vendredi 31 mai 2013

Tracing userspace memory allocation

You may wonder what are the actual memory allocations in a program according to time. Here is a method to trace these operations with the LTTng User Space Tracer (UST). Events are recorded by overloading malloc() and free() with a library that is preloaded with LD_PRELOAD. This library is called "". To easy the actual tracing session setup, this feature has been added to the lttng-simple utility of workload-kit v0.1.13. By specifying the option "--enable-libc-wrapper", then the environment is set automatically. Here is quick demo to look at the memory allocations of gnome-calculator. (output of babeltrace is simplified for clarity)

// trace the program
$ lttng-simple -u --enable-libc-wrapper -- gnome-calculator

// display events
$ babeltrace gnome-calculator-u/
[10:48:49.808255177] malloc: { size = 40, ptr = 0xCA8220 }
[10:48:49.808256389] free: { ptr = 0xCA8220 }

// count events and look for lost events
$ javeltrace analyze -i ctf gnome-calculator-u/...
number of streams                             1
number of packets                         1,058
number of events                        181,879
number of CPUs                                8
number of lost events                         0

This experiment shows that there are about 182 thousands memory allocation events to start a calculator!

Checking for lost events with javeltrace is recommended. The default buffer size in UST is quite small, and in consequence lost events occurs even with very light load, like starting the calculator with memory allocation tracing. The tool lttng-simple setups buffers of 16MB to make it safer.

The next step is of course to display this data according to time and compare it to physical pages allocated by the operating system with a kernel trace.

Happy hacking!