Running jmap getting Unable to open socket file
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.
Before connecting to Java process 1234,
jmap
creates a file.attach_pid1234
at the working directory of the target process or at/tmp
.Then
jmap
sendsSIGQUIT
to the target process. When JVM catches the signal and finds.attach_pid1234
, it startsAttachListener
thread.AttachListener
thread creates UNIX domain socket/tmp/.java_pid1234
to listen to commands from external tools.For security reasons when a connection (from
jmap
) is accepted, JVM verifies that credentials of the socket peer are equal toeuid
andegid
of JVM process. That's whyjmap
will not work if run by different user (even by root).jmap
connects to the socket, and sendsdumpheap
command.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
orjstack
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.
jmap -F
invokesPTRACE_ATTACH
on the target JVM. The target process is unconditionally suspended in response toSIGSTOP
signal.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.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 ifjmap
comes from the same JDK as the target Java process.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.
Java - AttachNotSupportedException: Unable to open socket file: HotSpot VM not loaded
Common reasons for this problem:
- Attach socket
/tmp/.java_pid1234
has been removed (e.g. by a scheduled job that periodically cleans up /tmp). - Target JVM is started with
-XX:+DisableAttachMechanism
option. - Garbage Collection or other long VM operation (e.g. Heap Dump) is in progress.
- JVM cannot reach safepoint within attach timeout. This happens rarely, and the problem is typically intermittent.
jstack: Target process not responding
I got it working by doing two things:
- Changed call to:
sudo -u tomcat6 jstack -J-d64 -m pid
- Replaced OpenJDK with Sun's original sun-6-jdk and sun-6-jre packages
Explanation for part 1: I switched to 64-bit mode, used sudo
and run the command as Tomcat user.
Note: Part 2 might not be necessary. For some users it seems like part 1 is enough. In fact, try to add just the sudo
command first. It might already do the trick.
com.sun.tools.attach.AttachNotSupportedException: Unable to open socket file: target process not responding or HotSpot VM not loaded
Work around for now.
Adding '-XX:+StartAttachListener'
to jvm argument fixed the issue.
A similar issue is discussed here at https://code.google.com/p/jmockit/issues/detail?id=136 and http://mail.openjdk.java.net/pipermail/macosx-port-dev/2013-October/006098.html (which talks about a possible regression in jdk7 build)
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