java：如何在 32 位 JVM 中使用超过 4 GB 内存的堆

Question

We have a product that runs currently on a 32 bit 1.6 JRE. We're using Berkeley DB which consumes about 2.5 GB RAM of the 4 GB address space. That leaves us about 750 MB of memory for the JVM address space.

We're currently hitting OutOfMemory issues with the current setup. It would be nice to increase our JVM heap size to 1.5 GB while still retaining Berkeley DB's 2.5 GB space. Is there any way to access more than 4 GB RAM/heap in a 32-bit JVM ? I've considering the following solutions
1) use a JVM which does better GC -- this will give me marginal results -- I could get about 50-100 MB working memory
2) something like memcached or "out of process ehcache " -- this can get me as much as the hardware allows with the overhead of IPC/serialization.

Are there other solutions to increasing an application's addressable memory ?

The solution should work on solaris 10 running sparc.

* UPDATE : Because of using native shared libraries, right now, we're unable to switch to a 64-bit JVM even though the OS is 64-bit *

thank you,

Answer 1

Are there other solutions to increasing an application's addressable memory ?

1. Split single application in several processes with non-shared memory (not a threads; but processes). First process can run DB and second can run other part of the project. You can use RMI or shared memory or sockets to communicate between processes.
2. Lower memory, reserved for OS. Eg on x86-32 PAE allows OS to reserve a smaller than 1 GB of 4 GB virtual address space. (eg "4GB / 4Gb split", supported on Oracle Linux )
3. Put some data to the disk. The disk can be a RAM-disk to better speed; or it can be real disk and OS will speed up access to files using "page cache".

Also, every real SPARC (not ancient SuperSparc or poor-man LION) is 64-bit really. So, it can be easier to switch to 64-bit version of OS. I don't know about Solaris, but in linux there is possible to run 32-bit applications on top of 64-bit OS. And 64bit OS will allow you to run 64-bit JVM.

UPDATE: There are ramdisks in Solaris http://wikis.sun.com/display/BigAdmin/Talking+about+RAM+disks+in+the+Solaris+OS and I think you should try them for storing database (or temporary files of DB). There will be no extra serialization/IPC like in case (1); only extra read/write or mmap/munmap. But Ramdisk is order faster than SSD and 3-4 orders faster than HDD.

Answer 2

32-bit programs are incapable of handling more than 4GB of memory addresses. They just don't have enough bits to represent more memory.

2^32 = 4 294 967 296

Your best bet would be to upgrade to a 64-bit JRE.

Answer 3

I suggest you run your 32-bit shared native libraries in a 32-bit JVM and run everything else in a 64-bit JVM. You can have the 64-bit JVM call the 32-bit JVM to do whatever it does. I assume the bulk of your data/memory requirement can be moved to a 64-bit JVM.