Freeing arrays is very slow under the debugger

[John Schroedl]

Hi all,

I have a MFC-based application which is behaving very slowly when closing
certain windows which manage a large amount of data.

We've recreated the problem in a simple dialog-based test application.
Create one of those and add a Button click handler and the following:

class MyClass
{
public:
MyClass() { myData = new char[20]; }
~MyClass() {delete [] myData;}
private:
char * myData;
};

void CSlowMemoryFreeDlg::OnBnClickedOk2()
{
int start = GetTickCount();
MyClass * myArray = new MyClass[200000];
int allocTime = GetTickCount();
delete [] myArray;
int deleteTime = GetTickCount();

wchar_t msg[128];
_snwprintf_s(msg, 128, L"%d to allocate, %d to delete", allocTime - start,
deleteTime - allocTime);
OutputDebugString(msg);
}

----

Timing Output - Release build run under the debugger:

281 to allocate, 25070 to delete


----

Timing Output - Release build run outside the debugge:
(output captured with Sysinternals DebugView):

[4880] 32 to allocate, 0 to delete

----

Notice that the delete is MANY times slower when running under the debugger.

This is Release build (with pdb generation); the same exact .exe. I have not
changed any build settings. It's also fast if I run then Attach the debugger
but we don't want to have to do that every time.

My profiling shows that the time is spent overwriting the freed memory with
a sentinal value. How/Why does this automatically occur when running under a
debugger (vs. running outside the debugger)?

What visual studio/debugger/crt?? setting can I use to get the fast behavior
whether under the debugger or not?

---

I tried the following which does speed up the delete[] but I'm not sure if
the memory is really freed and what other side-effects may result. This does
not seem like a good solution especially in our Release build. We mainly want
to be able to debug the release build in a reasonable amount of time (it can
take minutes to close these windows)

// Get the current debug flags
int tmp = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);

// FAST FREE
_CrtSetDbgFlag(tmp|_CRTDBG_DELAY_FREE_MEM_DF);

delete [] myArray;

// Reset flag
_CrtSetDbgFlag(tmp);


Any guidance is greatly appreciated.

John

[David Lowndes]

Post by John Schroedl
...
Notice that the delete is MANY times slower when running under the debugger.

John,

Have a look at the comments on this (non) bug report, I think it
explains the situation you're encountering.

https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=401862&wa=wsignin1.0

Dave

[John Schroedl]

David, this seem to be exactly the issue we're facing and setting the
environment variable does indeed speed things up again.

Thanks for your fast and accurate reply!!
John