Any concept of shared memory in Java

Question

AFAIK, memory in Java is based on heap from which the memory is allotted to objects dynamically and there is no concept of shared memory.

If there is no concept of shared memory, then the communication between Java programs should be time consuming. In C where inter-process communication is quicker via shared memory compared to other modes of communication.

Correct me if I'm wrong. Also what is the quickest way for 2 Java progs to talk to each other.

Answer 1

Since there is no official API to create a shared memory segment, you need to resort to a helper library/DDL and JNI to use shared memory to have two Java processes talk to each other.

In practice, this is rarely an issue since Java supports threads, so you can have two "programs" run in the same Java VM. Those will share the same heap, so communication will be instantaneous. Plus you can't get errors because of problems with the shared memory segment.

Answer 2

Shared memory is sometimes quick. Sometimes its not - it hurts CPU caches and synchronization is often a pain (and should it rely upon mutexes and such, can be a major performance penalty).

Barrelfish is an operating system that demonstrates that IPC using message passing is actually faster than shared memory as the number of cores increases (on conventional X86 architectures as well as the more exotic NUMA NUCA stuff you'd guess it was targeting).

So your assumption that shared memory is fast needs testing for your particular scenario and on your target hardware. Its not a generic sound assumption these days!

Answer 3

There's a couple of comparable technologies I can think of:

1. A few years back there was a technology called JavaSpaces but that never really seemed to take hold, a shame if you ask me.
2. Nowadays there are the distributed cache technologies, things like Coherence and Tangosol.

Unfortunately neither will have the out right speed of shared memory, but they do deal with the issues of concurrent access, etc.

Answer 4

One thing to look at is using memory-mapped files, using Java NIO's FileChannel class or similar (see the map() method). We've used this very successfully to communicate (in our case one-way) between a Java process and a C native one on the same machine.

I'll admit I'm no filesystem expert (luckily we do have one on staff!) but the performance for us is absolutely blazingly fast -- effectively you're treating a section of the page cache as a file and reading + writing to it directly without the overhead of system calls. I'm not sure about the guarantees and coherency -- there are methods in Java to force changes to be written to the file, which implies that they are (sometimes? typically? usually? normally? not sure) written to the actual underlying file (somewhat? very? extremely?) lazily, meaning that some proportion of the time it's basically just a shared memory segment.

In theory, as I understand it, memory-mapped files CAN actually be backed by a shared memory segment (they're just file handles, I think) but I'm not aware of a way to do so in Java without JNI.