How does virtual memory work?

[WillMak]

I am very confused right now. Activity monitor says that I am using 15.55 GB of Virtual memory...but I only have 11 gigs left on my hard drive?

 

[poppe]

I am very confused right now. Activity monitor says that I am using 15.55 GB of Virtual memory...but I only have 11 gigs left on my hard drive?

My Dell always Said... Windows doesn't have enough and needs to make more... Like it notifies me... why can't it just make more...

What ever it is in the first place...

 

[gekko513]

A possible explanation is that several of the processes have memory pages that are equal and "shared" until one of the processes writes to those pages. I found support for that explanation on Apple's developer pages:

Copy-on-write allows multiple blocks of code (including different processes) to share a page as long as none write to that page.

Most applications link to and use the same frameworks from Apple. It would make sense if the memory for these frameworks weren't physically duplicated, but since the memory is mapped from all processes, it may still show in each ones virtual memory usage.

I don't know if this is the case, though. It's just a theory of mine.

 

[Mitthrawnuruodo]

You need to go easy on the Widgets, there...

The "VM size" refers to virtual memory, a system of putting information in RAM or caching it to your hard disk as needed. Thus Mac OS X can "virtually" use more memory than the amount of RAM you have. The hard disk is much slower than RAM, so the virtual memory system automatically distributes information between disk space and RAM for efficient performance. "Page ins/outs" refers to the number of times Mac OS X has moved information between RAM and disk space.

Note: Unlike most UNIX-based operating systems, Mac OS X does not use a preallocated swap partition for virtual memory. Instead, it uses all of the available space on the machine’s boot partition.

In my experience, whenever the VM size exceeds the free space on your boot drive your Mac slows down, because then the system needs to search for space to page into... so you need to downsize your VM size and/or free up some hard drive space...