Commit Charge < Physical Memory?

Question

If Commit Charge is the total amount of physical memory and virtual memory; how can Commit Charge ever be less than Physical Memory?

For example; I have in System Information in Process Explorer registering 1.8GB for Commit, but 1.9GB for Physical.

Answer 1

From Wikipedia:

"commit charge is a term used in Microsoft Windows operating systems to describe the total amount of pageable virtual address space."

"Total is the amount of pagefile-backed virtual address space in use, i.e., the current commit charge. This is composed of main memory (RAM) and disk (pagefiles)."

I think where you are going astray is that it does not include ALL your memory, only that which composes the virtual address space. You don't say if that 1.9GB physical is Total or available, but there is memory that is not allocated to paging, like for the OS, and other hardware.

Here is my current Process Explorer screenshot (I updated my version since your previous question), and I have limit of 6.2GB of Commit Charge, but 7GB of total memory and swap space. My current readings are similar in nature to yours: 2.4GB of Commit Charge in use, and 3.1GB of total memory.

http://en.wikipedia.org/wiki/Commit_charge

http://en.wikipedia.org/wiki/Virtual_address_space

Answer 2

Easy: Commit charge is NOT the total of physical and virtual memory.

Commit charge is the amount of virtual memory for which the backing store is the pagefile. That doesn't mean all of commit charge is in the pagefile. The "backing store" is where portions of it would be put if it can't all be kept in RAM.

There is also virtual memory with other backing stores (mapped files).

Commit limit is the sum of current pagefile size and RAM (minus a bit to allow for nonpageable stuff).

Do not think of the pagefile as "virtual memory," despite what the Windows dialog for configuring the pagefile says. Doing so will make it tough to understand this stuff. The pagefile is just the pagefile.