.NET调试利器：Windbg与SOS.dll修复问题

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"Tess, if broken it is fix it you should - .NET Debug SOS help is a pdf reference file for .NET debug in Windbg" 这篇文档是针对.NET开发者的一份调试指南，特别强调了在遇到问题时利用调试器解决问题的重要性。文档由Tess Ferrandez撰写，旨在帮助用户理解并解决.NET应用中的各种故障，特别是内存泄漏和性能问题。Tess Ferrandez是微软的一位著名工程师，她通过此文档分享了她的调试技巧和经验。 1. **调试器的力量**（Why I love the debugger）文档开篇表达了作者对调试器的热爱，因为调试器可以帮助开发者深入到应用程序的内部，找到问题的根源，解决那些看似无解的问题。 2. **内存泄漏的识别与诊断**（I have a memory leak! What do I do?）文档中提到了如何识别和定位内存泄漏，介绍了使用`!dumpheap –stat`命令来分析.NET内存泄漏的情况，帮助开发者找到占用内存最多的对象类型。 3. **崩溃原因分析**（What on earth caused my process to crash?）讨论了如何调查导致程序崩溃的原因，这通常涉及分析堆栈跟踪、异常信息以及可能的系统状态。 4. **大量异常的警告**（Are you aware that you have thrown over 40,000 exceptions in the last 3 hours?）这部分提醒开发者注意应用程序中异常的频繁抛出可能是性能问题或错误的信号，需要及时调查和处理。 5. **关联Windbg与.dmp文件**（Associate windbg with .dmp files）教导读者如何设置将Windbg作为.dmp（内存转储）文件的默认打开程序，以便于进行离线调试。 6. **符号解析**（Why do I get weird function names on my stack? A discussion on symbols）解释了符号文件（pdb）在调试过程中的重要性，以及如何理解和解决符号加载问题，以正确显示堆栈上的函数名称。 7. **调试ASP.NET问题时的注意事项**（Things to ignore when debugging an ASP.NET hang）提供了在调试ASP.NET应用程序挂起时应忽略的一些常见干扰因素。 8. **锁和临界区导致的挂起场景**（Hang Scenario, Locks and Critical Sections）分析了由于线程同步问题导致的应用程序挂起，讲解了如何使用Windbg检查锁定和临界区的状态。 9. **获取内存转储的基础知识**（Back to Basics - How do I get the memory dumps in the first place? And what is SOS.dll?）阐述了如何收集内存转储，并介绍SOS.dll，这是一个用于.NET调试的强大扩展，提供了如`!gcroot`、`!dumpobj`等命令，用于分析.NET对象和垃圾回收的状态。 10. **调试技巧**（Debugging Tips: Use windbg as a calculator, a chat client, and more…）展示了一些高级的Windbg用法，如将其用作计算器和其他实用工具，以提高调试效率。 11. **反馈请求**（Request for feedback）文档末尾鼓励读者提供反馈，以改进和扩展这份调试指南。此外，文档还包含了一个.NET内存泄漏的案例研究，讨论了事件处理器可能导致内存膨胀的问题，以及关于ASP.NET缓存管理和`foreach`循环的性能影响的示例。最后，文档中还穿插了一些数学相关的调试小贴士，以轻松的方式展现了调试的乐趣。

If any questions, please feel free to email: charju@microsoft.com

What on earth caused my process to crash?

So you got a 1000 w3wp.exe stopped unexpectedly in the eventviewer or your

process just exited in some weird undefined way and you don't know why.

When a process crashes or exits a special event will be fired called EPR (Exit

PRocess) so with a debugger like windbg.exe we can attach to the process,

wait for epr to be thrown and take a memory dump. When you install the

debugging tools for windows you get a vbs script called adplus

(http://support.microsoft.com/default.aspx?scid=kb;en-us;286350) that will

do this automatically for you, and print logs for most exceptions that occurr

during the process life.

Debugging tip!: When you open a dump taken in -crash mode, you will

automatically be positioned on the thread that was active when the crash

occurred (the most likely suspect). If you switch threads and want to get back

to the faulting thread, type ~ to list all threads and the faulting thread will be

marked with a dot.

