64 bit - Memory limit to a 32-bit process running on a 64-bit Linux OS

Question

How much virtual memory can a 32-bit process have on 64-bit Linux (i.e. how much memory can I allocate and use with malloc() before I start getting a NULL pointer)?

I tried it on my 32-bit Linux and reached about 3 GB limit. Will I be able to get more on 64-bit Linux?

Answer 1

In the standard 32-bit x86 smp kernel, each process can use 3GB of the 4GB address space and 1GB is used by the kernel (shared in the address space of each process).

With the 4G/4G split "hugemem" 32-bit x86 kernel, each process can use (almost) the entire 4GB of address space and the kernel has a separate 4GB of address space. This kernel was supported by Red Hat in RHEL 3 and 4, but they dropped it in RHEL 5 because the patch was not accepted into the mainline kernel and most people use 64-bit kernels now anyway.

With the 64-bit x86_64 kernel, a 32-bit process can use the entire 4GB address space, except for a couple pages (8KB) at the end of the 4GB address space which are managed by the kernel. The kernel itself uses a part of the address space that is beyond the 4GB accessible to 32-bit code, so it does not reduce the user address space. A 64-bit process can use much more address space (128TB in RHEL 6).

Note that some of the address space will be used by the program code, libraries, and stack space, so you won't be able to malloc() your entire address space. The size of these things varies by program. Take a look at /proc/<pid>/maps to see how the address space is being used in your process; the amount you can malloc() will be limited by the largest unused address range.