A simple “Hello World” needs 10G virtual memory on a 64-bit machine vs 1G at 32-bit?

Question

Running a simple Java program on our production machine, I noticed that this program eats up more 10G virt. I know that virtual memory is not that relevant, but at least I would

Answer 1

Virtual memory really doesn't matter to you.

The basic difference between 32-bit and 64-bit is that the address space in 64-bit is incredibly large. If 10 GiB looks like a lot to you, note that .NET on 64-bit can use TiBs of memory like this. Yet on 32-bit, .NET is much more conservative (and so is JVM) - the address space is 4 GiB total - that's not a lot.

But it's irrelevant - it doesn't matter. It's just a thing that greatly simplifies programming, and has no negative effect on the host OS whatsoever. It creates a continuous address space for the VM to use, which means that you don't have to fragment the heap (or worse, the stack, where it's more or less impossible - but those tend to be only a MiB or so) as you get to require more "real" memory. When you finally commit the virtual memory, it becomes slightly more real - at that point, it more or less has to be backed by some data storage - be it the page (swap) file or physical RAM.

The point is, the physical location of the memory isn't necessarily continuous, but that's done outside of your reach, and the mapping is generally very fast. On the other hand, having to, say, index an array, that's actually fragmented over 10 different virtual address memory blocks, that's (completely unnecessary) work.

So there you have it - virtual memory is almost free on 64-bit. The basic approach is "if it's there, use it". You're not limiting the other applications, and it saves you quite a bit of work if you do actually end up using it. But until that point comes, you've only got a reservation. It doesn't translate to any physical memory at all. You don't pay for the friends that might come tonight and sit at your table, but you still have the space for them to sit if they do come - and only when they finally come do you actually get "charged".

See this question for more information about the way Java behaves on different machines and with different versions: What is the default maximum heap size for Sun's JVM from Java SE 6? The maximum heap size also determines the amount of virtual memory reserved, because the heap has to be a continuous address space. If it weren't pre-reserved, it could happen that the heap could not expand to this maximum value, because someone else reserved a region of address space in the place the heap has to expand.