记一次 .NET 某消防物联网 后台服务 内存泄漏分析

一：背景

1. 讲故事

去年十月份有位朋友从微信找到我，说他的程序内存要炸掉了。。。截图如下：

时间有点久，图片都被清理了，不过有点讽刺的是，自己的程序本身就是做监控的，结果自己出了问题，太尴尬了🤣🤣🤣

二：Windbg 分析

1. 托管还是非托管

这个是甄别内存问题的第一步，通过 !address -summary 和 !eeheap -gc 两个命令基本就可以断定。

0:000> !address -summary                             
Mapping file section regions...
Mapping module regions...
Mapping PEB regions...
Mapping TEB and stack regions...
Mapping heap regions...
Mapping page heap regions...
Mapping other regions...
Mapping stack trace database regions...
Mapping activation context regions...

--- Usage Summary ---------------- RgnCount ----------- Total Size -------- %ofBusy %ofTotal
Free                                    237     7ffc`1e222000 ( 127.985 TB)           99.99%
<unknown>                               594        3`b9b20000 (  14.901 GB)  95.96%    0.01%
Heap                                    370        0`12a2a000 ( 298.164 MB)   1.88%    0.00%
Image                                  1248        0`0ee5a000 ( 238.352 MB)   1.50%    0.00%
Stack                                   315        0`06780000 ( 103.500 MB)   0.65%    0.00%
Other                                    13        0`001d7000 (   1.840 MB)   0.01%    0.00%
TEB                                     105        0`000d2000 ( 840.000 kB)   0.01%    0.00%
PEB                                       1        0`00001000 (   4.000 kB)   0.00%    0.00%

--- Type Summary (for busy) ------ RgnCount ----------- Total Size -------- %ofBusy %ofTotal
MEM_PRIVATE                            1178        3`ce03d000 (  15.219 GB)  98.00%    0.01%
MEM_IMAGE                              1409        0`0f6fd000 ( 246.988 MB)   1.55%    0.00%
MEM_MAPPED                               59        0`04694000 (  70.578 MB)   0.44%    0.00%

--- State Summary ---------------- RgnCount ----------- Total Size -------- %ofBusy %ofTotal
MEM_FREE                                237     7ffc`1e222000 ( 127.985 TB)           99.99%
MEM_COMMIT                             2326        3`c7543000 (  15.115 GB)  97.33%    0.01%
MEM_RESERVE                             320        0`1a88b000 ( 424.543 MB)   2.67%    0.00%

0:000> !eeheap -gc
Number of GC Heaps: 1
generation 0 starts at 0x0000009902B57670
generation 1 starts at 0x0000009902A56810
generation 2 starts at 0x00000095318C1000
ephemeral segment allocation context: (0x0000009902D724A8, 0x0000009902D724C0)
         segment             begin         allocated         committed    allocated size    committed size
...
00000098FFBE0000  00000098FFBE1000  0000009902D724A8  0000009902D7D000  0x31914a8(51975336)  0x319c000(52019200)
Large object heap starts at 0x00000095418C1000
         segment             begin         allocated         committed    allocated size    committed size
00000095418C0000  00000095418C1000  00000095475D8D98  00000095475D9000  0x5d17d98(97615256)  0x5d18000(97615872)
Total Allocated Size:              Size: 0x398e6cbc8 (15450164168) bytes.
Total Committed Size:              Size: 0x398e7b000 (15450222592) bytes.
------------------------------
GC Allocated Heap Size:    Size: 0x398e6cbc8 (15450164168) bytes.
GC Committed Heap Size:    Size: 0x398e7b000 (15450222592) bytes.     

从输出信息看，好家伙。。。进程提交内存是 15G, 托管堆差不多也是 15G，这就说明当前是相对简单的 托管内存泄漏。

2. 到底是什么在泄漏

要想知道到底是什么在泄漏，可以先到托管堆上看看有没有什么异常的对象，使用 !dumpheap -stat 命令。

0:000> !dumpheap -stat
Statistics:
              MT    Count    TotalSize Class Name
...
00007ff8815d0f88  7260233    290409320 System.Collections.ArrayList
00007ff8815e6830  7313696    326240826 System.String
000000952fbbd2b0 12141398    509369998      Free
00007ff880685cf0  7254983    928637824 System.Diagnostics.ProcessInfo
00007ff88065f7d0  7256845   2031916600 System.Diagnostics.Process
00007ff8815e6ea8  7391338   2230744600 System.Object[]
00007ff88068fa70 186800748   8966435904 System.Diagnostics.ThreadInfo

