VerifyHeap and GC Interference in Visual Studio .NET

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VerifyHeap and GC Interference
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We have seen how the VerifyHeap command can make troubleshooting managed heap corruptions more efficient by walking the heap and reporting inconsistencies that can be a result of a heap corruption There are times, however, when VerifyHeap can yield results that may not be as a result of a heap corruption An example of that is if the CLR is in the middle of doing a garbage collection During a garbage collection, the GC may end up compacting the heap, which involves moving objects around For example, if a move was currently in progress, the VerifyHeap command may very well fail or give inaccurate information due to the heap being reorganized One of the built-in diagnostic aids that the garbage collector includes is the capability to perform heap verification before and after garbage collection occurs To enable these diagnostics, set the environment variable COMPLUS_HeapVerify=1
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Debugging Managed Heap Corruptions
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The sample application we used to demonstrate how the managed heap can become corrupted was based on using the interoperability services to invoke native code Depending on how the heap is corrupted by the native code, as well as the timing of garbage collections, there may not be any signs of a heap corruption being present until much later after the native code has already done the damage, making it difficult to backtrack to the source of the problem To aid in this troubleshooting process, an MDA was added called the gcUnmanagedToManaged MDA Essentially, the MDA aims at reducing the time gap between when the corruption actually occurs in native code and when the next GC occurs The way this is accomplished is by forcing a garbage collection when the interoperability call transitions back from unmanaged to managed code, thereby pinpointing the problem much earlier in the process Let s enable the MDA (please see 1, Introduction to the Tools on how to enable MDAs) and rerun our sample application under the debugger to see if we can trap the heap corruption earlier:
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Press any key to invoke native method ModLoad: 71190000 711ab000 C:\ADNDBin\05Nativedll ModLoad: 63f70000 64093000 C:\Windows\WinSxS\x86_microsoftvc90 debugcrt_1fc8b3b9a1e18e3b_90210228_none_96748342450f6aa2\MSVCR90Ddll (19d8258c): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling This exception may be expected and handled eax=3b3a3938 ebx=02d81010 ecx=00960184 edx=01d8598c esi=00020000 edi=00001000 eip=79f66846 esp=0025ec54 ebp=0025ec74 iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010202 mscorwks!WKS::gc_heap::mark_object_simple+0x16c: 79f66846 0fb708 movzx ecx,word ptr [eax] ds:0023:3b3a3938= 0:000> k ChildEBP RetAddr 0025ec74 79f66932 mscorwks!WKS::gc_heap::mark_object_simple+0x16c 0025ec88 79fbc552 mscorwks!WKS::GCHeap::Promote+0x8d 0025eca0 79fbc3c9 mscorwks!PinObject+0x10 0025ecc4 79fc37b9 mscorwks!ScanConsecutiveHandlesWithoutUserData+0x26 0025ece4 79fba942 mscorwks!BlockScanBlocksWithoutUserData+0x26 0025ed08 79fba917 mscorwks!SegmentScanByTypeMap+0x55 0025ed60 79fba807 mscorwks!TableScanHandles+0x65 0025edc8 79fbb9a2 mscorwks!HndScanHandlesForGC+0x10d 0025ee0c 79fbaaf8 mscorwks!Ref_TracePinningRoots+0x6c 0025ee30 79f669f6 mscorwks!CNameSpace::GcScanHandles+0x60 0025ee70 79f65d57 mscorwks!WKS::gc_heap::mark_phase+0xae
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Managed Heap and Garbage Collection
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0025ee94 79f6614c mscorwks!WKS::gc_heap::gc1+0x62 0025eea8 79f65f5d mscorwks!WKS::gc_heap::garbage_collect+0x261 0025eed4 79f6dfa1 mscorwks!WKS::GCHeap::GarbageCollectGeneration+0x1a9 0025eee4 79f6df4b mscorwks!WKS::GCHeap::GarbageCollectTry+0x2d 0025ef04 7a0aea3d mscorwks!WKS::GCHeap::GarbageCollect+0x67 0025ef8c 7a12addd mscorwks!MdaGcUnmanagedToManaged::TriggerGC+0xa7 0025f020 79e7c74b mscorwks!FireMdaGcUnmanagedToManaged+0x3b 0025f030 79e7c6cc mscorwks!CallDescrWorker+0x33 0025f0b0 79e7c8e1 mscorwks!CallDescrWorkerWithHandler+0xa3 0:000> !ClrStack OS Thread Id: 0x258c (0) ESP EIP 0025efdc 79f66846 [NDirectMethodFrameStandalone: 0025efdc] AdvancedNETDebugging5HeapInitBuffer(Byte[], Int32) 0025efec 00a80165 AdvancedNETDebugging5HeapRun() 0025f018 00a800a7 AdvancedNETDebugging5HeapMain(SystemString[]) 0025f240 79e7c74b [GCFrame: 0025f240]
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We can see here that the native stack trace that caused the access violation looks a lot different than our earlier stack trace It now looks like we are hitting the problem during a garbage collection Where in our managed code flow did the garbage collection occur If we look at the managed code stack trace, we can see that we now get the access violation during our call to the native InitBuffer API If you ever suspect that a heap corruption might be taking place due to a native API invocation, enabling the gcUnmanagedtoManaged MDA can save a ton of debugging time
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Debugging Managed Heap Fragmentation
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Earlier in the chapter, we described a phenomenon known as heap fragmentation, in which free and busy blocks are arranged and interleaved on the managed heap in such a way that they can cause problems in applications that surface as OutOfMemory exceptions; in reality, enough memory is free, just not in a contiguous fashion The CLR heap manager utilizes a technique known as compacting and coalescing to reduce the risk of heap fragmentation In this section, we will take a look at an example that can cause heap fragmentation to occur and how we can use the debuggers to identify that a heap fragmentation is in fact occurring and the reasons behind it The example is shown in Listing 5-8
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