Inspect pmap -X output of a java process, requires Java11, may not be 100% accurate

JavaPmapInspector.java

/*


Moved here => https://github.com/bric3/java-pmap-inspector

*/

public class JavaPmapInspector {
    public static void main(String[] args) throws IOException {
        System.err.println("Go to:");
        System.err.println("   https://github.com/bric3/java-pmap-inspector");
    }
}

can you please explain why MALLOC_ARENA has the following format? I tested locally -- calling malloc in cpp via JNI, and pmap shows results rw-p for the block I malloc, and the address is continuous.

lowerAddressBlock.permissions.equals("rw-p")
                && higherAddressBlock.permissions.equals("---p")

Hi @hfu5,

This is a a good question. I do have have all the piece, but here's my understanding of the phenomena.

It appears on 64bit Linux systems when the program uses the malloc implementation of the glibc. And glibc happens to manage memory using a well known strategy that is called region based memory management (a region is also called arena).
These region are created by glibc to avoid contention, and this depends on the number of CPU. Calling malloc once or two in the same thread won't stress enough the native allocator, in order to see multiple arenas, the program really need to allocate a bunch of data inferior to the 64 MB threshold, and in a multithreaded fashion.

Homnestly, I only saw those on running jvm processes in production. I couldn't reproduce it locally but I likely didn't tried hard enough, if you succeed I'll be happy to learn how you did it.

I started to write a blog post a while ago on this topic and I need to go back writing it.