运行 jmap 得到无法打开套接字文件

我必须运行 jmap来获取进程的堆转储。但是 jvm返回:

Unable to open socket file: target process not responding or HotSpot VM not loaded
The -F option can be used when the target process is not responding

所以我用了 -F:

./jmap -F -dump:format=b,file=heap.bin 10330
Attaching to process ID 10331, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.51-b03
Dumping heap to heap.bin ...
  1. 使用 -F可以接受堆转储吗？
  2. 我已经等了20分钟了，还没有完成。你知道为什么吗？
82441 次浏览
小开

I just found that jmap (and presumably jvisualvm when using it to generate a heap dump) enforces that the user running jmap must be the same user running the process attempting to be dumped.

in my case the jvm i want a heap dump for is being run by linux user "jboss". so where sudo jmap -dump:file.bin <pid> was reporting "Unable to open socket:", i was able to grab my heap dump using:

sudo -u jboss jmap -dump:file.bin <pid>
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最佳答案

jmap vs. jmap -F, as well as jstack vs. jstack -F use completely different mechanisms to communcate with the target JVM.

jmap / jstack

When run without -F these tools use Dynamic Attach Mechanism. This works as follows.

  1. Before connecting to Java process 1234, jmap creates a file .attach_pid1234 at the working directory of the target process or at /tmp.

  2. Then jmap sends SIGQUIT to the target process. When JVM catches the signal and finds .attach_pid1234, it starts AttachListener thread.

  3. AttachListener thread creates UNIX domain socket /tmp/.java_pid1234 to listen to commands from external tools.

  4. For security reasons when a connection (from jmap) is accepted, JVM verifies that credentials of the socket peer are equal to euid and egid of JVM process. That's why jmap will not work if run by different user (even by root).

  5. jmap connects to the socket, and sends dumpheap command.

  6. This command is read and executed by AttachListener thread of the JVM. All output is sent back to the socket. Since the heap dump is made in-process directly by JVM, the operation is really fast. However, JVM can do this only at safepoints. If a safepoint cannot be reached (e.g. the process is hung, not responding, or a long GC is in progress), jmap will timeout and fail.

Let's summarize the benefits and the drawbacks of Dynamic Attach.

Pros.

  • Heap dump and other operations are run collaboratively by JVM at the maximum speed.
  • You can use any version of jmap or jstack to connect to any other version of JVM.

Cons.

  • The tool should be run by the same user (euid/egid) as the target JVM.
  • Can be used only on live and healthy JVM.
  • Will not work if the target JVM is started with -XX:+DisableAttachMechanism.

jmap -F / jstack -F

When run with -F the tools switch to special mode that features HotSpot Serviceability Agent. In this mode the target process is frozen; the tools read its memory via OS debugging facilities, namely, ptrace on Linux.

  1. jmap -F invokes PTRACE_ATTACH on the target JVM. The target process is unconditionally suspended in response to SIGSTOP signal.

  2. The tool reads JVM memory using PTRACE_PEEKDATA. ptrace can read only one word at a time, so too many calls required to read the large heap of the target process. This is very and very slow.

  3. The tool reconstructs JVM internal structures based on the knowledge of the particular JVM version. Since different versions of JVM have different memory layout, -F mode works only if jmap comes from the same JDK as the target Java process.

  4. The tool creates heap dump itself and then resumes the target process.

Pros.

  • No cooperation from target JVM is required. Can be used even on a hung process.
  • ptrace works whenever OS-level privileges are enough. E.g. root can dump processes of all other users.

Cons.

  • Very slow for large heaps.
  • The tool and the target process should be from the same version of JDK.
  • The safepoint is not guaranteed when the tool attaches in forced mode. Though jmap tries to handle all special cases, sometimes it may happen that target JVM is not in a consistent state.

Note

There is a faster way to take heap dumps in forced mode. First, create a coredump with gcore, then run jmap over the generated core file. See the related question.

小开

Just like ben_wing said, you can run with:

sudo -u jboss-as jmap -dump:file.bin <pid>

(in my case the user is jboss-as, but yours could be jboss or some other.)

But it was not enough, because it asked me for a password ([sudo] password for ec2-user:), although I could run sudo without prompting me for a password with other commands.

I found the solution here, and I just needed to add another sudo first:

sudo sudo -u jboss-as jmap -dump:file.bin <pid>

It works with other commands like jcmd and jinfo too.

小开

If your application is runing as a systemd service.You should open service file that under /usr/lib/systemd/system/ and named by your service name. Then check whether privateTmp attribute is true.

If it is true,you shoud change it to false,then refresh service by command as follow: systemctl daemon-reload systemctl restart [servicename] If you want runing jmap/jcmd before restart, you can make use of the execStop script in the service file. Just put command in it and to execute systemctl stop [service name]

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It's usually solved with -F.

As stated in the message:

The -F option can be used when the target process is not responding

I encountered a situation where the full GC made it impossible to execute the command.


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