Saturday, September 18, 2010

Congratulate me, I’m the winner!

Several articles from this blog have participated in Tell Your Windows Debugging Story Annual Competition. Few days ago Dmitry Vostokov introduced the winners of the competition. I’m glad to tell you that this blog is among them!

The way to find a hotspot

Today I want to describe the way to find a hotspot without profilers. I often use this approach and find it effective and simple enough. The only tools you need are any Windows debugger (I prefer WinDbg, but you can use your favorite one) which should always be in your hands if you decided to investigate some performance issue (as well as any other problem with your software) and tool like Process Explorer which available free from Windows Sysinternals. You can use this method without profiler at all or in conjunction with profiler to clarify the result of profilers work.

Before explaining the approach, I want to define the term “hotspot”. There are different definitions which can be applied in different contexts. In this article we consider a hotspot as a limited set of instructions on which execution spends significant CPU time or more CPU time than you expect from it.

The process is divided into the three steps. If you used a profiler and have a call graph, you can miss the first two steps and jump to step number three to verify the profilers result and find out the hotspot most precisely. Nevertheless, I recommend you to go through all steps because the results returned by profilers sometimes can be very obscure and you will lose time rolling around a call graph or other related information. Even if your profiler was absolutely right, you’ll have one more confirmation of its result. Sometimes it’s better to check twice :)

Thursday, September 9, 2010

The bug in MFC tool tip control

Yesterday, I encountered the bug in MFC’s tool tip control implementation.
The bug dozes in the following code:

LRESULT CToolTipCtrl::OnAddTool(WPARAM wParam, LPARAM lParam)
    TOOLINFO ti = *(LPTOOLINFO)lParam;     <----- HERE
    if ((ti.hinst == NULL) && (ti.lpszText != LPSTR_TEXTCALLBACK)
        && (ti.lpszText != NULL))
        void* pv;
        if (!m_mapString.Lookup(ti.lpszText, pv))
            m_mapString.SetAt(ti.lpszText, NULL);
        // set lpszText to point to the permanent memory associated
        // with the CString
        VERIFY(m_mapString.LookupKey(ti.lpszText, ti.lpszText));
    return DefWindowProc(TTM_ADDTOOL, wParam, (LPARAM)&ti);

Friday, September 3, 2010

To untangle a rope

Last weekend I was launching a kite with my daughter and her friend. It was great fun, but when I was veering the kite down its string entangled. I spent about 30 minutes (maybe even more – time flies fast :)) to untangle the string. I followed the string loops, analyzed the types of knots, imagined what will happen to string if I pass it through one loop or another. I went through a lot of different combinations inside my mind and devised more and more new techniques. When I finished finally, I felt satisfaction. I felt no tiredness or irritation, furthermore I had the feeling like I untangle the strings all my life. Then I thought about it I realized that it’s not far from the truth. The process of untangling string (as well as rope, chain and so on) is very similar with the process of software defects researching (the term was first introduced by Dmitry Vostokov). So if you in doubt to be software researcher or not – try to untangle a rope first :)

Thursday, September 2, 2010

Can lightning strike the same place twice? (Or can a bomb to hit an elephant?)

In the old soviet movie about World War II was mentioned the story about the old Professor of Mathematics. When Leningrad has been bombing he is never descended to bombshelter along with other people. He told that he calculated the probability of bomb hitting his house and the probability was too small to hiding from bombs. But one day his neighbors saw him in a bombshelter and asked why is he hiding with others. He replied that on that day a bomb hit the only elephant in the cities zoo, and probability of hitting the elephant was absolutely equal with probability of been hit himself.

Why did I remember this story? Because during the last week I encountered twice with the failures which was the result of circumstances those probability was very low. Like people say: “If gun is hanging on the wall it will shoot one day or another”.