22 September 2015

Visual Studio slow debugging and _NO_DEBUG_HEAP

Verify you assumptions about tools you use!

Some time ago I was tracing a perf problem (UI code + some custom logic). I needed to track what module was eating most of the time in one specific scenario. I prepared release version of the app and I added some profiling code. I’ve used Visual Studio 2013. The app used OutputDebugString so I needed to run the debugging (F5) in order to be able to see logs in the output window (I know I know, I could use DebugView as well…)
But, my main assumption was that when I run F5 in release mode, only a little performance hit would occur. What was my astonishment when I noticed it was a wrong idea! My release-debug session pointed to a completely different place in the code…

Note: this article relates to Visual Studio up to VS2013, in VS2015 debug head is fortunately disabled by default.

Story continuation

What was wrong with the assumption? As it appeared when I was starting the app with F5, even in release mode Visual Studio is attaching a special debug heap! The whole application runs slower, because every system memory allocation gets additional integrity checks.
My code used win32 UI and thus every list addition, control creation was double checked by this special heap. When running using F5 the main bottleneck seemed to be happening in that UI code. When I disabled the additional heap checking (or when I simply run my application without debugger attached) the real bottleneck appeared in a completely different place.

Those kind of bugs have even their name Heisenbug, those are bugs that disappear (or are altered) by tools that are used to track the problem. As in our situation: debugger was changing the performance of my application so I was not able to find a real hot spot…

Let’s learn from the situation! What is this debug heap? Is it really useful? Can we live without it?

Example

Let’s make a simple experiment:

for (int iter = 0; iter < NUM_ITERS; ++iter)
{
  for (int aCnt = 0; aCnt < NUM_ALLOC; ++aCnt)
  {
    vector<int> testVec(NUM_ELEMENTS);
    unique_ptr<int[]> pTestMem(new int[NUM_ELEMENTS]);
  }
}

Full code located here: fenbf/dbgheap.cpp

The above example will allocate (and delete) memory NUM_ITERS x NUM_ALLOC times.

For NUM_ITERS=100 and NUM_ALLOC=100 and NUM_ELEMENTS=100000 (~400kb per allocation) I got

Release mode, F5: 4987 milliseconds
Release mode, running exe: 1313 milliseconds  

So by running using F5, we get ~3.7 slower memory allocations!

Let’s compare calls stacks:

call stacks when running F5 and attaching debugger

To prepare the above images I run the app using F5 and I paused at random position. There were lots of allocations, so I usually entered some interesting code. Of course, producing the second view (without F5) was a bit harder, so I set a breakpoint using _asm int 3 (DebugBreak() also would work), then I got debugger attached so I could also pause at random. Additionally, since the second version runs much faster, I needed to increase number of allocations happening in the program.

Running with F5 I could easily break in some deep allocation method (and as you can see there is a call to ntdll.dll!_RtlDebugAllocateHeap@12 ()). When I attached debugger (the second call stack) I could only get into vector allocation method (STD).

Debug Heap

All dynamic memory allocation (new, malloc, std containers, etc, etc…) at some point must ask system to allocate the space. Debug Heap adds some special rules and ‘reinforcements’ so that memory will not be corrupt.
It might be useful when coding in raw C winApi style (when you use raw HeapAlloc calls), but probably not when using C++ and CRT/STD.

CRT has its own memory validation mechanisms (read more at msdn) so windows Debug Heap is doing additional, mostly redundant checks.

Options

What can we do about this whole feature? Fortunately, we have an option to disable it!

Disabling debug heap in Visual Studio

Any drawbacks of this approach?

Obviously there is no additional checking… but since you’ve probably checked your app in Debug version, and since there are additional checks in CRT/STD no problems should occur.

Also, in the latest Visual Studio 2015 this feature is disabled by default (it is enabled in the previous versions). This suggests that we should be quite safe.

On the other hand, when you rely solely on WinAPI calls and do some advanced system programming then DebugHeap might help…

Summary

Things to remember:
Use "_NO_DEBUG_HEAP" to increase performance of your debugging sessions!.

As I mentioned in the beginning, I was quite surprised to see so different results when running F5 in release mode VS running the app alone. Debugger usually adds some performance hit, but not that huge! I can expect a slow down in a debug build, but not that much in release version of the application.

Debug Heap is attached every time: in debug builds and in release as well. And it’s not that obvious. At least we can disable it.

Fortunately Debug Heap is disabled by default in Visual Studio 2015 - this shows that MS Team might be wrong when they enabled Debug Heap by default in the previous versions of Visual Studio.

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