C# memory leak?? Gen 0 and 1 increasing constantly in perfmon - What does it mean?

Question

I am using C# 2.0 for a multi-threaded application that receives atleast thousand callbacks per second from an unmanaged dll and periodically send messages out of socket. GUI remains on main thread.

My application mostly creates object at the startup and periodically during the execution for a short lived period.

The problem I am experiencing is periodic latency spike (measured by time stamping a function at start and end) which I suppose happen when GC run.

I ran perfmon and here are my observations...

Gen0 Heap Size is flat with a spike every few seconds with periodic spike.

Gen1 Heap Size is always on the roll. Up and down

Gen2 Heap Size follows a cycle. It keep increasing till it becomes flat for a while and then drops.

Gen 0 and 1 Collections are always increasing in a range of 1 to 5 units. Gen 2 collections is constant.

Answer 1

I remember reading a blog post about memory performance in .NET (specifically, XNA on the XBox 360) a while ago (unfortunately I can't find said link anymore).

The nutshell of achieving low latency memory performance was to make sure you never run gen 2 GC's at a performance critical time (although it is OK to run them when latency is not important; there are a bunch of notification callback functions on the GC class in more recent versions of the framework which may help with this). There are two ways to make sure this happens:

1. Don't allocate anything that escapes to gen 2. It's alarmingly easy for objects to escape to gen 2 when you don't realise it, so this often translates into: don't allocate anything in performance critical code. Because no objects escape to gen 2, the GC doesn't need to collect.
2. Allocate everything you need upfront and use object pooling. Your gen 2 heap will be big but because nothing is being added to it, the GC doesn't need to collect it.

It may be helpful to look into some XNA or Silverlight related performance articles because games and resource constrained devices are often very latency sensitive. (Note that you have it easy because the XBox 360 and, until Mango, Windows Phone only had a single generation GC (mark-and-sweep collector)).

Answer 2

So as @Jalf says, there's no evidence of a memory "leak" as such: what you discuss is closer to latency caused by garbage collection.

Others may disagree but I'd suggest anything above a few hundred callbacks per second is going to stretch a general purpose language like C#, especially one that manages memory on your behalf. So you're going to have to get smart with your memory allocation and give the runtime some help.

First, get a real profiler. Perfmon has its uses but even the profiler in later versions of Visual Studio can give you much more information. I like the SciTech profiler best (http://memprofiler.com/); there are others including a well respected one from RedGate reviewed here: http://devlicio.us/blogs/scott_seely/archive/2009/08/23/review-of-ants-memory-profiler.aspx

Once you know your baseline, aim to eliminate gen2 collections. They'll be the really slow ones. Work hard in any tight loops to eliminate as much memory allocation as you can -- strings are the usual offenders.

Some useful tips are in an old but still relevant MSDN article here: http://msdn.microsoft.com/en-us/library/ms973837.aspx.

It's also good to read Tess Ferrandez's (outstanding) blog series on debugging ASP.NET applications - book a day out of the office and start here: http://blogs.msdn.com/b/tess/archive/2008/02/04/net-debugging-demos-information-and-setup-instructions.aspx.

Answer 3

I recommend using a memory profiler in order to know if you have a real memory leak or not. There are many available and they will allow you to quickly isolate any issue.

The garbage collector is adaptive and will modify how often it runs in response to the way your application is using memory. Just looking at the generation heap sizes is going to tell you very little in terms of isolating the source of any problem. Second quessing how it works is a bad idea.

RedGate Ants Memory Profiler

SciTech .NET Memory Profiler

EQATEC .NET Profiler

CLR Profiler (Free)