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I'm writing a data structure and if I set <gcServer enabled="true" /> in my app.config file, the program adds 500,000 items in 200 milliseconds. If I set <gcServer enabled="false" /> it takes 300 milliseconds. That is, setting this flag to false makes it take 50% longer consistently, as measured by a Stopwatch.

I'm wondering why this is because I am not doing any garbage collecting. I know it is done automatically sometimes but after profiling with CLRProfiler, I can confirm 0 collections are occurring:

enter image description here

Does anyone know why this is happening? If the garbage collector isn't even running, then why is a server garbage collector so much faster? Here is the code where I am checking the speed differences:

Stopwatch sw = Stopwatch.StartNew();

foreach (string s in items)
{
    dataStructure.Add(s, s + "a");
}

sw.Stop();
valiano
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Ryan Peschel
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  • Just because you don't have any objects collected, does not mean that the GC algorithm is not running & trying to find objects in graph that it could collect. Perhaps this paper will shed some light about the differences; https://msdn.microsoft.com/en-us/library/ee787088(v=vs.110).aspx – Erti-Chris Eelmaa Apr 04 '15 at 22:46
  • Why is it that when I do `GCSettings.LatencyMode = GCLatencyMode.LowLatency;` before starting the stopwatch that it takes 600 milliseconds then? – Ryan Peschel Apr 04 '15 at 22:49
  • Just have to ask; are you running in release mode & without debugger attached? (Ctrl+F5) – Erti-Chris Eelmaa Apr 04 '15 at 22:50
  • Yes I am sure to do Ctrl+F5 every time in release mode. – Ryan Peschel Apr 04 '15 at 22:51

1 Answers1

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The are five things here I need to cover.

First is understanding this flag does not turn the garbage collector on or off, but rather merely determines which mode it uses. Both examples have garbage collection enabled; only the first (slower) example used server garbage collection.

Second is understanding just because no collections were performed, it does not mean the GC never stopped to check whether it needs to run a collection.

Third is this excerpt from the gcServer docs:

For single-processor computers, the default workstation garbage collection should be the fastest option.

Fourth is understanding "single processor" in the above context really does refer to processors, and not to cores.

Put all this together, and the behavior from the question exactly matches the documentation. Move along; nothing to see here.

Fifth, and finally, is understanding the values at play and how server loads differ from desktop loads. The primary importance in a server system is stability. This is true even over performance; performance doesn't matter if the system crashes all the time. Additionally, servers are more likely to run workloads where a process has a long life-time... even months or years at a time without restarting. So a server admin may sacrifice some performance to get a GC that runs a little more often and runs a more complete check, which may include memory compaction to reclaim address space, and thus avoid issues with OutOfMemoryExceptions that can otherwise cause problems in long-lived .Net applications.

Joel Coehoorn
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