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Talk: dtrace, Python and You

Speaker: Mark Allen / [email protected] / @bytemeorg / http://byte-me.org / github:mrallen1

What is DTrace?

Focused on dynamic tracing (as opposed to static tracing). No more print "got here!"!

DTrace can help you examine everything from application code all the way down to the kernel.

Examples:

  • Trace socket accepts and debug your web application's "real" code execution pathway
  • Profile why NFS gets super busy at 0430 and a directory tree traversal takes 2 hours instead of 2 minutes

Use dtrace with a profiler, and with a debugger, not instead of one. Dtrace only runs on a few OSes, including FreeBSD and OSX (as of mountain lion). What about linux? There's a dtrace4linux, but only supports kernel-level tracing. Don't install the dtrace you find for Ubuntu… it's not safe in a production environment.

DTrace Terms

  • Provider: manages probes in a subsystem, ex. the python binary itself
  • Probes: DTrace instrumentation or "observables" [?]
  • Consumer: a user-mode program that calls into DTrace, ex. a D script

Dtrace has it's own scripting language called "D".

D Language

An awk-like scripting language. Define a probe, optional predicate, and optional actions. Supports BEGIN and END blocks, local variables, aggregate/associative variables (prefixed with '@').

dtrace -n 'probe /predicate/ {action}'

    $ cat subentry.d
    
    #!/usr/sbin/dtrace -qFZs
    
    function-entry,
    function-return
    {
        /* arg1 is a subroutine name */
        trace(copyinstr(arg1))
    }

function-entry and function-return are python-specific probes.

    function-entry
    {
        /* Count sub entries by package and sub name
         * arg0 = source file name
         * arg1 = function name */
        @[strjoin(strjoin(copyinstr(arg0), "-"), copinstr(arg1))] = count()
    }
    
    END
    {
        /* Give me the top 10 highest counts, throw away rest */
        trunc(@, 10)
    }

Aggregate Functions

avg, count, sum, min, max, lquantize (linear), and quantize (log - power of 2)

These operate on what the '@' symbol is in front of. The quantize functions give you histograms so you can see outliers, which avg wouldn't give you.

DTrace and Python

Python is one of the few programming languages that doesn't come with dtrace support out of the box. So you have to brew install python --with-dtrace.

Python Probes

  • function-entry
  • function-return
  • line (FILE, FUNCTION, LINE)
  • gc-start (GENERATION) (similar to gc.collect())
  • gc-done (OBJECTS-COLLECTED)
  • instance-new-start, instance-new-done, instance-delete-start, instance-delete-done (CLASS_NAME, FILE)

in parentheses is args for that probe, i.e. (arg0, …)

You can make your own DTrace probes in Python by using: http://tmetsch.github.io/python-dtrace (also avail. on PyPI).

Resources