从卦象上来看，真的太奇怪了，如果大家了解 Process 类，应该知道 ProcessInfo 和 ThreadInfo 都是挂在 Process 下的，而且 ThreadInfo 对象高达 1.8亿，真的太🐂👃了，看样子程序是在不断的做 Process.Start 操作吧。

接下来要探究的问题是 ThreadInfo 到底正在被谁持有？？？ 可以挑几个看看它们的 !gcroot， 首尾法查了几个，都是没有引用根，如下所示：

0:000> !gcroot 0000009531e8f760
Found 0 unique roots (run '!GCRoot -all' to see all roots).
0:000> !gcroot 0000009531e8f670 
Found 0 unique roots (run '!GCRoot -all' to see all roots).
0:000> !gcroot 0000009531e8f378
Found 0 unique roots (run '!GCRoot -all' to see all roots).

3. 无引用根为什么不被回收

既然对象没有引用根，为什么 GC 不回收它呢？ 这里就需要谈经验了，在我之前分析的很多关于内存泄漏 的dump中，我都是从 生产端 找问题，貌似还没有出现一个从 消费端 找问题的案例，参考模型如下：

生产者既然没问题，那消费端能有什么问题呢？大家可以想一想，托管堆的消费端起码有两个角色。

1. GC
2. Finalizer 线程

GC 肯定是不会出问题的,那就只能怀疑 Finalizer 线程出了什么问题，可以用 !t 命令把所有线程调出来看看。

0:000> !t
ThreadCount:      9566
UnstartedThread:  0
BackgroundThread: 88
PendingThread:    0
DeadThread:       9471
Hosted Runtime:   no
                                                                                                        Lock  
       ID OSID ThreadOBJ           State GC Mode     GC Alloc Context                  Domain           Count Apt Exception
   0    1 40e18 000000952fbdfe50    26020 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
   2    2 41ce8 000000952fc0e4d0    2b220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Finalizer) 
   4    4 41cb4 000000954c324970    27220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
   5    5 41cb8 000000954c36d1e0  2027220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
   6    6 41c58 000000954c33f070  2027220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
   7    7 41c38 000000954c33f840    27220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
   8    8 41e0c 000000954c333580    27220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
   9    9 41e2c 000000954c354440    27220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
  10   10 41f24 000000954c355840    27220 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     STA 
...
XXXX 9446    0 000000957a233410  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 
XXXX 9447    0 0000009579f83e30  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 
XXXX 9450    0 000000957a46dcf0  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 
XXXX 9449    0 000000957967c6e0  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 
XXXX 9448    0 000000957aee0010  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 
XXXX 9452    0 00000095796824a0  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 
XXXX 9451    0 000000957af05df0  8039820 Preemptive  0000000000000000:0000000000000000 000000952fbd2e40 0     MTA (Threadpool Completion Port) 

这不看不知道，一看吓一跳，当前进程有 9566 个，死线程高达 9471 个 ，以过往经验，这小子不用线程池，用 new Thread 咯, 🐂👃🦆 ，吐槽结束，再看下 Finalizer 线程正在做什么,使用 ~2s & !dumpstack

