How does the operating system know how much memory my app is using? (And why doesn't it do garbage collection?)

Question

When my task manager (top, ps, taskmgr.exe, or Finder) says that a process is using XXX KB of memory, what exactly is it counting, and how does it get updated?

In terms of memory allocation, does an application written in C++ "appear" different to an operating system from an application that runs as a virtual machine (managed code like .NET or Java)?

And finally, if memory is so transparent - why is garbage collection not a function-of or service-provided-by the operating system?

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As it turns out, what I was really interested in asking is WHY the operating system could not do garbage collection and defrag memory space - which I see as a step above "simply" allocating address space to processes.

These answers help a lot! Thanks!

Answer 1

This is a big topic that I can't hope to adequately answer in a single answer here. I recommend picking up a copy of Windows Internals, it's an invaluable resource. Eric Lippert had a recent blog post that is a good description of how you can view memory allocated by the OS.

Memory that a process is using is basically just address space that is reserved by the operating system that may be backed by physical memory, the page file, or a file. This is the same whether it is a managed application or a native application. When the process exits, the operating system deletes the memory that it had allocated for it - the virtual address space is simply deleted and the page file or physical memory backings are free for other processes to use. This is all the OS really maintains - mappings of address space to some physical resource. The mappings can shift as processes demand more memory or are idle - physical memory contents can be shifted to disk and vice versa by the OS to meet demand.

What a process is using according to those tools can actually mean one of several things - it can be total address space allocated, total memory allocated (page file + physical memory) or memory a process is actually using that is resident in memory. Task Manager has a separate column for each of these possibilities.

The OS can't do garbage collection since it has no insight into what that memory actually contains - it just sees allocated pages of memory, it doesn't see objects which may or may not be referenced.

Whereas the OS handles allocates at the virtual address level, in the process itself there are other memory managers which take these large, page-sized chunks and break them up into something useful for the application to use. Windows will return memory allocated in 64k boundaries, but then the heap manager breaks it up into smaller chunks for use by each individual allocation done by the program via new. In .Net applications, the CLR will hand off new objects off of the garbage collected heap and when that heap reaches its limits, will perform garbage collection.