How can I tell if I have a memory leak?

Question

I am using ANTS memory profiler and am somewhat baffled at the moment. If I load my site up and hook ANTS up to the process I can see the Private Bytes around 90mb, I then run the same routine a number of time with the following results:

1. 109mb
2. 120mb
3. 125mb
4. 126mb
5. 123mb
6. 126mb

and it basically stays around 126mb for each try after. My understanding is that if I had a memory leak then it would keep going up and not settle but what I don't understand is why it grows slowly until 126mb. Does .NET have an amount of memory it is allowed to reserve and it is just hitting that limit?

Simple question: Memory is growing up to a point then stopping. Is this normal for a .NET app?

EDIT: Just realised that I probably should have posted this at programmers.stackexchange.com - Apologies.

Answer 1

"Memory leak" is when memory you think should not be allocated is allocated.

It is not possible to simply look at amount of consumed memory and say "you have memory leak". I.e. what if your application collects logs in memory for 3 days - in this case memory consumption will grow, but it is not an indication of a leak. On other hand if your application simply prints a line a minute, but memory usage constantly grows it likley is a leak.

Answer 2

In my experience, if I see memory grow and plateau, it's generally from caching. As stated before, a memory leak is simply when something remains in memory that you feel should've been released. Using a profiler is a great way to determine leaks (vs using the task manager) because it generally will ensure a 2nd generation garbage collection has occurred, allowing you to see everything that's still in memory.

When I profile, I generally will execute the commands that I want to test a couple times to ensure all caching has occurred, and then I'll create a before and after snapshot and compare the delta of memory. If you're using a managed language like C#, it's not uncommon to have a delta of +/- 10KB. Repeat that process several times and if your delta is consistently positive, you more than likely have a leak (assuming you're not intentionally allocating more memory).