If your dump only shows one active thread in the process and that thread is the

main thread, the process was likely killed by something external (health

monitoring, low system memory, iisreset etc.)

I'll go into more depth of some scenarios in some later posts but I'm a very

top-down person so I wanted to start off with a post about some common

situations when you will see a managed process exit so that you have some

idea of what to look for in the dump.

In no particular order, here are some of the ones we see most in support:

Stack Overflow Exceptions

A stack overflow exception will occur when the memory allocated for the stack

for a thread is used up. By default it's 1 MB so your call stack can be pretty deep,

so most of the time when this happens it's because of infinite

recursion, i.e. FunctionA calls FunctionB which calls FunctionB again which

calls FunctionB again... and there is no stop condition.

A bit of an obscure infinite recursion situtaion that unfortunately is pretty

common is incorrect usage of the Exception Handling application block.

Imagine this scenario: Your application gets an exception, the exception

handler kicks in and you have set it up to log to a file. While logging you get an

If any questions, please feel free to email: charju@microsoft.com

exception of some kind (say access denied) and you have set the exception

handler to handle this exception too. In this case you will end up in an infinite

recursive loop where you handle an exception, throw another, handle it, throw

another... you get the gist. The moral of the story? Don't use the exception

handler to handle exceptions in the exception handler:)

If you run "kb 2000" (to see the native stack) and "!clrstack" (from sos.dll to

see the managed stack) you can spot the recursive pattern to track down

where/why the recursion occurred.

Out Of Memory Exceptions

Most of the time when an Out Of Memory exception occurrs it is caused by a

design problem, where either too much memory is stored in cache or session

scope. Caching can be great for increasing performance if used in the right way,

i.e. you cache the data that will be accessed the most and you don't cache it for

longer than you need. In old ASP you got problems if you stored objects in

session scope and believe me, it was a blessing in disguise because developers

stored only the most necessary items in session scope. Storing large datasets in

session scope for example is often counter-productive as you reduce the

number of concurrent users your web site can handle and when memory gets

high enough you may have more overhead in terms of time needed to do

garbage collections and searches through cache than you would requesting the

data from the database when you actually need it.

There is no one-size fits all solution here on when you should store something

in session/cache and when you shouldn't. The best thing to do is at an early

stage, determine how many users your application needs to be able to handle,

and based on that determine how much you can allow for storage per user. And

then stress-test with more than your maximum number of users to make sure

you can cope. Preferably, stress test with and without the objects in session

state to see what the perf. difference is for different amounts of users.

Memory issues are very hard to fix when you are in production since they often

require a lot of re-design, so a penny spent in the early stages will save a lot of

pennies later.

Debugging tip!: Run !dumpheap -type System.Web.Caching.Cache to get the

cache roots and then do !objsize on the addresses for these to find out how

much you are storing in cache for the different applications. (Note: InProc

Session state is stored in the cache as well)

For a more indepth discussion on why you get Out of Memory exceptions see

my earlier post.

If any questions, please feel free to email: charju@microsoft.com

Unhandled exception in COM Component

If your application calls a native COM component, you can crash if you get an

unhandled exception in it. For example if you are referencing memory that

has already been freed or similar.

kb 2000 will then show the COM component on the stack so you can narrow it

down from there.

Native heap corruption

This, along with GC holes are some of the nastiest issues to troubleshoot. A

native heap corruption when someone writes to a location that it was not

supposed to. The problem with this is that you will not see the error when the

code that writes to the wrong address executes but rather when someone else

is trying to access that memory address in the correct way. In other words,

long after the "thief" was there. The location that is written to can be the heap

or even worse, a location where code is stored (so instructions are overwritten)

or the stack so your code calls into the middle of nowhere. Most frequently

this occurrs when you write outside of the bounds of a buffer or something to

that effect.

For an introduction to heap corruption read Geoff Gray's article on Heap

Corruption. When you get a crash because of heap corruption, the faulting

stack will often be in a heap allocation function in ntdll, and to resolve it you

need to run with GFlags or PageHeap to be able to catch the thief in the act.

However, the reason why it is so hard to catch is that these issues are very

random and very hard to reproduce.

