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The Black Box of .NET

Tuesday, April 17, 2012

How to determine which garbage collector is running

You can determine which version of GC you're running via 2 methods:
  1. calling the System.Runtime.GCSettings.IsServerGC property
  2. attaching to the process using WinDbg and checking how many GC threads you have using the command "!sos.threads" without the quotes and (according to the below criteria)...
If you are running a Console app, WinForm app or a Windows Service, you will get the Workstation GC. Just because you are running on a Server OS doesn't mean that you will get the Server version of GC.
  • If your app is non-hosted on a multi-proc machine, you will get the Workstation GC - Concurrent by default.
  • If your app is hosted on a multi-proc machine, you will get the ServerGC by default.
The following apply to any given .NET Managed Process:

Workstation GC

  • Uni-processor machine
  • Always suspends threads
  • 1 Ephemeral GC Heap (SOH), 1 LOH GC Heap
  • Runs on thread that triggered GC
  • Thread priority is the same as the thread that triggered GC

Workstation GC - Concurrent

  • Only runs concurrent in Gen2/LOH (full collection)
  • Mutually exclusive with Server Mode
  • Slightly larger working set
  • GC Thread expires if not in use after a while
  • 1 Ephemeral GC Heap (SOH), 1 LOH GC Heap
  • Has a dedicated GC Thread
  • Thread priority is Normal

Server GC

  • Larger segment sizes
  • Faster than Workstation GC
  • Always suspends threads
  • 1 Ephemeral GC Heap (SOH) for each logical processor (this includes hyperthreaded), 1 LOH GC Heap for each logical processor (this includes hyperthreaded)
  • Runs on thread that triggered GC
  • Thread priority is THREAD_PRIORITY_HIGHEST
There is only 1 Finalizer thread per managed process regardless of GC Mode. Even during a concurrent GC, managed threads are suspended (blocked) twice to do some phases of the GC.

A seldom known fact is that even if you try to set the Server mode of GC, you might not be running in Server GC; the GC ultimately determines which mode will be optimal for your app and WILL override your settings if it determines your ServerGC setting will negatively impact your application. Also, any hosted CLR app will have any manual GC settings overridden.

In CLR 4.0, things change just a little bit

  • Concurrent GC is now Background GC
  • Background GC only applies to Workstation GC
  • Old (Concurrent GC):
    • During a Full GC Allowed allocations up to end of ephemeral segment size
    • Otherwise, suspends all other threads
  • New (Background GC):
    • Allows for ephemeral GC’s simultaneously with Background GC if necessary
    • Performance is much faster
  • Server GC always blocks threads for collection of any generation

In CLR 4.5, things change just a little bit...again

  • Background Server GC
    • Server GC no longer blocks. Instead, it uses dedicated background GC threads that can run concurrently with user code - see MSDN: Background Server GC
Thus, in .NET 4.5+, all applications now have background GC available to them, regardless of which GC they use.

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