Windbg-Cheat-Sheet

A practical guide to analyze memory dumps of .Net applications by using Windbg.

• Environment
• Dump Generation
  • Manual Dump Generation
  • Automatic Dump Generation
• Memory Leak
  • General Heap Check
  • Check Finalizer Queue and Finalizer Thread
• High CPU Usage
• Deadlocks
• Static Class,Field etc. Access
• Command List
• Articles

Environment

• Install SOS.dll(Son of strike) to same path with WinDBG.
• Install psscor4
• Set symbol path

.sympath srv*d:\dumps\symbols;https://msdl.microsoft.com/download/symbols

• Run command of .loadby sos clr to load sos.

Dump Generation

1. Manual Dump Generation

There are multiple manners to generate dumps ranging from task manager and debug diagnostic. I prefer to use procdump for manual generation. To generate dump manually, follow instructions below.

• Download procdump from this link.
• Run command prompt as administrator and switch the path of procdump. Following is an example from my local.

cd \BrkDev\Procdump

• Procdump provides a variety of parameters which change characteristic of generated dump. List of parameters can be seen in the link above. I will use following command to get full memory dump of all process memory.

procdump -ma [process identifier] [folder path]

2. Automatic Dump Generation

Sometimes it is not possible to find a chance to collect dump before application crashes. Therefore, Windows Error Reporting can be configured so that dumps can be collected in a situation of crash; however, applications handling their own custom crash reporting are not supported by this feature. Ultimately, in order to collect dumps automatically, follow the steps below.

• Open Registery Editor(Regedit).
• Go to following record.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Windows Error Reporting\LocalDumps

• If the record does not exist, create it. Otherwise, check the values of following parameters.

• Locate and delete following registry entries.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug\Debugger
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework\DbgManagedDebugger
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\AeDebug\Debugger
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\.NETFramework\DbgManagedDebugger

Memory Leak

1. General Heap Check

• Verify dump file. Make sure that heap is not corrupted.

!verifyheap

• Look at commited memory with following command. The best command to look at memory as whole.

!address -summary

Sections in result

• Image: memory used for executables and dlls.

• Stack: stack space for threads.

• https://blogs.msdn.microsoft.com/webtopics/2010/04/02/address-summary-explained/

• Following command shows gc heap and loader heap usages. Look at the amount of space that heaps allocate. (Reminder: gcheap is collected by gc but loader heap is not. Loader heap is for static objects.) Make sure that both values are under certain size.

!eeheap

• Run following command to see full list of objects in memory(name, count, size etc.)

!dumpheap -stat

• If there is an object looking suspicious(having more size or count) dump that object.

!dumpheap -type [class name space]

• Pick the one allocationg more space than others and go to its details.

!do [address]

2. Check Finalizer Queue and Finalizer Thread

• list all the objects in memory

!dumpheap -stat

• Take the suspicious one. following command only gives addresses of objects of that type.

!dumpheap -type [type of object] -short

• To see what keeps the reference to those objects run following command. the example below is valid for bytearray. Find the one(above) waiting for being finalized.

.foreach(bytearr {!dumpheap -type System.Byte[] -short}){!gcroot bytearr; .echo - - - - - }

• check the finalizer queue with following command.

!finalizequeue

• If there are many objects waiting for finalizing analyze the finalizer thread stack. Switch the finalizer thread.

!clrstack

High CPU Usage

• Check threadpool whether CPU utilization is greater than 80%. If it is greater, no more .Net thread will be created and Garbage Collector will run.

!threadpool

• Check uptime of threads

!runaway

• Get list of an arbitrary number of threads on top of the list

• Switch thread contexes one by one

• Check their clr stack

• If all of them waits for the same method. Analyze the method.

Deadlock

Static Field Access

• List application domains.

!dumpdomain

• Delve into selected module

!dumpmodule  -mt [module identifier]

• Dump class

!dumpclass [class identifier]

• Get value of static field

!do [field identifier]

Command List

• Evaluates given expression.

?
Example usage: `?e186fa28`

• Gives list of threads.

!threads

• Switches thread context to specified id's context.

~[id]s
~5s

• Gives detailed information about that thread pool.

!threadpool

• Check this if you look for high cpu usage. You can see which thread works for how long.

!runaway

• Lists all .net call stacks. Lists contexts of all threads.

!eestack

• Lists exceptions of current thread.

!printexception

• Shows usage statistics of heaps. Heap count equals to core counts.

!heapstat

• If you do not dispose object, it is alive during 2 GC process. If you dispose, it is alive during 1 GC process.

!fq(finalizer queue)

• Kernel time: cpu operations like siscall, interrupt, tcp call etc. User time: cpu operations like read, multiplication.

.time

• First command related to memory.

!address -summary

• Gives process info like computer name. Process environment block.

!peb

• Gives managed heap usage.(.net heap) Size of heaps should be nearly equal to each other.

!eeheap -gc

• Lists all objects on heap.

!dumpheap -stat

• Gives address list of specified type of objects.

!dumpheap -mt [address]

• Lists the objects refers to specified address. If it says that there is no unique root found, the object will be collected in next gc cycle.

!gcroot [address]

• Gives details of method.

!dumpmd

• Gives call stack of current thread.

!clrstack

• Shows thread locks if there is.

!syncblk

• Lists modules loaded by application.

lm

• Makes windbg more talkative - shows detailed messages

.srcnoisy 3

Articles

• Guides from Tess Fernandez