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Devel::NYTProf - Powerful feature-rich perl source code profiler


  # profile code and write database to ./nytprof.out
  perl -d:NYTProf

  # convert database into a set of html files, e.g., ./nytprof/index.html

  # or into comma seperated files, e.g., ./nytprof/*.csv


Devel::NYTProf is a powerful feature-rich perl source code profiler.

  • Performs per-line statement profiling for fine detail

  • Performs per-subroutine statement profiling for overview

  • Performs per-block statement profiling (the first profiler to do so)

  • Accounts correctly for time spent after calls return

  • Performs inclusive and exclusive timing of subroutines

  • Subroutine times are per calling location (a powerful feature)

  • Can profile compile-time activity, just run-time, or just END time

  • Uses novel techniques for efficient profiling

  • Sub-microsecond (100ns) resolution on systems with clock_gettime()

  • Very fast - the fastest statement and subroutine profilers for perl

  • Handles applications that fork, with no performance cost

  • Immune from noise caused by profiling overheads and I/O

  • Program being profiled can stop/start the profiler

  • Generates richly annotated and cross-linked html reports

  • Trivial to use with mod_perl - add one line to httpd.conf

  • Includes an extensive test suite

  • Tested on very large codebases

NYTProf is effectively two profilers in one: a statement profiler, and a subroutine profiler.

Statement Profiling

The statement profiler measures the time between entering one perl statement and entering the next. Whenever execution reaches a new statement, the time since entering the previous statement is calculated and added to the time associated with the line of the source file that the previous statement starts on.

By default the statement profiler also determines the first line of the current block and the first line of the current statement, and accumulates times associated with those. NYTProf is the only Perl profiler to perform block level profiling.

Another innovation unique to NYTProf is automatic compensation for a problem inherent in simplistic statement-to-statement timing. Consider a statement that calls a subroutine and then performs some other work that doesn't execute new statements, for example:

  foo(...) + mkdir(...);

In all other statement profilers the time spent in remainder of the expression (mkdir in the example) will be recorded as having been spent on the last statement executed in foo()! Here's another example:

  while (<>) {

After the first time around the loop, any further time spent evaluating the condition (waiting for input in this example) would be be recorded as having been spent on the last statement executed in the loop!

NYTProf avoids these problems by intercepting the opcodes which indicate that control is returning into some previous statement and adjusting the profile accordingly.

The statement profiler naturally generates a lot of data which is streamed out to a file in a very compact format. NYTProf takes care to not include the measurement and writing overheads in the profile times (some profilers produce 'noisy' data due to periodic stdio flushing).

Subroutine Profiling

The subroutine profiler measures the time between entering a subroutine and leaving it. It then increments a call count and accumulates the duration. For each subroutine called, separate counts and durations are stored for each location that called the subroutine.

Subroutine entry is detected by intercepting the entersub opcode. Subroutine exit is detected via perl's internal save stack. The result is both extremely fast and very robust.

Subroutine Recursion

For subroutines that recurse directly or indirectly, such as Error::try, the inclusive time is only measured for the outer-most call.

The inclusive times of recursive calls are still measured and are accumulated separately. Also the 'maximum recursion depth' per calling location is recorded.

Application Profiling

NYTProf records extra information in the data file to capture details that may be useful when analysing the performance. It also records the filename and line ranges of all the subroutines.

NYTProf can profile applications that fork, and does so with no loss of performance. There's (now) no special 'allowfork' mode. It just works. NYTProf detects the fork and starts writing a new profile file with the pid appended to the filename.

Fast Profiling

The NYTProf profiler is written almost entirely in C and great care has been taken to ensure it's very efficient.

Apache Profiling

Just add one line near the start of your httpd.conf file:

  PerlModule Devel::NYTProf::Apache

By default you'll get a /tmp/nytprof.$$.out file for the parent process and a /tmp/nytprof.$parent.out.$$ file for each worker process.

NYTProf takes care to detect when control is returning back from perl to mod_perl so time spent in mod_perl (such as waiting for the next request) does not get allocated to the last statement executed.

Works with mod_perl 1 and 2. See Devel::NYTProf::Apache for more information.


Usually you'd load Devel::NYTProf on the command line using the perl -d option:

  perl -d:NYTProf

To save typing the ':NYTProf' you could set the PERL5DB env var

  PERL5DB='use Devel::NYTProf'

and then just perl -d would work:

  perl -d

Or you can avoid the need to add the -d option at all by using the PERL5OPT env var:


That's also very handy when you can't alter the perl command line being used to run the script you want to profile.


The behavior of Devel::NYTProf may be modified by setting the environment variable NYTPROF. It is possible to use this environment variable to effect multiple setting by separating the values with a :. For example:

  export NYTPROF=trace=2:start=init:file=/tmp/nytprof.out


Append the current process id to the end of the filename.

This avoids concurrent, or consecutive, processes from overwriting the same file.


Set trace level to N. 0 is off (the default). Higher values cause more detailed trace output.


Specify at which phase of program execution the profiler should be enabled:

  start=begin - start immediately (the default)
  start=init  - start at begining of INIT phase (after compilation)
  start=end   - start at begining of END phase
  start=no    - don't automatically start

The start=no option is handy if you want to explicitly control profiling by calling DB::enable_profile() and DB::disable_profile() yourself.


Disable the perl optimizer.

By default NYTProf leaves perl's optimizer enabled. That gives you more accurate profile timing overall, but can lead to odd statement counts for individual sets of lines. That's because the perl's peephole optimizer has effectively rewritten the statements but you can't see what the rewritten version looks like.

For example:

  1     if (...) {
  2         return;
  3     }

may be rewritten as

  1    return if (...)

so the profile won't show a statement count for line 2 in your source code because the return was merged into the if statement on the preceeding line.

Using the optimize=0 option disables the optimizer so you'll get lower overall performance but more accurately assigned statement counts.

If you find any other examples of the effect of optimizer on NYTProf output (other than performance, obviously) please let us know.


Set to 0 to disable the collection of subroutine caller and timing details.


Set to 0 to disable the determination of block and subroutine location per statement. This makes the profiler about 50% faster (as of July 2008) and produces smaller output files, but you loose some valuable information. The extra cost is likely to be reduced in later versions anyway, as little optimization has been done on that part of the code.


Set to 0 to disable the statement profiler. (Implies blocks=0.) The reports won't contain any statement timing detail.

This significantly reduces the overhead of the profiler and can also be useful for profiling large applications that would normally generate a very large profile data file.


Set to 0 to disable the extra work done to allocate times accurately when returning into the middle of statement. For example leaving a subroutine and returning into the middle of statement, or re-evaluting a loop condition.

This feature also ensures that in embedded environments, such as mod_perl, the last statement executed doesn't accumulate the time spent 'outside perl'.

NYTProf is the only line-level profiler to measure these times correctly. The profiler is fast enough that you shouldn't need to disable this feature.


Set to 1 to enable use of the traditional DB::DB() subroutine to perform profiling, instead of the faster 'opcode redirection' technique that's used by default. Also effectively sets leave=0 (see above).

The default 'opcode redirection' technique can't profile subroutines that were compiled before NYTProf was loaded. So using use_db_sub=1 can be useful in cases where you can't load the profiler early in the life of the application.


Save a copy of all source code into the profile data file. This makes the file self-contained, so the reporting tools no longer depend on having the original source code files available. So it also insulates you from later changes to those files that would normally make the reports out of sync with the data.

By default NYTProf saved some source code: the arguments to the perl -e option, the script fed to perl via STDIN when using perl -, and the source code of string evals. (Currently string eval source code isn't available in the reports. Patches welcome.)

If you're using perl 5.10.0 or 5.8.8 (or earlier) then you need to also enable the use_db_sub=1 option otherwise perl doesn't make the source code available to NYTProf. Perl 5.8.9 and 5.10.1+ don't require that.


Measure user CPU + system CPU time instead of the real elapsed 'wall clock' time (which is the default).

Measuring CPU time has the advantage of making the measurements independant of time spent blocked waiting for the cpu or network i/o etc. But it also has the severe disadvantage of having typically far less accurate timings.

Most systems use a 0.01 second granularity. With modern processors having multi- gigahertz clocks, 0.01 seconds is like a lifetime. The cpu time clock 'ticks' happen so rarely relative to the activity of a most applications that you'd have to run the code for many hours to have any hope of reasonably useful results.

A better alternative would be to use the clock=N option to select a high-resolution cpu time clock, if available on your system.


Specify the output file to write profile data to (default: './nytprof.out').


Specify the compression level to use, if NYTProf is compiled with compression support. Valid values are 0 to 9, with 0 disabling compression. The default is 6 as higher values yield little extra compression but the cpu cost starts to rise significantly. Using level 1 still gives you a significant reduction in file size.

If NYTProf was not compiled with compression support, this option is silently ignored.


Systems which support the clock_gettime() system call typically support several clocks. By default NYTProf uses CLOCK_MONOTONIC.

This option enables you to select a different clock by specifying the integer id of the clock (which may vary between operating system types). If the clock you select isn't available then CLOCK_REALTIME is used.

See "CLOCKS" for more information.


You can profile only parts of an application by calling DB::disable_profile() to stop collecting profile data, and calling DB::enable_profile() to start collecting profile data.

Using the start=no option lets you leave the profiler disabled initially until you call DB::enable_profile() at the right moment.

You can finish profiling completely by calling DB::finish_profile(). This may be useful if perl is exiting abnormally, leaving the profile data file in an incomplete state,

Multiple Output Files

You can pass a filename argument to DB::enable_profile() to make NYTProf write future profile data to that file. The current output file, if any, is closed. Any existing file with the new name will be deleted before being written to. When combined with DB::disable_profile() this lets you profile individual sections of code.


The Devel::NYTProf::Data module provides a low-level interface for loading the profile data.

The Devel::NYTProf::Reader module provides an interface for generating arbitrary reports. This means that you can implement your own output format in perl. (Though the module is in a state of flux and may be deprecated soon.)

Included in the bin directory of this distribution are two scripts which implement the Devel::NYTProf::Reader interface:


creates comma delimited profile reports


creates attractive, richly annotated, and fully cross-linked html reports (including statistics, source code and color highlighting)


Here we discuss the way NYTProf gets high-resolution timing information from your system and related issues.

POSIX Clocks

These are the clocks that your system may support if it supports the POSIX clock_gettime() function. Other clock sources are listed in the "Other Clocks" section below.

The clock_gettime() interface allows clocks to return times to nanosecond precision. Of course few offer nanosecond accuracy but the extra precision helps reduce the cumulative error that naturally occurs when adding together many timings. When using these clocks NYTProf outputs timings as a count of 100 nanosecond ticks.


CLOCK_REALTIME is typically the system's main high resolution 'wall clock time' source. The same source as used for the gettimeofday() call used by most kinds of perl benchmarking and profiling tools.

If your system doesn't support clock_gettime() then NYTProf will use gettimeofday(), or the nearest equivalent,

The problem with real time is that it's far from simple. It tends to drift and then be reset to match 'reality', either sharply or by small adjustments (via the adjtime() system call).

Surprizingly, it can also go backwards, for reasons explained in


CLOCK_MONOTONIC rrepresents the amount of time since an unspecified point in the past (typically system start-up time). It increments uniformally independent of adjustments to 'wallclock time'.


CLOCK_VIRTUAL increments only when the CPU is running in user mode on behalf of the calling process.


CLOCK_PROF increments when the CPU is running in user or kernel mode.


CLOCK_PROCESS_CPUTIME_ID represents the amount of execution time of the process associated with the clock.


CLOCK_THREAD_CPUTIME_ID represents the amount of execution time of the thread associated with the clock.

Finding Available POSIX Clocks

On unix-like systems you can find the CLOCK_* clocks available on you system using a command like:

  grep -r 'define *CLOCK_' /usr/include

Look for a group that includes CLOCK_REALTIME. The integer values listed are the clock ids that you can use with the clock=N option.

A future version of NYTProf should be able to list the supported clocks.

Other Clocks

This section lists other clock sources that NYTProf may use.


This is the traditional high resolution time of day interface for most unix-like systems. It's used on platforms like Mac OS X which don't (yet) support clock_gettime().

With this clock NYTProf outputs timings as a count of 1 microsecond ticks.


On systems which don't support clock_gettime() or gettimeofday() NYTProf falls back to using the Time::HiRes module. With this clock NYTProf outputs timings as a count of 1 microsecond ticks.

Clock References

Relevant specifications and manual pages:

Why 'realtime' can appear to go backwards:



Devel::NYTProf is not currently thread safe. If you'd be interested in helping to make it thread safe then please get in touch with us.

For perl < 5.8.8 it may change what caller() returns

For example, the Readonly module croaks with "Invalid tie" when profiled with perl versions before 5.8.8. That's because Readonly explicitly checking for certain values from caller(). The NEXT module is also affected.

For perl < 5.10.1 it can't see calls made via operator overloading

For perl versions prior to 5.8.9 and 5.10.1, calls made via operator overloading are not noticed by the subroutine profiler. Though the statements executed by the code in the overload subs are profiled.

The effect is that time in the subroutines that handle overloading is accumulated by any subroutines that trigger overloading (so it is measured, but the cost is dispersed across possibly many calling locations).


The goto &foo; isn't recognised as a subroutine call by the subroutine profiler.

#line directives

The reporting code currently doesn't handle #line directives, but at least it warns about them. Patches welcome.


SMP Systems

Systems with multiple processors, which includes most modern machines, have

From Linux docs (though applicable to most SMP systems):

  many platforms using timers from the CPUs (TSC on i386, AR.ITC on Itanium).
  These registers may differ between CPUs and as a consequence these clocks may
  return bogus results if a process is migrated to another CPU.

  If the CPUs in an SMP system have different clock sources then there is no way
  to maintain a correlation between the timer registers since each CPU will run
  at a slightly different frequency. If that is the case then
  clock_getcpuclockid(0) will return ENOENT to signify this condition. The two
  clocks will then only be useful if it can be ensured that a process stays on a
  certain CPU.

  The processors in an SMP system do not start all at exactly the same time and
  therefore the timer registers are typically running at an offset. Some
  architectures include code that attempts to limit these offsets on bootup.
  However, the code cannot guarantee to accurately tune the offsets. Glibc
  contains no provisions to deal with these offsets (unlike the Linux Kernel).
  Typically these offsets are small and therefore the effects may be negligible
  in most cases.

In summary, SMP systems are likely to give 'noisy' profiles. Setting a "Processor Affinity" may help.

Processor Affinity

Processor affinity is an aspect of task scheduling on SMP systems. "Processor affinity takes advantage of the fact that some remnants of a process may remain in one processor's state (in particular, in its cache) from the last time the process ran, and so scheduling it to run on the same processor the next time could result in the process running more efficiently than if it were to run on another processor." (From

Setting an explicit processor affinity can avoid the problems described in "SMP Systems".

Processor affinity can be set using the taskset command on Linux.

Future versions of NYTProf could support setting processor affinity automatically (e.g. via sched_setaffinity() on Linux). Patches welcome!

Note that processor affinity is inherited by child processes, so if the process you're profiling spawns cpu intensive sub processes then your process will be impacted by those more than it otherwise would.

Virtual Machines

I recommend you don't do performance profiling while running in a virtual machine. If you do you're likely to find inexplicable spikes of real-time appearing at unreasonable places in your code. You should pay less attention to the statement timings and rely more on the subroutine timings. They will still be noisy but less so than the statement times.

You could also try using the clock=N option to select a high-resolution cpu-time clock instead of a real-time one. That should be much less noisy, though you will loose visibility of wait-times due to network and disk I/O, for example.

If your system doesn't support the clock=N option then you could try using the usecputime=1 option. That will give you cpu-time measurements but only at a very low 1/100th of a second resolution.




Screenshots of nytprofhtml v2.01 reports can be seen at and A writeup of the new features of NYTProf v2 can be found at and the background story, explaining the "why", can be found at

Mailing list and discussion at

Blog posts and

Public SVN Repository and hacking instructions at

nytprofhtml is a script included that produces html reports. nytprofcsv is another script included that produces plain text CSV reports.

Devel::NYTProf::Reader is the module that powers the report scripts. You might want to check this out if you plan to implement a custom report (though it's very likely to be deprecated in a future release).

Devel::NYTProf::ReadStream is the module that lets you read a profile data file as a stream of chunks of data.


Adam Kaplan, <akaplan at>. Tim Bunce, and Steve Peters, <steve at>.


  Copyright (C) 2008 by Adam Kaplan and The New York Times Company.
  Copyright (C) 2008 by Tim Bunce, Ireland.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or, at your option, any later version of Perl 5 you may have available.


A bit of history and a shameless plug...

NYTProf stands for 'New York Times Profiler'. Indeed, this module was initially developed from Devel::FastProf by The New York Times Co. to help our developers quickly identify bottlenecks in large Perl applications. The NY Times loves Perl and we hope the community will benefit from our work as much as we have from theirs.

Please visit, our open source blog to see what we are up to, to see some of our open projects and then check out for the latest news!


Subroutine-level profilers:

  Devel::DProf        | 1995-10-31 | ILYAZ
  Devel::AutoProfiler | 2002-04-07 | GSLONDON
  Devel::Profiler     | 2002-05-20 | SAMTREGAR
  Devel::Profile      | 2003-04-13 | JAW
  Devel::DProfLB      | 2006-05-11 | JAW
  Devel::WxProf       | 2008-04-14 | MKUTTER

Statement-level profilers:

  Devel::SmallProf    | 1997-07-30 | ASHTED
  Devel::FastProf     | 2005-09-20 | SALVA
  Devel::NYTProf      | 2008-03-04 | AKAPLAN
  Devel::Profit       | 2008-05-19 | LBROCARD

Devel::NYTProf is a (now distant) fork of Devel::FastProf, which was itself an evolution of Devel::SmallProf.

Adam Kaplan took Devel::FastProf and added html report generation (based on Devel::Cover) and a test suite - a tricky thing to do for a profiler. Meanwhile Tim Bunce had been extending Devel::FastProf to add novel per-sub and per-block timing, plus subroutine caller tracking.

When Devel::NYTProf was released Tim switched to working on Devel::NYTProf because the html report would be a good way to show the extra profile data, and the test suite made development much easier and safer.

Then he went a little crazy and added a slew of new features, in addition to per-sub and per-block timing and subroutine caller tracking. These included the 'opcode interception' method of profiling, ultra-fast and robust inclusive subroutine timing, doubling performance, plus major changes to html reporting to display all the extra profile call and timing data in richly annotated and cross-linked reports.

Steve Peters came on board along the way with patches for portability and to keep NYTProf working with the latest development perl versions. Nicholas Clark added zip compression. Jan Dubois contributed Windows support.

Adam's work is sponsored by The New York Times Co. Tim's work was partly sponsored by Shopzilla.