This summer, I read a great article "coz: This Is the Profiler You're Looking For" in USENIX ;login: about how to profile multi-threaded programs. It presented a system for profiling software by running experiences in the running program, testing how run time performance is affected by "speeding up" parts of the code to various degrees compared to a normal run. It does this by slowing down parallel threads while the "faster up" code is running and measure how this affect processing time. The processing time is measured using probes inserted into the code, either using progress counters (COZ_PROGRESS) or as latency meters (COZ_BEGIN/COZ_END). It can also measure unmodified code by measuring complete the program runtime and running the program several times instead.
The way the system work is fairly simple. To run an coz experiment on a binary with debug symbols available, start the program like this:
coz run --- program-to-run
This will create a text file profile.coz with the instrumentation information. To show what part of the code affect the performance most, use a web browser and either point it to http://plasma-umass.github.io/coz/ or use the copy from git (in the gh-pages branch). Check out this web site to have a look at several example profiling runs and get an idea what the end result from the profile runs look like. To make the profiling more useful you include <coz.h> and insert the COZ_PROGRESS or COZ_BEGIN and COZ_END at appropriate places in the code, rebuild and run the profiler. This allow coz to do more targeted experiments.
A video published by ACM presenting the Coz profiler is available from Youtube. There is also a paper from the 25th Symposium on Operating Systems Principles available titled Coz: finding code that counts with causal profiling.
The source code for Coz is available from github. It will only build with clang because it uses a C++ feature missing in GCC, but I've submitted a patch to solve it and hope it will be included in the upstream source soon.
Please get in touch if you, like me, would like to see this piece of software in Debian. I would very much like some help with the packaging effort, as I lack the in depth knowledge on how to package C++ libraries.