Reasons for seeing high" % Time in GC" in Perf Mon

Question

While monitoring our application in Perf Mon I noticed that the % of Time In GC is anywhere from 20 - 60% while our application is performing a long running process (varies between 30 seconds to 1.5 minutes). This seems a bit excessive to me. This raises two important questions.

1. Am I correct that this excessive?
2. Routes I can take to figure out why so much GC is happening?

Answer 1

Yes, this does sound excessive. Reducing the amount of GC would probably be the single best step you could take to reducing the runtime of your application (if that is your goal).

A high "% time in GC" is typically caused by allocating and then throwing away thousands or millions of objects. A good way to find out what's going on is to use a memory profiler tool.

Microsoft provides the free CLR Profiler. This will show you every allocation, but will make your app run 10-60 times slower. You may need to run it on less input data so that it can finish analyzing in a reasonable amount of time.

A great commercial tool is SciTech's .NET Memory Profiler. This imposes much less runtime overhead, and there is a free trial available. By taking multiple snapshots while your process is running, you can find out what type of objects are being frequently allocated (and then destroyed).

Once you've identified the source of the allocations, you then need to examine the code and figure out how those allocations can be reduced. While there are no one-size-fits-all answers, some things I've encountered in the past include:

• String.Split can create hundreds of small short-lived strings. If you're doing a lot of string manipulation, it can help to process the string by walking it character-by-character.
• Creating arrays or lists of thousands of small classes (say, under 24 bytes in size) can be expensive; if those classes can be treated as value types, it can (sometimes) greatly improve things to change them to structs.
• Creating thousands of small arrays can increase memory usage a lot (because each array has a small amount of overhead); sometimes these can be replaced with one large array and indexes into a sub-section of it.
• Having a lot of finalizable objects (particularly if they're not being disposed) can put a lot of pressure on the garbage collector; ensure that you're correctly disposing all IDisposable objects, and note that your own types should (almost) never have finalizers.
• Microsoft has an article with Garbage Collection Guidelines for improving performance.

Answer 2

Another reason could be lots of gen-1 or gen-2 collections, each of which takes MUCH more time and is caused by hanging on to objects a longer time.

I've seen this happen in web apps when buggy objects hang onto actual page objects - forcing the page to live as long as the other objects referring to them.

Breaking the link between objects and pages (in this case) caused GC to drop to very low values. Our site now has 100+ hits/second and GC time is typically 1% or less.