Shared memory usage for Java native libraries

[Dino Marin]

I have a large Java application that has to use the services of a (similarly large) 32-bit C++ library. This application and the associated library are memory intensive. And it runs out of memory already :-)

My question is: does it make sense to try and start multiple instances of the application on the same machine and find a way of sharing the load between them?
Why I ask this question: I have heard that all 32-bit libraries share the same memory area - is that true? If the memory area is shared then I don't gain anything by starting multiple processes, the many library instances will just share the same area that was previously used by a single instance. If on the other hand the memory is not shared and the library gets a separate memory area in each process then it's worth the trouble of distributing the load.

Thank you,
Dino

[Jeff Schussele · Accepted solution]

Hi Dino,

In a nutshell, If they share the same libs, then they MUST occupy the same memory area.
If they didn't then you *could* run them in seperate memory windows. Else you're constricted to a 1.75Gb shared area.

See the whitepaper on this in
/usr/share/doc/mem_wndws.txt
And do mans on
getmemwindow
setmemwindow
for detailed explanations on how/why to use memory windows on 32-bit apps.

HTH,
Jeff

[Sridhar Bhaskarla]

Hi Dino,

By default, you will get a maximum of 1.75 GB of shared memory. Even less in case the allocation is fragmented.

You are right. All 32-bit processes share the same area.

However, you can make use of memory windows and create seperate pools of shared memory. If your application does not use IPC_GLOBAL while creating the shared memory window, you will not be able to access the shared objects of other processes that are using a different memory window.

/usr/share/doc/memwindows.txt has more information.

-Sri

[Dino Marin]

I red /usr/share/doc/mem_wndwns.txt and also the man pages for setmemwindow/getmemwindow and the information there is very useful.

Thanks again,
Dino

[Mike Stroyan]

This question and the replies seem unclear on just what memory areas are being discussed.

The normal process data area used by malloc is private to a process.

The java heap used by java new operations is private to a process.

Explicitly created System V shared memory and shared mmap regions are shared between processes.

Shared libraries have both text and data areas. A shared library's text area can be shared between multiple processes. Its data area is always private. Calls to malloc from shared library code use the same private data area that calls from the a.out
use.

Only the shared memory areas use a system-wide address space. The other data areas are private to a process and only compete with other processes for swap space.

So, multiple 32 bit processes can have more data than a single 32 bit process.

[Sridhar Bhaskarla]

Mike?. I believe it was the subject that made us to think about shared memory area.

-Sri

[Mike Stroyan]

While the title contained the magic words "shared memory", the question really seems to be "What memory is shared?".
The correct answer is that most memory is not shared between processes.