When is it acceptable to call GC.Collect?


Question

The general advise is that you should not call GC.Collect from your code, but what are the exceptions to this rule?

I can only think of a few very specific cases where it may make sense to force a garbage collection.

One example that springs to mind is a service, that wakes up at intervals, performs some task, and then sleeps for a long time. In this case, it may be a good idea to force a collect to prevent the soon-to-be-idle process from holding on to more memory than needed.

Are there any other cases where it is acceptable to call GC.Collect?

2
155
1/25/2009 8:04:21 PM

Accepted Answer

If you have good reason to believe that a significant set of objects - particularly those you suspect to be in generations 1 and 2 - are now eligible for garbage collection, and that now would be an appropriate time to collect in terms of the small performance hit.

A good example of this is if you've just closed a large form. You know that all the UI controls can now be garbage collected, and a very short pause as the form is closed probably won't be noticeable to the user.

UPDATE 2.7.2018

As of .NET 4.5 - there is GCLatencyMode.LowLatency and GCLatencyMode.SustainedLowLatency. When entering and leaving either of these modes, it is recommended that you force a full GC with GC.Collect(2, GCCollectionMode.Forced).

As of .NET 4.6 - there is the GC.TryStartNoGCRegion method (used to set the read-only value GCLatencyMode.NoGCRegion). This can itself, perform a full blocking garbage collection in an attempt to free enough memory, but given we are disallowing GC for a period, I would argue it is also a good idea to perform full GC before and after.

Source: Microsoft engineer Ben Watson's: Writing High-Performance .NET Code, 2nd Ed. 2018.

See:

147
7/2/2018 8:13:45 PM

Exper Answer

I use GC.Collect only when writing crude performance/profiler test rigs; i.e. I have two (or more) blocks of code to test - something like:

GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestA(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestB(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
...

So that TestA() and TestB() run with as similar state as possible - i.e. TestB() doesn't get hammered just because TestA left it very close to the tipping point.

A classic example would be a simple console exe (a Main method sort-enough to be posted here for example), that shows the difference between looped string concatenation and StringBuilder.

If I need something precise, then this would be two completely independent tests - but often this is enough if we just want to minimize (or normalize) the GC during the tests to get a rough feel for the behaviour.

During production code? I have yet to use it ;-p

48
3/20/2017 6:01:17 PM

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