Why does my STL code run so slowly when I have the debugger/IDE attached?
c++, ide, performance, stl, visual-studio
Solution
Running under a Microsoft debugger (windbg, kd, cdb, Visual Studio Debugger) by default forces Windows to use the debug heap instead of the default heap. On Windows 2000 and above, the default heap is the Low Fragmentation Heap, which is insanely good compared to the debug heap. You can query the kind of heap you are using with HeapQueryInformation.
To solve your particular problem, you can use one of the many options recommended in this KB article: Why the low fragmentation heap (LFH) mechanism may be disabled on some computers that are running Windows Server 2003, Windows XP, or Windows 2000
For Visual Studio, I prefer adding `_NO_DEBUG_HEAP=1` to `Project Properties->Configuration Properties->Debugging->Environment`. That always does the trick for me.
Problem
I'm running the following code, using Visual Studio 2008 SP1, on Windows Vista Business x64, quad core machine, 8gb ram. If I build a release build, and run it from the command line, it reports 31ms. If I then start it from the IDE, using F5, it reports 23353ms. Here are the times: (all Win32 builds) - DEBUG, command line: 421ms - DEBUG, from the IDE: 24,570ms - RELEASE, command line: 31ms - RELEASE, from IDE: 23,353ms code: ``` #include <windows.h> #include <iostream> #include <set> #include <algorithm> using namespace std; int runIntersectionTestAlgo() { set<int> set1; set<int> set2; set<int> intersection; // Create 100,000 values for set1 for ( int i = 0; i < 100000; i++ ) { int value = 1000000000 + i; set1.insert(value); } // Create 1,000 values for set2 for ( int i = 0; i < 1000; i++ ) { int random = rand() % 200000 + 1; random *= 10; int value = 1000000000 + random; set2.insert(value); } set_intersection(set1.begin(),set1.end(), set2.begin(), set2.end(), inserter(intersection, intersection.end())); return intersection.size(); } int main(){ DWORD start = GetTickCount(); runIntersectionTestAlgo(); DWORD span = GetTickCount() - start; std::cout << span << " milliseconds\n"; } ```