Does garbage collection happen at the process leve

问题:

FullGC normaly pauses all Threads while running. Having two AppDomains, each running several threads. When GC runs, will all threads be paused, or only those of either one AppDomain?

回答1:

Hard to answer, best thing to do is just test it:

using System;
using System.Reflection;

public class Program : MarshalByRefObject {
    static void Main(string[] args) {
        var dummy1 = new object();
        var dom = AppDomain.CreateDomain("test");
        var obj = (Program)dom.CreateInstanceAndUnwrap(Assembly.GetExecutingAssembly().FullName, typeof(Program).FullName);
        obj.Test();
        Console.WriteLine("Primary appdomain, collection count = {0}, gen = {1}",
            GC.CollectionCount(0), GC.GetGeneration(dummy1));
        Console.ReadKey();

    }
    public void Test() {
        var dummy2 = new object();
        for (int test = 0; test < 3; ++test) {
            GC.Collect();
            GC.WaitForPendingFinalizers();
        } 
        Console.WriteLine("In appdomain '{0}', collection count = {1}, gen = {2}",
            AppDomain.CurrentDomain.FriendlyName, GC.CollectionCount(0),
            GC.GetGeneration(dummy2));
    }
}

Output:

In appdomain 'test', collection count = 3, gen = 2
Primary appdomain, collection count = 3, gen = 2

Good evidence that a GC affects all AppDomains on the default CLR host. This surprised me.

回答2:

From this thread here: Is the garbage collector in .net system-wide or application-wide?, it occurs on the process level. All threads in that process will be paused but not across multiple processes.

One or multiple app domains can exist within a process but app domains are not shared between processes. Per: http://blogs.msdn.com/b/tess/archive/2008/08/19/questions-on-application-domains-application-pools-and-unhandled-exceptions.aspx, "all appdomains in the process share the same GC." Accordingly, GC should affect all app domains when a GC is triggered.

However, a CPU performance hit can occur with too many processes spending time doing GC which can negatively affect the performance of other processes not involved in a GC.

This link also explains the fundamentals of GC too:

http://msdn.microsoft.com/en-us/library/ee787088.aspx