分配的内存大小
return getThreadAllocatedMemory0(
Thread.currentThread().getId() == id ? 0 : id);
}
return -1;
}

//getThreadAllocatedMemory0 是一个 native 方法
private static native long getThreadAllocatedMemory0(long id);

这个 native 方法对应的 JVM 源码是：

ThreadImpl.c

JNIEXPORT jlong JNICALL
Java_sun_management_ThreadImpl_getThreadAllocatedMemory0
(JNIEnv *env, jclass cls, jlong tid)
{
//实际实现方法是 GetOneThreadAllocatedMemory
return jmm_interface->GetOneThreadAllocatedMemory(env, tid);
}

management.cpp

JVM_ENTRY(jlong, jmm_GetOneThreadAllocatedMemory(JNIEnv *env, jlong thread_id))
if (thread_id < 0) {
THROW_MSG_(vmSymbols::java_lang_IllegalArgumentException(),
"Invalid thread ID", -1);
}
//获取当前线程
if (thread_id == 0) { // current thread
//调用 cooked_allocated_bytes，和采集 jdk.ThreadAllocationStatistics 调用的底层方法一样
return thread->cooked_allocated_bytes();
}

//根据线程号获取线程
ThreadsListHandle tlh;
JavaThread* java_thread = tlh.list()->find_JavaThread_from_java_tid(thread_id);

if (java_thread != NULL) {
//调用 cooked_allocated_bytes，和采集 jdk.ThreadAllocationStatistics 调用的底层方法一样
return java_thread->cooked_allocated_bytes();
}
return -1;
JVM_END

可以看出，其实底层调用的和 JFR 采集 jdk.ThreadAllocationStatistics 事件一样，都是调用 Thread 的

cooked_allocated_bytes()

针对这个 JFR 事件的一些思考

首先，提出一个观点，jdk.ThreadAllocationStatistics 这个事件并不太消耗性能。原因有二：

1. 统计线程分配大小是近似统计，并不需要进入全局安全点统计。同时也代表，jdk.ThreadAllocationStatistics 同一时间的不同线程事件的分配大小实际并不是同一时间点的，因为没有进入安全点暂停所有线程。
2. 统计仅仅是原子读取每个线程的分配对象大小这个变量，之后加上每个线程当前 TLAB 分配对象大小（这个大小是通过读取两个指针地址获取的），可以看出操作是很轻量级的。即使有很多线程，也不会增加多少性能负担。

然后，默认配置的采集周期，并不能满足我们的需求。默认的采集周期是 everyChunk，默认的 chunk 大小（maxchunksize）是 12M，也就是每采集 12M 的 JFR 事件之后，采集一次 jdk.ThreadAllocationStatistics。这是不太可控的，我一般配置为每过 5s 采集一次。这样对于我们上面提到的那两个需要这个事件的场景也是很适合的。

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