I am playing with an MSDN sample to do memory stress testing (see: http://msdn.microsoft.com/en-us/magazine/cc163613.aspx) and an extension of that tool that specifically eats physical memory (see http://www.donationcoder.com/Forums/bb/index.php?topic=14895.0;prev_next=next). I am obviously confused though on the differences between Virtual and Physical Memory. I thought each process has 2 GB of virtual memory (although I also read 1.5 GB because of “overhead”. My understanding was that some/all/none of this virtual memory could be physical memory, and the amount of physical memory used by a process could change over time (memory could be swapped out to disc, etc.)I further thought that, in general, when you allocate memory, the operating system could use physical memory or virtual memory. From this, I conclude that dwAvailVirtual should always be equal to or greater than dwAvailPhys in the call GlobalMemoryStatus. However, I often (always?) see the opposite. What am I missing.
I apologize in advance if my question is not well formed. I’m still trying to get my head around the whole memory management system in Windows. Tutorials/Explanations/Book recs are most welcome!
Andrew
The amount of virtual memory is limited by size of the address space – which is 4GB per process on a 32-bit system. And you have to subtract from this the size of regions reserved for system use and the amount of VM used already by your process (including all the libraries mapped to its address space).
On the other hand, the total amount of physical memory may be higher than the amount of virtual memory space the system has left free for your process to use (and these days it often is).
This means that if you have more than ~2GB or RAM, you can’t use all your physical memory in one process (since there’s not enough virtual memory space to map it to), but it can be used by many processes. Note that this limitation is removed in a 64-bit system.