Investigate High CPU usage or Infinite Loop via WinDbg


Confirm the Issue

We can investigate it using TaskMgr, if the CPU is around 25%, 50% or 100% (depends on how many cores you have, e.g. for 4 cores, 25% mean the program used up 1 core.)

If the CPU rate is not high but the program hangs, it probably indicates a deadlock.

Find the Thread

Note: WinDbg will suspend all thread, so we need to run the g command frequently if the program has something needs to run continuously, for example the program may disconnect from the server if suspended too long.


Open WinDbg and the Command window(alt+1)

  • Attach to the process that you want to inspect using File-->Attach to a process(F6). And run g` in Command window immediately.
  • See the instructions here to setup the Symbol File Path and don’t forget to tick the reload check box. If you process cannot be suspended to long, then I would suggest just include the minimal necessary symbol files, especially you should exclude the Microsoft Symbol Server.

Find out the Thread

We can use !runaway to inspect the time each thread consumes. Now break the program (It may take a while to reload the symbol files after symbol file paths changed) and run the following command:


The ;g above is to resume the program immediately after the !runaway command been executed.

0:004> .reload //reloads the symbol files as the symbol file paths changed.
Reloading current modules
0:004> !runaway;g
User Mode Time
Thread       Time
2:7b0       0 days 0:00:20.203
0:790       0 days 0:00:00.015
4:eb8       0 days 0:00:00.000
3:8a4       0 days 0:00:00.000
1:648       0 days 0:00:00.000

As shown above, we have 5 threads.

In the first column, the 2 in 2:7b0 is the index of the thread (depends on when it was created), and 7b0 is the ThreadId.

The second column shows the CPU time each thread consumes.

wait for a while and break the program again and run the !runaway;g command again.

0:004> !runaway;g User Mode Time Thread Time 2:7b0 0 days 0:00:25.484 0:790 0 days 0:00:00.015 4:d80 0 days 0:00:00.000 3:8a4 0 days 0:00:00.000 1:648 0 days 0:00:00.000

Compare the two results, we can see the thread 2 consumes all the CPU time.

Inspect the Stack Track

Break the program and run the ~2s;kb;g command. ~2s is to set the thread 2 as current thread, kb is to print the stack trace of the current thread, g is to resume the program.

And then we can get the stack trace:

0:004> ~2s eax=00401480 ebx=0013f7fc ecx=00000000 edx=7c9585ec esi=000379b8 edi=000006d8 eip=00401480 esp=0111ff18 ebp=0111ff84 iopl=0 nv up ei pl nz na po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000202 testwhile!ThreadFun: 00401480 ebfe jmp testwhile!ThreadFun (00401480) 0:002> kb ChildEBP RetAddr Args to Child 0111ff14 755422cb 00000000 00000000 00038b40 testwhile!ThreadFun [E:\testwhile\testwhileDlg.cpp @ 173] 0111ff84 77b9b530 0013f7fc 00000000 00000000 MFC42!Ordinal1184+0x15b 0111ffb8 7c82482f 00038aa8 00000000 00000000 msvcrt!endthreadex+0xa3 0111ffec 00000000 77b9b4bc 00038aa8 00000000 kernel32!GetModuleHandleA+0xdf 0:002> g

Usually, I will wait for a while and repeat the ~2s;kb;g to see if the function is still doing the same thing. And then repeat again and again so I can confirm the problem or get an idea of what’s wrong.

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