Slow performance on x64 free

Question

I have a class like this (assume all malloc's succeedes)

class CMyClass
{
public:
  CMyClass()
  {
  lpData = malloc(128);
  };

  ~CMyClass()
  {
  free(lpData);
  };

public:
  LPVOID lpData;
};

then I execute this:

CMyClass *lpList = new CMyClass[32768];

delete [] lpList;

The problem is that in x86 the code works fine and fast (some milliseconds to complete in debug and release builds) but in x64 the delete call takes about 15 seconds to free all memory.

O.S. is Win7 x64.

Hints will be appreciated.

Regards, Mauro.

Answer 1

It is possible that if you are running your test app through a debugger that you are hitting some performance issue with the Windows debug heap. Add _NO_DEBUG_HEAP=1 to the environment settings for the debuggee (in the Project Properties->Configuration Properties->Debugging->Environment property under Visual Studio 20xx) and see if that improves your deallocation perf.

Answer 2

I just tested this myself, using gcc 4.6.1-1 on Debian (after adding typedef void *LPVOID). There is no difference; both execute instantly, even without any optimization turned on.

I upped the array length to 1048576 to get a measurable runtime (0.161s), which was the same for both IA32 and AMD64. I turned on optimizations (-O3), and the time remained the same, but decreased to 0.157s. -Os (optimize for size) had the same result.

Is it possible you used different build options, like maybe you have some sort of memory access debugging enabled on AMD64?