Garbage Collector Internals in .NET

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Garbage Collector Internals
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Allocation Request
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Allocate memory and advance current allocation pointer
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Collect objects that are not rooted
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5 MANAGED HEAP AND GARBAGE COLLECTION
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Figure 5-5 High-level overview of generational garbage collection algorithm
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5
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Managed Heap and Garbage Collection
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collecting objects with no roots in generation 1 and promoting objects with roots to generation 2 The process repeats itself for generation 2 If, while promoting to generation 2, the GC cannot collect any objects and the budget for generation 2 is exceeded, the CLR heap manager tries to allocate another segment that will hold generation 2 objects If the creation of a new segment fails, an OutOfMemoryException is thrown The CLR heap manager also releases segments if they are not in use anymore; we will discuss this process in more detail later in the chapter
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WHAT ELSE CAN TRIGGER A GARBAGE COLLECTION In addition to a garbage collection occurring due to the allocation of memory and exceeding the thresholds for generation 0, 1, and 2, respectively, a couple of other scenarios exist that can cause it to happen First, a garbage collection can be forced via the GCCollect and related APIs Secondly, the garbage collector is very cognizant of memory usage in the system as a whole Through careful collaboration with the operating system, the garbage collector can kick start a collection if the system as a whole is found to be under extreme memory pressure
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Let s take a practical look at how an object is collected and promoted Listing 5-2 shows the source code behind the application we will use to illustrate the generational concepts Listing 5-2 Example source code to illustrate generational concepts
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using System; using SystemText; using SystemRuntimeRemoting; namespace AdvancedNETDebugging5 { class Name { private string first; private string last; public string First { get { return first; } } public string Last { get { return last; } } public Name(string f, string l) { first = f; last = l; } }
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Garbage Collector Internals
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class Gen { static void Main(string[] args) { Name n1 = new Name("Mario", "Hewardt"); Name n2 = new Name("Gemma", "Hewardt"); ConsoleWriteLine("Allocated objects"); ConsoleWriteLine("Press any key to invoke GC"); ConsoleReadKey(); n1 = null; GCCollect(); ConsoleWriteLine("Press any key to invoke GC"); ConsoleReadKey(); GCCollect(); ConsoleWriteLine("Press any key to exit"); ConsoleReadKey(); } } }
5 MANAGED HEAP AND GARBAGE COLLECTION
The source code and binary for Listing 5-2 can be found in the following folders:
Source code: C:\ADND\5\Gen Binary: C:\ADNDBin\05Genexe
In Listing 5-2, we have defined a simple type called Name In the Main method, we instantiate two instances of the Name type, both of which end up going to generation 0 as new allocations When the user has been prompted to Press any key to invoke GC, we set the n1 instance to null, which indicates that it can be garbage collected because it no longer has any roots Next, the garbage collection occurs and collects n1 and promotes n2 to generation 1 Finally, the last garbage collection promotes n2 to generation 2 because it is still rooted Let s run the application under the debugger and see how we can verify our theories on how n1 and n2 are collected and promoted When the application is running under the debugger, resume execution until the first Press any key to invoke GC prompt At that point, we need to break execution and find the addresses
5
Managed Heap and Garbage Collection
to the two object instances, which can easily be done via the ClrStack command as shown in the following:
0:000> !ClrStack -a OS Thread Id: 0x1c0c (0) ESP EIP 0028f3b4 77709a94 [NDirectMethodFrameSlim: 0028f3b4] MicrosoftWin32Win32NativeReadConsoleInput(IntPtr, InputRecord ByRef, Int32, Int32 ByRef) 0028f3cc 793e8f28 SystemConsoleReadKey(Boolean) PARAMETERS: intercept = 0x00000000 LOCALS: <no data> 0x0028f3dc = 0x00000001 <no data> <no data> <no data> <no data> <no data> <no data> <no data> <no data> 0028f40c 793e8e33 SystemConsoleReadKey() 0028f410 003000f3 AdvancedNETDebugging5GenMain(SystemString[]) PARAMETERS: args = 0x01c55818 LOCALS: <CLR reg> = 0x01da5938 <CLR reg> = 0x01da5948 0028f65c 79e7c74b [GCFrame: 0028f65c]
The addresses of the two objects on the managed heap are 0x01da5938 and 0x01da5948 How can we figure out which generation objects on the managed heap belong to The answer to that lies in understanding the correlation between managed heap segments and generations As previously discussed, each managed heap consists of one or more segments where the objects reside Furthermore, part of the segment(s) is dedicated to a given generation Figure 5-6 shows an example of a hypothetical managed heap segment In Figure 5-6, the managed heap segment is divided into three generations, each with its own starting address managed by the CLR heap manager Generations 0 and 1 are part of a single segment known as the ephemeral segment where short-lived objects live Because the GC goes under the assumption that most objects are short