使用live选项时，jmap是否强制垃圾回收？ [英] Does jmap force garbage collection when the live option is used?

问题描述

我一直在试用 jmap -histo 和 jmap -dump 今天

按此顺序运行

  jmap -dump：format = b，file = heap.1 [pid] 
 jmap -dump：live，format = b，file = heap.2 [pid] 
 jmap -dump：format = b，file = heap.3 [pid] 
  

heap.3 类似于 heap.2 大于 heap.1 。特别是，我对 heap.1 感兴趣的死对象在 heap.3 。

看到这个，我开始寻找可以告诉我我应该期待什么的文档。我设法得到的最接近的是这个讨论，其中来自briand和alanb的评论意味着在实践中当我使用live选项时，我可以期待这个GC发生;但答案是五年，并且对论坛帖子看起来有点不正式。

我在哪里可以找到记录的当前行为？

$为了确定活跃性，Java 必须运行完整的GC，所以是的，它的确如此。

---

为了让这个问题入睡......这里是答案，如果有人需要深入挖掘。感受自由。

部分/hotspot/agent/src/share/vm/services/attachListener.cpp 取自

openjdk http://download.java.net/ openjdk / jdk7 /
，您必须接受 http：// www .gnu.org / licenses / gpl-2.0.html

  //执行inspectheap 命令
 // 
 //输入参数： -  
 // arg0：-live或-all
 static jint heap_inspection（AttachOperation * op，outputStream * out ）{
 bool live_objects_only = true; //默认为true，以保持行为之前的行为
 const char * arg0 = op-> arg（0）; （arg0！= NULL&&（strlen（arg0）> 0））{
 if（strcmp（arg0，-all）！= 0&& strcmp（arg0 ，-live）！= 0）{
 out-> print_cr（检查堆操作的无效参数：％s，arg0）; 
 return JNI_ERR; 
} 
 live_objects_only = strcmp（arg0，-live）== 0; 
} 
 VM_GC_HeapInspection heapop（out，live_objects_only / * request full gc * /，true / * need_prologue * /）; 
 VMThread :: execute（& heapop）; 
 return JNI_OK; 
} 
  

在vmGCOperations.hpp中是定义

 `VM_GC_HeapInspection（outputStream * out，bool request_full_gc，
 bool need_prologue）：`
  

I've been experimenting with jmap -histo and jmap -dump today

When run in this sequence

jmap -dump:format=b,file=heap.1 [pid]
jmap -dump:live,format=b,file=heap.2 [pid]
jmap -dump:format=b,file=heap.3 [pid]

heap.3 resembles heap.2 more than heap.1. In particular, the "dead" objects that I'm interested in in heap.1 are absent from heap.3.

Seeing this, I started looking for documentation that would tell me what I should expect. The closest I managed to get was this discussion, where the comments from briand and alanb imply that in practice I can expect this GC to occur when I use the live option; but the answers are five years old, and posts to a forum seem a bit informal for a specification.

Where can I find the current behavior documented?

解决方案

In order to determine liveness, Java has to run full GC, so yes, it does.

---

To put the question to sleep... here is the answer, if anyone needs to dig deeper. Feel free.

part of /hotspot/agent/src/share/vm/services/attachListener.cpp taken from

openjdk http://download.java.net/openjdk/jdk7/ and you must accept http://www.gnu.org/licenses/gpl-2.0.html

// Implementation of "inspectheap" command
//
// Input arguments :-
//   arg0: "-live" or "-all"
static jint heap_inspection(AttachOperation* op, outputStream* out) {
  bool live_objects_only = true;   // default is true to retain the behavior before this change is made
  const char* arg0 = op->arg(0);
  if (arg0 != NULL && (strlen(arg0) > 0)) {
    if (strcmp(arg0, "-all") != 0 && strcmp(arg0, "-live") != 0) {
      out->print_cr("Invalid argument to inspectheap operation: %s", arg0);
      return JNI_ERR;
    }
    live_objects_only = strcmp(arg0, "-live") == 0;
  }
  VM_GC_HeapInspection heapop(out, live_objects_only /* request full gc */, true /* need_prologue */);
  VMThread::execute(&heapop);
  return JNI_OK;
}

in vmGCOperations.hpp it's the definition

`VM_GC_HeapInspection(outputStream* out, bool request_full_gc,
                   bool need_prologue) :`

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