One great feature of VisualVM is that it can read heap dump files. Heap dumps are useful to diagnose memory leaks. See this post for more details about memory leaks and how to solve them.
Why VisualVM is slow with heap dump
Another great feature of VisualVM is that you can read a huge heap dump file and VisualVm will consume a minimal amount of memory to do so. For instance, you will be able to read a 8 Gigabytes heap dump file with VisualVM running on a development workstation having only 2 Gigabytes of RAM. In order to achieve that, VisualVM will parse the heap dump file and will create a work file on disk in the default system temp folder (/tmp by default on Linux). In theory that’s great, but in practice, VisualVM becomes painfully slow because it constantly have to do disk I/O’s to process the information.
This behavior is even more frustrating if you happen to have a server with 12 Gigabytes of RAM available for you. A simple solution for that is to create a ramdisk and tell VisualVM to use that ramdisk as the tmp folder.
The solution: use a RAMDisk
First, create the RAMDisk (tmpfs). Here I am on a linux development server and I create a tmp folder in my home. Then I create (mount) the ramdisk in the tmp folder I just created:
$ mkdir /tmp/ramdisk
$ sudo mount -t tmpfs none /tmp/ramdisk
Then I launch VisualVM and I modify the java.io.tmpdir VM arg that tells VisualVM where the system tmp folder is.
$ jvisualvm -J-Djava.io.tmpdir=/tmp/ramdisk
Now VisualVM is much much faster and I can investigate and find the root cause of that memory leak much faster.
Author: Jonathan Demers