How to measure memory usage

>> Is the Mem Usage the total memory used by the process?

That should be the total amount of memory that the process is currently using, yes.


>> What is VM Size column?

That would be the part of the process's memory that has been paged to the disk (to the page file).

Well, I know that personally, I would not trust the Task Manager for something as critical as memory leak verification.  Reliable, probably.  How to interpret what you see?  That is more difficult.

If you have overloaded new and delete, and are doing your own memory tallying, then I wold trust that more than I would trust the task manager.  As you said, your application is run in a special way.  How can you then use the task manager?  If you suspect a memory leak from looking at the task manager, it may not even be in your code, it may be in the libraries you use.

Why don't you use some form of statistics in your own code, for the new and delete, and then your code can report any outstanding allocated memory at exit.  This would be much more reliable than the task manager.  Doing that, and also using some developer tools, would be a lot more reliable and efficient than using the task manager.

The simplest way to check if your code is leaking without resorting to memory profiling is just to enable the CRT (C Runtime) memory debug tools. Then, when you run your application you'll get a report of what, if any, memory leaks were detected. You can then set break-points on the allocations to see where the leak starts from.

You can use _CrtSetDbgFlag to enable CRT heap allocation debugging. This should be at the very start of your program.
E.g. _CrtSetDbgFlag ( _CrtSetDbgFlag( _CRTDBG_REPORT_FLAG ) | _CRTDBG_LEAK_CHECK_DF );
http://msdn2.microsoft.com/en-us/library/974tc9t1(VS.80).aspx

You can use _CrtDumpMemoryLeaks to generate an error report if the application failed to free all the memory it allocated. This should be at the very end of your program.
http://msdn2.microsoft.com/en-us/library/d41t22sb(VS.80).aspx

Use can use _CrtSetBreakAlloc or _crtBreakAlloc to set break points where specific heap is allocated (as reported by CrtDumpMemoryLeaks) so that you can see where the problem starts
http://support.microsoft.com/kb/151585

Memory leak detection and isolation: http://msdn2.microsoft.com/en-us/library/x98tx3cf(VS.80).aspx

The memory usage shown my Task Manager is not accurate (I had a link to a great article but right now I can't find it but if I do I'll be sure to post it for you). If you just want to monitor memory usage then consider using Perfmon, which is a tool that comes with Windows for monitor performance, including memory usage, of a process. You probably want to watch private bytes.
http://technet.microsoft.com/en-us/library/bb490957.aspx
http://msdn.microsoft.com/en-us/library/aa645516.aspx

Alternatively, the excellent Process Explorer can be used to perform quick memory usage checked.
http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx