Profilers identify bottlenecks in Perl programs

Not Without Side Effects

Just like any other profiler, Devel::NYTProf also involves some overhead that can completely falsify the measured results in some cases. The profiler's activities are particularly invasive if a program is not waiting for external events, such as network traffic or disk access, which are magnitudes of scale slower.

For a program running at full CPU speed, the run time can be 10 times longer if you enable the profiler.

Figure 7 shows the effect the NYT profiler has on the short l4ptest test program (Listing 4). The program configures Log4perl for the $INFO logging priority and then issues $DEBUG messages, which should be suppressed because of their lower priority. Log4perl has optimized this case; after all, a disabled logging system shouldn't have any measurable effect.

Figure 7: NYTProf can slow down fast programs – considerably in some cases.

Listing 4


#!/usr/local/bin/perl -w
use strict;
use Log::Log4perl qw(:easy);
for(1..100_000) {
    DEBUG "waah!";

The script running without the profiler achieves around 100,000 calls in about 100msec; the script run time is about 10 times longer with NYTProf enabled.

This by no means detracts from the quality of the profiler, but it is important to take this into consideration when measuring script run time and interpreting the results.

The module also is quite at home in the Apache server's mod_perl. Adding PerlModule Devel::NYTProf::Apache to the configuration loads the module and tells it to append profile data to the /tmp/nytprof.$$.out file for incoming requests, where $$ represents the PID for the Apache process handling the request.

Calling nytprofhtml again creates a collection of web pages for in-depth analysis of the web application's performance that could very well point you to critical code sections that need improvement.


  1. Listings for this article:
  2. Amazon Web Services (developer token application):
  3. Devel::NYTProf:
  4. Tim Bunce blog "NYTProf v2 – the background story,"
  5. Tim Bunce at OSCON 2008:

The Author

Michael Schilli works as a software engineer with Yahoo! in Sunnyvale, California. He is the author of Goto Perl 5 (German) and Perl Power (English), both published by Addison-Wesley, and he can be contacted at Michael's homepage is at

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