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Devel::NYTProf - Powerful fast 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
  # and open a web browser on the nytprof/index.html file
  nytprofhtml --open

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

A screencast about profiling perl code, including a detailed look at how to use NYTProf and how to optimize your code, is available at


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

  • Performs per-line statement profiling for fine detail

  • Performs per-subroutine statement profiling for overview

  • Performs per-opcode profiling for slow perl builtins

  • 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 supported systems

  • 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

  • Captures source code, including string evals, for stable results

  • 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.

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! (Until perl bug #60954 is fixed this problem still applies to some loops. For more information see

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.

Goto &Subroutine

Perl implements a goto &destination as a return followed by a call to &destination, so that's how it will appear in the report.

The goto will be shown with a very short time because it's effectively just a return. The &destination sub will show a call not from the location of the goto but from the location of the call to the sub that performed the goto.


The perl built-in accept() function waits listening for a connection on a socket, and so is a key part of pure-perl network service applications.

The time spent waiting for a remotely initiated connection can be relatively high but is not relevant to the performance of the application. So the accept() function is treated as a special case. The subroutine profiler discounts the time spent in the accept() function. It does this in a way that also discounts that time from all the callers up the call stack. The effect on the reports is that all accept() calls appear to be instant.

The statement profiler still shows the time actually spent in the statement that executed the accept() call.

Application Profiling

NYTProf records extra information in the data file to capture details that may be useful when analyzing 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. 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. Usually you'll want to enable the "addpid=1" option to ensure any nested invocations of perl don't overwrite the 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

Any colon or equal characters in a value can be escaped by preceding them with a backslash.


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. Trace output is written to STDERR or wherever the "log=F" option has specified.


Specify the name of the file that "trace=N" output should be written to.


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

  start=begin - start immediately (the default)
  start=init  - start at beginning of INIT phase (after compilation)
  start=end   - start at beginning 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 preceding line.

Also 'empty' statements like 1; are removed entirely. Such statements are empty because the optimizer has already removed the pointless constant in void context. It then goes on to remove the now empty statement (in perl >= 5.13.7).

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 lose 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 by the statement profiler 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-evaluating 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'.


Force NYTProf to recalculate the name of the caller of the each sub instead of 'inheriting' the name calculated when the caller was entered. (Rarely needed, but might be useful in some odd cases.)


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.

Another side effect of use_db_sub=1 is that it enables recording of the source code of the perl -e '...' and perl - input for old versions of perl. See also "savesrc=0".


Disable the saving of source code.

By default NYTProf saves 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 unmodified source code files available.

With savesrc=0 some source code is still saved: the arguments to the perl -e option, the script fed to perl via STDIN when using perl -, and the source code of string evals.

Saving the source code of string evals requires perl version 5.8.9+, 5.10.1+, or 5.12 or later.

Saving the source code of the perl -e '...' or perl - input requires either a recent perl version, as above, or setting the "use_db_sub=1" option.


Profile perl opcodes that can be slow. These include opcodes that make system calls, such as print, read, sysread, socket etc., plus regular expression opcodes like subst and match.

If N is 0 then slowops profiling is disabled.

If N is 1 then all the builtins are treated as being defined in the CORE package. So times for print calls from anywhere in your code are merged and accounted for as calls to an xsub called CORE::print.

If N is 2 (the default) then builtins are treated as being defined in the package that calls them. So calls to print from package Foo are treated as calls to an xsub called Foo::CORE:print. Note the single colon after CORE.

The opcodes are currently profiled using their internal names, so printf is prtf and the -x file test is fteexec. This may change in future.

Opcodes that call subroutines, perhaps by triggering a FETCH from a tied variable, currently appear in the call tree as the caller of the sub. This is likely to change in future.


Measure user CPU + system CPU time instead of the real elapsed 'wall clock' time (which is the default). But there are better ways to do this, read on.

