Can a 32-bit OS machine use up all 8GB RAM + 20GB page file?

Solution 1:

What I understand about 32-bit OS is, the address is expressed in 32 bits, so at most the OS could use 2^32 = 4GB memory space

The most that the process can address is 4GB. You are potentially confusing memory with address space. A process can have more memory than address space. That is perfectly legal and quite common in video processing and other memory intensive applications. A process can be allocated dozens of GB of memory and swap it into and out of the address space at will. Only 2 GB can go into the user address space at a time.

If you have a four-car garage at your house, you can still own fifty cars. You just can't keep them all in your garage. You have to have auxiliary storage somewhere else to store at least 46 of them; which cars you keep in your garage and which ones you keep in the parking lot down the street is up to you.

Does this mean any 32-bit OS, be it Windows or unix, if the machine has RAM + page file on hard disk more than 4GB, for example 8GB RAM and 20GB page file, there will never be "memory used up"?

Absolutely it does not mean that. A single process could use more memory than that! Again the amount of memory a process uses is almost completely unrelated to the amount of virtual address space a process uses. Just like the number of cars you keep in your garage is completely unrelated to the number of cars you own.

Moreover, two processes can share non-private memory pages. If twenty processes all load the same DLL, the processes all share the memory pages for that code. They don't share virtual memory address space, they share memory.

My point, in case it is not clear, is that you should stop thinking of memory and address space as the same thing, because they're not the same thing at all.

if this 32-bit OS machine has 2GB RAM and 2GB page file, increasing the page file size won't help the performance. Is this true?

You have fifty cars and a four-car garage, and a 100 car parking lot down the street. You increase the size of the parking lot to 200 spots. Do any of your cars get faster as a result of you now having 150 extra parking spaces instead of 50 extra parking spaces?

Solution 2:

It is true that the CPU can only address maximum 4Gb of RAM. However, current CPU's use an MMU (Memory management unit) to translate process-specific memory addresses into physical memory addresses.

This MMU is used for all sorts of different tricks, from memory isolation (process A cannot manipulate memory of process B) to memory sharing (process A can access the same physical memory region as process B and can exchange data this way).

Although 32-bit CPU's only support 4Gb of memory per process, it can address up to 64Gb of RAM when using Physical Address Extension. This allows process A to use the first 4Gb of memory, while process B uses the next 4Gb. In total, more than 4Gb of physical memory is used, but the total amount of memory a single process uses is still capped at 4Gb.

PAE is supported on Linux since kernel version 2.3.23 and on some 32-bit flavours of Windows Server, but not on 32-bit Windows XP, Vista or 7.

If your CPU does not support PAE you will be limited to 4GB of physical memory (or less depending on other factors).

Please note your operating system can still evict parts of physical memory to the disk (page file) regardless of the CPU supporting PAE. This ensures you can start multiple processes who use more than 4Gb combined. The only impact PAE has is whether you can keep the 4Gb of process B in physical memory while running process A.