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5 MANAGED HEAP AND GARBAGE COLLECTION
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Listing 5-6 Sample application using pinning (continued)
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public void Run() { SByte[] b1 = null; SByte[] b2 = null; SByte[] b3 = null; ConsoleWriteLine("Press any key to alloc"); ConsoleReadKey(); b1 = new SByte[100]; b2 = new SByte[200]; b3 = new SByte[300]; GCHandle h1 = GCHandleAlloc(b1, GCHandleTypePinned); GCHandle h2 = GCHandleAlloc(b2, GCHandleTypePinned); GCHandle h3 = GCHandleAlloc(b3, GCHandleTypePinned); ConsoleWriteLine("Press any key to GC"); ConsoleReadKey(); GCCollect(); ConsoleWriteLine("Press any key to exit"); ConsoleReadKey(); h1Free(); h2Free(); h3Free(); } } }
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The source code and binary for Listing 5-6 can be found in the following folders:
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Source code: C:\ADND\5\Pinning Binary: C:\ADNDBin\05Pinningexe
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The sample application shown in Listing 5-6 illustrates how to use the GCHandle type to pin objects The Run method declares three arrays of the SByte type and creates GCHandles for each of the allocations specifying that the objects be pinned The application then forces a garbage collection and exits Let s run the application under the debugger and see if we can track the allocated memory and how it gets pinned
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Resume execution of the application until you see the Press any key to GC prompt At this point, we manually break execution and use a command called GCHandles The GCHandles command displays a list of all the handles available in the process:
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0:004> !GCHandles GC Handle Statistics: Strong Handles: 15 Pinned Handles: 7 Async Pinned Handles: 0 Ref Count Handles: 0 Weak Long Handles: 0 Weak Short Handles: 1 Other Handles: 0 Statistics: MT Count TotalSize 790fd0f0 1 12 790feba4 1 28 790fcc48 2 48 790fe17c 1 72 790fe0e0 1 72 790fe044 1 72 790fed00 1 100 790fe704 2 112 79100a18 4 144 790fe284 2 144 7912ee44 3 636 7912d8f8 4 8736 Total 23 objects
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Class Name SystemObject SystemSharedStatics SystemReflectionAssembly SystemExecutionEngineException SystemStackOverflowException SystemOutOfMemoryException SystemAppDomain SystemThreadingThread SystemSecurityPermissionSet SystemThreadingThreadAbortException SystemSByte[] SystemObject[]
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5 MANAGED HEAP AND GARBAGE COLLECTION
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The GCHandles command walks the handle tables and looks for all types of different handles (strong, weak, pinned, etc) and displays a summary of the results as well as a statistical section with detailed information on each type found In the preceding output, we can see that we have 15 strong handles, 7 pinned handles, and 1 weak short handle In addition, in the Statistics section, we can see the three SByte arrays that we allocated and pinned The GCHandles command provides a good overview of the handle activity in any given process, but if further information is required, such as the type of handle for each of the types listed in the Statistics section, we have to use an additional command called objsize One of the functions of the objsize command is to output the size of the object passed in as an argument If no arguments are specified, it scans all the referenced objects in the process and outputs the size as well as other useful information:
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0:004> !objsize Scan Thread 0 OSTHread 2558 ESP:2fed54: sizeof(01d9599c) = 20 ( 0x14) bytes (MicrosoftWin32SafeHandlesSafeFileHandle) ESP:2fee18: sizeof(01d96d9c) = 312 ( 0x138) bytes (SystemSByte[]) ESP:2fee20: sizeof(01d96c58) = 112 ( 0x70) bytes (SystemSByte[]) ESP:2fee24: sizeof(01d96cc8) = 212 ( 0xd4) bytes (SystemSByte[]) ESP:2fee30: sizeof(01d958b4) = 12 ( 0xc) bytes (AdvancedNETDebugging5Pinning) Scan Thread 2 OSTHread 2c80 DOMAIN(004DFD10):HANDLE(Strong):1c119c: sizeof(01d958a4) = 16 ( 0x10) bytes (SystemObject[]) DOMAIN(004DFD10):HANDLE(WeakSh):1c12fc: sizeof(01d91de8) = 56 ( 0x38) bytes (SystemThreadingThread) DOMAIN(004DFD10):HANDLE(Pinned):1c13e4: sizeof(01d96d9c) = 312 ( 0x138) bytes (SystemSByte[]) DOMAIN(004DFD10):HANDLE(Pinned):1c13e8: sizeof(01d96cc8) = 212 ( 0xd4) bytes (SystemSByte[]) DOMAIN(004DFD10):HANDLE(Pinned):1c13ec: sizeof(01d96c58) = 112 ( 0x70) bytes (SystemSByte[]) DOMAIN(004DFD10):HANDLE(Pinned):1c13f0: sizeof(02d93030) = 708 ( 0x2c4) bytes (SystemObject[]) DOMAIN(004DFD10):HANDLE(Pinned):1c13f4: sizeof(02d92020) = 4276 ( 0x10b4) bytes (SystemObject[]) DOMAIN(004DFD10):HANDLE(Pinned):1c13f8: sizeof(01d9118c) = 12 ( 0xc) bytes (SystemObject) DOMAIN(004DFD10):HANDLE(Pinned):1c13fc: sizeof(02d91010) = 19332 ( 0x4b84) bytes (SystemObject[])
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The output has been abbreviated, but clearly shows that our SByte arrays have been pinned as shown by HANDLE(Pinned) Although the notion of pinning objects solves the problem of movable objects during native code invocations, it does present a problem to the garbage collector; the problem is that of fragmentation (one of the problems that compaction is meant to solve) If there are a lot of interleaved pinned objects on the managed heap, situations may occur where there isn t enough contiguous free space available Figure 5-11 shows a hypothetical example of a fragmented managed heap due to excessive pinning In the layout illustrated in Figure 5-11, we can see that we have several free smaller blocks intertwined with live objects (Obj A through D) If a garbage collection
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