Managed Heap Corruption

Managed heap corruption is a heap corruption that occurrs on the managed

heaps. Again, here, you get the problem long after it occurred. A managed

heap corruption occurrs if someone overwrites a piece of the managed heap

that it wasn't allowed to. Normally you can't buffer overflow in managed code.

If you have a byte[] and try to write outside the bounds you will get an

IndexOutOfRange exception or similar. The most common reason for

a managed heap corruption is that the code called PInvoke and passed in a

buffer of some sort but the buffer was too small. The PInvoked function writes

to the buffer but writes beyond the bounds of the buffer and onto the next

object on the managed heap. Later on when the garbage collector is doing its

work, it tries to go through the heap and things go bad so the process crashes.

If any questions, please feel free to email: charju@microsoft.com

If you get a crash and the top of the active stack contains a GC function, start

looking for PInvokes in your code and see if you might be passing buffers that

are too small.

Fatal Execution Engine Exceptions

Fatal Execution Engine Exceptions are fairly rare, but when they occurr it's

normally a bug. This means that for some reason we went into some piece of

code that we were not supposed to in the CLR and the CLR has decided that in

the unlikely event that someone comes in here, let's throw a Fatal Execution

Engine exception and die because we can't recover from this point. In the

eventlog this will be logged as Execution Engine Exception occurred and the

address listed will tell exactly where in the code it occurred. If you reach one

of these and can't find a knowledge base article about it, contact support,

preferably with a crash dump available since that will speed up the time to

resolve the case tremendously.

GC Holes

These are fairly rare too. The unmanaged portion of the CLR has a pointer to

managed memory and "forgot" to tell the GC about it so the GC doesn't know to

keep it around (if there are no other roots) or to track movement of it, which

means that if a GC happens at the "wrong time" the pointer could be pointing

basically anywhere causing a lot of havoc. Yun Jin talks a little about it here

http://blogs.msdn.com/yunjin/archive/2004/02/08/69906.aspx

To this date I have had little or no issues involving GC Holes so it wouldn’t be

the first thing to look for in a "crash" dump.

No available memory left

If your process dies and at the same time you see a huge dip in the

memory/available MBytes, to the point where its 0, this can cause the

process to crash. Here of course the task will be to check

what process stole the memory.

External process kills/Recycles the process

Countless times when I have had a customer with adplus attached in crash

mode i have gotten memory dumps where the process was killed by something

external. I wanted to add this gotcha since normally when you troubleshoot a

crash, this is not the actual reason for the crash you are troubleshooting (only

in rare cases), but rather that someone ran iisreset or killed the process not

knowing that you had a debugger attached waiting for the crash. So if you are

If any questions, please feel free to email: charju@microsoft.com

troubleshooting a crash and get a memory dump. Check the eventlog to make

sure that noone ran an iisreset so this was the reason for your crash dump.

That will save you some hours trying to find the culprit of the crash:)

Another curious one is where some kind of monitoring software is installed to

monitor that the server servs pages as expected and shuts down the process if

it doesn't. But usually this logs an event of the occurrance in the eventviewer as

well.

Health monitoring settings

This one might be a doh!!! of course i know that we are set to recycle the

process every 24 hours, or if the server has been idle for more than 20 minutes,

but it's worth mentioning anyways because we get these issues pretty

frequently. Again here, normally an event will be logged in the eventlog

stating why the process was recycled but the moral of the story is to have a

brief look at your health monitoring settings for the application pool in iis or in

machine.config to see what recycling opitons you have turned on so you don't

get a surprise later.

There are more reasons for crashes than the above but they are usually pretty

obscure so I hope this post has given you a little insight in where to start

looking if your process suddenly dies.

Over and out...

Wednesday, November 30, 2005 12:12 PM by Tess

Are you aware that you have thrown over 40,000 exceptions

in the last 3 hours?

This may seem like a preposterous statement, but unfortunately it’s all too common.

In my work I go through a lot of dumps, somewhere in the neighborhood of 5-20 in a day:)

Since the information is readily available to me, I usually do a quick check for the number of

exceptions that the application has thrown and what types of exceptions they are so I can make

the customers aware. More often than not, the applications are throwing a lot more exceptions

than the developers expected, or they were expecting them but didn’t know that they can

actually cause a problem since they don’t seem to have a direct effect on the application.

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