PAE (Physical Address Extension) was introduced in CPUs back in 1994. This allows a 32-bit processor to access 64 GB of memory instead of 4 GB. Linux kernels offer support for this starting with 2.3.23. Assume I am booting one of these kernels, and want to write an application in C that will access more than 3 GB of memory (why 3 GB? See this).
How would I go about accessing more than 3 GB of memory? Certainly, I could fork off multiple processes; each one would get access to 3 GB, and could communicate with each other. But that's not a realistic solution for most use cases. What other options are available?
Obviously, the best solution in most cases would be to simply boot in 64-bit mode, but my question is strictly about how to make use of physical memory above 4 GB in an application running on a PAE-enabled 32-bit kernel.
You don't, directly -- as long as you're running on 32-bit, each process will be subject to the VM split that the kernel was built with (2GB, 3GB, or if you have a patched kernel with the 4GB/4GB split, 4GB).
One of the simplest ways to have a process work with more data and still keep it in RAM is to create a
shmfs and then put your data in files on that fs, accessing them with the ordinary seek/read/write primitives, or mapping them into memory one at a time with
mmap (which is basically equivalent to doing your own paging). But whatever you do it's going to take more work than using the first 3GB.