R – Do Small Memory Leaks Matter Anymore

A memory leak simply refers to when your program is allocating memory that it then loses track of. The operating system still considers this memory to be in the address space of the program, and thus it will be reused when the program finishes.

All modern operating systems use a mechanism called virtual memory to keep track of a programs memory.

This is where I learned about virtual memory in quite good detail CS3231.

Basically it is where the operating system can put chunks of an applications memory anywhere in physical memory, while maintaining a mapping to where these chunks should point to.

From the applications point of view, it gets full access to memory (4gig on 32-bit OS, something massive on 64-bit), and can keep allocating until it hits the hardware limit, even if physical memory is less than this limit (this requires the OS to store some of the contents of memory on disk, usually in a swap file)

This is facilitated by the hardware on the CPU, a module usually called the MMU (Memory Management Unit), and sometimes there is also a TLB (Translation Lookaside Buffer) to speed up virtual memory operations.

Another page that explains a bit more about Memory Protection which details some more of the inner workings of Virtual Memory.

The short answer to this question is that none of these values are a reliable indicator of how much memory an executable is actually using, and none of them are really appropriate for debugging a memory leak.

Private Bytes refer to the amount of memory that the process executable has asked for - not necessarily the amount it is actually using. They are "private" because they (usually) exclude memory-mapped files (i.e. shared DLLs). But - here's the catch - they don't necessarily exclude memory allocated by those files. There is no way to tell whether a change in private bytes was due to the executable itself, or due to a linked library. Private bytes are also not exclusively physical memory; they can be paged to disk or in the standby page list (i.e. no longer in use, but not paged yet either).

Working Set refers to the total physical memory (RAM) used by the process. However, unlike private bytes, this also includes memory-mapped files and various other resources, so it's an even less accurate measurement than the private bytes. This is the same value that gets reported in Task Manager's "Mem Usage" and has been the source of endless amounts of confusion in recent years. Memory in the Working Set is "physical" in the sense that it can be addressed without a page fault; however, the standby page list is also still physically in memory but not reported in the Working Set, and this is why you might see the "Mem Usage" suddenly drop when you minimize an application.

Virtual Bytes are the total virtual address space occupied by the entire process. This is like the working set, in the sense that it includes memory-mapped files (shared DLLs), but it also includes data in the standby list and data that has already been paged out and is sitting in a pagefile on disk somewhere. The total virtual bytes used by every process on a system under heavy load will add up to significantly more memory than the machine actually has.

So the relationships are:

• Private Bytes are what your app has actually allocated, but include pagefile usage;
• Working Set is the non-paged Private Bytes plus memory-mapped files;
• Virtual Bytes are the Working Set plus paged Private Bytes and standby list.

There's another problem here; just as shared libraries can allocate memory inside your application module, leading to potential false positives reported in your app's Private Bytes, your application may also end up allocating memory inside the shared modules, leading to false negatives. That means it's actually possible for your application to have a memory leak that never manifests itself in the Private Bytes at all. Unlikely, but possible.

Private Bytes are a reasonable approximation of the amount of memory your executable is using and can be used to help narrow down a list of potential candidates for a memory leak; if you see the number growing and growing constantly and endlessly, you would want to check that process for a leak. This cannot, however, prove that there is or is not a leak.

One of the most effective tools for detecting/correcting memory leaks in Windows is actually Visual Studio (link goes to page on using VS for memory leaks, not the product page). Rational Purify is another possibility. Microsoft also has a more general best practices document on this subject. There are more tools listed in this previous question.

I hope this clears a few things up! Tracking down memory leaks is one of the most difficult things to do in debugging. Good luck.