Is the maximum memory space an application can use equals to (physical memory + swap space)?

Question

From this post, I know the swap space is correlated to physical memory. So assume the physical memory and the swap space are both 4 GB. Although theoretically, the memory space of the 64-bit application is near to 2^64 (certainly, the kernel will occupy some space), but per my understanding, the actual memory the application can use is only 8 GB.

So my question is: for an application running on Unix/Linux, Is the maximum memory space it can use equals to (physical memory + swap space)?

Answer 1

This is a complicated question.

First of all, the theoretical virtual memory space of 64-bit system is 2^64. But in fact, neither the OS nor the CPU supports so big virtual memory space or physical RAM.

Current x86-64 CPUs (aka AMD64 and Intel's current 64-bit chips) actually use 48-bit address lines (AMD64) and 42-bit address lines (Intel), theoretically allowing 256 terabytes of physical RAM.

And Linux allows 128TB of virtual memory space per process on x86-64, and can theoretically support 64TB of physical RAM.

To your question, in an ideal case, the maximum virtual memory space a Linux process can use is just the Linux limitation of virtual memory space above. Even if your system has run out of all the swap space, leaved only 100MB of free RAM, your process can also make use of the entire memory space.

But your system may have some limitations for the virtual memory space request (malloc, which call brk/sbrk syscall). For example, Linux has a vm.overcommit_memory and vm.overcommit_ratio options to determine whether malloc will refuse in a process. See http://www.win.tue.nl/~aeb/linux/lk/lk-9.html.

However, the virtual memory space is not the real RAM + swap. Considering real RAM + swap, your opinion is right: a process will never use more real RAM + swap than that your system has. But in most cases, there will be a lot of processes exist in your system, so the RAM + swap your process can use is shrinked. If all the physical RAM + swap are going to be exhausted, the OOM killer will choose some process to kill.