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I want to understand how a C++ program that was given to me works, and where it spends the most time.

For that I tried to use first gprof and then gprof2dot to get the pictures, but the results are sometimes kind of ugly.

How do you usually do this? Can you recommend any better alternatives?

P.D. Which are the open source solutions (preferably for Linux or Mac OS )X?

Open the way
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  • Can you clarify what you mean by "ugly"? Are you objecting to the effective content or just the layout? – Ira Baxter Mar 13 '10 at 17:48
  • the layout, I followd the first examples. the info is ok, but the way it is displayed is not kind of nice; I mean, I get a picture where the dimensions are 20 (width) x 2 (height). It can not be used in a report – Open the way Mar 13 '10 at 18:53
  • If you want a report, why are you trying to draw a graph? – Ira Baxter Mar 13 '10 at 21:25
  • I want to add some figures to the report, some of teh figures containint the call graph. i think this is not that unusual – Open the way Mar 14 '10 at 17:34

6 Answers6

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OProfile on Linux works fairly well, actually i like it better than GProf. There are a couple graphical tools that help visualize OProfile output.

kert
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See the SD C++ Profiler.

Other answers here suggest that probe-oriented profilers have high overhead (10x). This one does not.

Ira Baxter
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In my opinion there are two alternatives (on Windows):

  • Profilers that change the assembly instructions of your applications (that's called instrumenting) and record every detail. These profilers tend to be slow (applications running about 10 times slower), sometimes hard to set up, and often not-free, but they give you the best performance related information. Look for "Ration Quantity", "AQTime" and "Performance Validator" if you want a profiler of this type.
  • Profilers that don't instrument the application, but just look at a running application and collect 'samples' of it. These profilers are fast (no performance loss), often easy to set up, and you can find quite some free alternatives. Look for "Very Sleepy" and "Luke Stackwalker" if you want a profiler of this type.

Although I used commercial profilers like Rational Quantity and AQTime in the past, and was very satisfied with the results, I found that the disadvantages (hard to setup, unexplainable crashes, slow performance) outweighed the advantages.

Therefore I switched to the free alternatives and I am mainly using "Very Sleepy" at this moment.

Patrick
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If you want to look at the structure of your application (who calls what, references, call trees, ...) look at "Understand for C/C++". This application investigates your source code and allows you to query almost everything from the application's structure.

Patrick
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You can try KCachegrind. This is a program that visualizes samples acquired by Valgrind tool called Callgrind. KCachegrind may seem to be not actively maintained, but the graphs he produces are very useful.

pajton
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  • can you give a link on how to get the call graph info from valgrind before using kcachegrind? thanks – Open the way Mar 14 '10 at 16:54
  • Try something like `valgrind ­­tool=callgrind `. There are some options that can tune your results, often used include `­­dump­instr=yes ­­trace­jump=yes`. Then just start `kcachegrind` read the stats and have fun:-). HTH – pajton Mar 17 '10 at 00:34
-1

Same answer as ---

EDIT: @Steve suggested I give a less pithy answer.

I hear this all the time - "I want to find out where my program spends its time".
Let me suggest an alternate phrasing - "I want to find out why my program spends its time".

Maybe the difference isn't obvious. When a program executes an instruction, the reason why it is doing so is encoded in the entire state of the program, including the call stack.
Looking only at the program counter is like trying to see if a taxi ride is necessary by profiling the rotation angle of its wheels.
You need to look at the whole state of the program.

There's another myth I hear all the time - that you need to measure the execution time of methods, to find the "slow" ones. There are many ways for programs to take more time than they need to than by, say, doing a linear search instead of a binary search in some method, which might be the kind of thing people have in mind.

The way to think about it is this:

  • There isn't just one thing taking more time than necessary. There probably are several.
  • Each thing taking time is taking some fraction, like 10%, 50%, 90% or some such number. That means if the wall clock could be stopped during that time, that is how much less time the overall app could take.
  • You want to find out what those things are, whatever they are. Profilers (samplers) work by taking a lot of shallow samples (PC or call stack) and summarizing them to get measurements. But measurements are not what you need. What you need is finding out what it's doing, from a time perspective. It's better to get a small number of samples, like 10 or 20, and examine (not summarize) them. If some activity takes 20%, 50%, or 90% of the time, then that is the probability you will catch it in the act on each sample, so that is roughly the percent of samples on which you will see it. The important thing is finding out what it is, not getting an accurate measurement of something irrelevant.

So as a way to see what the program is doing, from a time perspective, here's how many people do it.

Community
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Mike Dunlavey
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