0:002> ~2s
ntdll!NtWaitForSingleObject+0xa:
00007ff8`8e220c8a c3              ret
0:002> !dumpstack
OS Thread Id: 0x41ce8 (2)
Current frame: ntdll!NtWaitForSingleObject+0xa
Child-SP         RetAddr          Caller, Callee
0000009549e4e120 00007ff88b591118 KERNELBASE!WaitForSingleObjectEx+0x94, calling ntdll!NtWaitForSingleObject
0000009549e4e1c0 00007ff88da3e334 combase!MTAThreadWaitForCall+0x54 [d:\9147\com\combase\dcomrem\channelb.cxx:5657], calling KERNELBASE!WaitForSingleObject
0000009549e4e210 00007ff88d8fe089 combase!MTAThreadDispatchCrossApartmentCall+0x75 [d:\9147\com\combase\dcomrem\chancont.cxx:193], calling combase!MTAThreadWaitForCall [d:\9147\com\combase\dcomrem\channelb.cxx:5619]
0000009549e4e240 00007ff88da3e13d combase!CRpcChannelBuffer::SendReceive2+0x64b [d:\9147\com\combase\dcomrem\channelb.cxx:4796], calling combase!MTAThreadDispatchCrossApartmentCall [d:\9147\com\combase\dcomrem\chancont.cxx:156]
0000009549e4e2b0 00007ff88e1bb6f7 ntdll!RtlAllocateHeap+0xd7, calling ntdll!RtlpLowFragHeapAllocFromContext
...
0000009549e4f5d0 00007ff8827d79cd clr!ManagedThreadBase_DispatchOuter+0x75, calling clr!ManagedThreadBase_DispatchMiddle
0000009549e4f5e0 00007ff8828601af clr!EEConfig::GetConfigDWORD_DontUse_+0x3b, calling clr!EEConfig::GetConfiguration_DontUse_
0000009549e4f660 00007ff8828574fa clr!FinalizerThread::FinalizerThreadStart+0x10a, calling clr!ManagedThreadBase_DispatchOuter
0000009549e4f6a0 00007ff8827d55b9 clr!EEHeapFreeInProcessHeap+0x45, calling kernel32!HeapFreeStub
0000009549e4f700 00007ff882882e8f clr!Thread::intermediateThreadProc+0x86
0000009549e4f780 00007ff882882e6f clr!Thread::intermediateThreadProc+0x66, calling clr!_chkstk
0000009549e4f7c0 00007ff88dcd13d2 kernel32!BaseThreadInitThunk+0x22
0000009549e4f7f0 00007ff88e2003c4 ntdll!RtlUserThreadStart+0x34

从堆栈信息看，原来是终结器线程卡死了，从 MTAThreadDispatchCrossApartmentCall 方法看，貌似是 MTA 向 STA 做了一个调用，到这里有经验的朋友应该知道，这和 com 组件有关系了，也就是说 Finalizer 线程无法释放由 STA 线程创建的 COM 对象，那到底是哪一个线程创建了没有被合理释放的 COM 组件呢？

4. 寻找创建 COM 组件的线程

说实话，这个对COM组件不了解的话，很难找出答案，但天无绝人之路，当我回头看 线程列表 的时候，发现居然有 38 个 STA线程，截图如下：

这里面肯定有问题，接下来抽一个线程看看调用栈如何。

0:004> !clrstack 
OS Thread Id: 0x41cb4 (4)
        Child SP               IP Call Site
000000954da1ee38 00007ff88e220c8a [HelperMethodFrame: 000000954da1ee38] System.Threading.Thread.SleepInternal(Int32)
000000954da1ef30 00007ff88138c20a *** WARNING: Unable to verify checksum for mscorlib.ni.dll
System.Threading.Thread.Sleep(Int32)
000000954da1ef60 00007ff82322437f xxx.CFileLogTask.DoWork()
000000954da1f160 00007ff8232234d6 xxx.CTask.InitStart()
000000954da1f240 00007ff8813c31d3 System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
000000954da1f310 00007ff8813c3064 System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
000000954da1f340 00007ff8813c3032 System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
000000954da1f390 00007ff8813bc812 System.Threading.ThreadHelper.ThreadStart()
000000954da1f5e8 00007ff8827d6bb3 [GCFrame: 000000954da1f5e8] 
000000954da1f938 00007ff8827d6bb3 [DebuggerU2MCatchHandlerFrame: 000000954da1f938] 

接下来反编译下 xxx.CFileLogTask.DoWork() 方法看看它是如何被 Thread 执行的。

到这里终于水落石出，罪魁祸首在 CurrThread.SetApartmentState(ApartmentState.STA); 这一句上，我也不知道为啥开个 Thread 还要给个 STA。。。

三：总结

本次事故主要是因为在 STA 线程上用到了 COM 组件，导致让 MTA 模型的 Finalizer 线程去释放时被卡死，而这个Thread又没有用 Application.Run 启动消息循环，STA也是 Sleep 状态，我个人感觉两者无法通讯，给到朋友的建议是去掉 Thread 的 STA。

其实这里有一个很好的点就是，当内存暴涨，不一定是 生产端 的问题，也有可能是 消费端 。