Visual Call Graph using DTrace

Here is an example of how incredibly powerful DTrace can be. I had to solve a problem where a nice high-level overview of the code flow in a single threaded userland application would save me some pain the backside. Lo and Behold, Dtrace – The Universal Hammer for every nail.

Here is a simple visual call graph generator using 5 lines of DTrace combined with the outstanding Graphviz graphics library.

$ cat callgraph.d
pid$target:a.out::entry
{
@call_stats[ufunc(ucaller), strjoin(strjoin(probemod,"`"), probefunc)] = count();
}
END
{
printf("digraph foo {\n");
printa(" \"%A\" -> \"%s\" [weight=%@d];\n", @call_stats);
printf("}\n");
}

Run the script using the following command line and feed the dtrace output to one of the graphviz graph generators such as dot or neato or circo.

$ dtrace -q -o graph.in -s ./callgraph.d -c /usr/bin/ls
$ dot -T jpg -o callgraph.jpg graph.in

Voila ! Here is the output:

Nice ! You know – Picture, 1024 words and all that stuff.

P.S: Only for the insanely curious

_start: This is the *real* entry point of any dynamically linked executable. This is the function that calls into main and does the atexit cleanup after main returns.

__fsr : This is the trap handling code for floating point. No idea why this is a named global. _start calls __fsr as function which is why the graph looks the way it does.

qsort: This is the solaris libc implementation of Quick Sort. The reason you see this as a disconnected subgraph is because the D script is tracing only functions from a.out (ls in this case). libc.so.1 functions are not traced. But they do get recorded because I look at the first level caller in the stack. And because qsort uses a callback to do the item comparision during sort, you get a graph edge that says

libc.so.1`qsort -> ls`compare

And you don’t record the edge that would have said

ls`pdirectory -> libc.so.1`qsort

So the link between and ls and libc.so.1 is cut-off. Of course, if you enable the probe for all libc.so.1 functions or all library functions and so on, you would get the “complete” picture. But that happens to be slightly overwhelming. Unsuitable for muggles who might chance to read this

Filed under: Open Source, Operating Systems, Programming, Solaris