What's the difference between reserved and committed memory?

I understand that memory has to be reserved before being committed. And when it's reserved, no other process can use it. However reserved memory does not count against available RAM. But shouldn't it? Because if no one else can use it, then what good is it being "available"?

Or is there some bigger difference?

Solution 1:

In the context of Win32, "reserved" means that the address space is allocated within the process that requested it. This may be used, for example, to reserve space for a large buffer that's not all needed right away, but when it's needed it would need to be contiguous.

Reserving memory does not interact with other processes at all, since each process has its own private address space. So the statement that "when it's reserved, no other process can use it" is meaningless, since processes can't normally allocate memory in the address space of another process anyway.

When the reserved pages are requested to be committed (backing store allocated for them), that operation can potentially fail due to lack of physical memory (or pagefile).

Solution 2:

I like to view Reserved as booking the address space so that no one else can allocate it (but I can't use memory at that address because it is not yet available). And Committed as mapping that address space to the physical memory so that it can be used.

Why would I want to reserve? Why not just get committed memory? There are several reasons I have in mind:

1. Some application needs a specific address range, say from 0x400000 to 0x600000, but does not need the memory for storing anything. It is used to trap memory access. E.g., if some code accesses such area, it will be caught. (Useful for some reason.)

2. Some thread needs to store progressively expanding data. And the data needs to be in one contiguous chunk of memory. It is preferred not to commit large physical memory at one go because it is not needed and would be such a waste. The memory can be utilized by some other threads first. The physical memory is committed only on demand.