Question

The man page says

If the given process is running on a 64-bit VM, you may need to specify the -J-d64 option

and that’s it. Some heap dump howtos do include it, without explanation.

More specifically, I’m trying to figure out if not specifying that option might be corrupting my heap dumps (jhat can’t read them). Experimenting blindly is costly, as the heap is large, the system is a live one, and there are specific points during operation at which I need the dump.

Answer 1

Snippet of jmap‘s help message:

λ > jmap 
Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable <core>
        (to connect to a core file)
    jmap [option] [server_id@]<remote server IP or hostname>
        (to connect to remote debug server)

where <option> is one of:
    <none>               to print same info as Solaris pmap
    -heap                to print java heap summary
    ... stuff ...
    -J<flag>             to pass <flag> directly to the runtime system

So you see the -J flag passes arguments directly to the JVM.

Look at the jvm help message:

λ > java
Usage: java [-options] class [args...]
           (to execute a class)
   or  java [-options] -jar jarfile [args...]
           (to execute a jar file)

where options include:
    -d32          use a 32-bit data model if available
    -d64          use a 64-bit data model if available (implies -server, only for x86_64)

---

So jmap -J-d64 is really telling jmap to start java -d64, using a 64-bit model instead of 32-bit.

If the process you’re running jmap on is not a 64-bit JVM, do not pass the -J-d64 argument.

edit since I have done a lot of jmap/jhat to troubleshoot

When you say corrupted, do you mean jhat actually reports a corrupted heap dump? Or is it because your heap dump is too large to read? jhat will attempt to load the entirety of the dump into memory, so you will need at least as much free memory as the size of the dump. You might need to specify -Xmx to jhat, as well as J-d64 to increase the heap space.

A much better alternative that I use is Eclipse Memory Analyzer Tool, which does selective loading of the heap dump rather than preload everything. It was much better on 6+ GB heap dumps than jhat was for me.