Measuring CPU time has the advantage of making the measurements independent of time spent blocked waiting for the cpu or network i/o etc. But the method used by Cusecputime=1> also has the severe disadvantage of having typically far less accurate timings.

Most systems have a 0.01 second granularity in the results from the times() sytem call. With modern processors having multi- gigahertz clocks, 0.01 seconds is like a lifetime. The 'ticks' of this CPU time clock 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 much better alternative is to use the "clock=N" option to select a high-resolution CPU time clock, if available on your system, because that'll give you higher resolution and work for the subroutine profiler as well.


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.


When perl exits normally it runs any code defined in END blocks. NYTProf defines an END block that finishes profiling and writes out the final profile data.

If the process ends due to a signal then END blocks are not executed. The sigexit option tells NYTProf to catch some signals (e.g. INT, HUP, PIPE, SEGV, BUS) and ensure a usable by executing:

    exit 1;

You can also specify which signals to catch in this way by listing them, separated by commas, as the value of the option (case is not significant):



The NYTProf subroutine profiler normally detects calls to POSIX::_exit() (which exits the process without running END blocks) and automatically calls DB::finish_profile() for you, so NYTProf 'just works'.

When using the subs=0 option to disable the subroutine profiler the posix_exit option can be used to tell NYTProf to take other steps to arrange for DB::finish_profile() to be called before POSIX::_exit().


When a perl process that is being profiled executes a fork() the child process is also profiled. The forkdepth option can be used to control this. If forkdepth is zero then profiling will be disabled in the child process.

If forkdepth is greater than zero then profiling will be enabled in the child process and the forkdepth value in that process is decremented by one.

If forkdepth is -1 (the default) then there's no limit on the number of generations of children that are profiled.


The 'file name' of a string eval is normally a string like "(eval N)", where N is a sequence number. By default NYTProf asks perl to give evals more informative names like "(eval N)[file:line]", where file and line are the file and line number where the string eval was executed.

The nameevals=0 option can be used to disable the more informative names and return to the default behaviour. This may be need in rare cases where the application code is sensitive to the name given to a eval. (The most common case in when running test suites undef NYTProf.)

The downside is that the NYTProf reporting tools are less useful and may get confused if this option is used.


The name of a anonymous subroutine is normally "__ANON__". By default NYTProf asks perl to give anonymous subroutines more informative names like "__ANON__[file:line]", where file and line are the file and line number where the anonymous subroutine was defined.

The nameanonsubs=0 option can be used to disable the more informative names and return to the default behaviour. This may be need in rare cases where the application code is sensitive to the name given to a anonymous subroutines. (The most common case in when running test suites undef NYTProf.)

The downside is that the NYTProf reporting tools are less useful and may get confused if this option is used.


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.

The profile output file can't be used until it's been properly completed and closed. Calling DB::disable_profile() doesn't do that. To make a profile file usable before the profiled application has completed you can call DB::finish_profile(). Alternatively you could call DB::enable_profile($newfile).



Stops collection of profile data.

Subroutine calls which were made while profiling was enabled and are still on the call stack (have not yet exited) will still have their profile data collected when they exit.



Enables collection of profile data. If $newfile is true the profile data will be written to $newfile (after completing and closing the previous file, if any). If $newfile already exists it will be deleted first.



Calls DB::disable_profile(), then completes the profile data file by writing subroutine profile data, and then closes the file. The in memory subroutine profile data is then discarded.

Normally NYTProf arranges to call finish_profile() for you via an END block.


NYTProf tries very hard to gather accurate information. The nature of the internals of perl mean that, in some cases, the information that's gathered is accurate but surprising. In some cases it can appear to be misleading. (Of course, in some cases it may actually be plain wrong. Caveat lector.)

If Statement and Subroutine Timings Don't Match

NYTProf has two profilers: a statement profiler that's invoked when perl moves from one perl statement to another, and a subroutine profiler that's invoked when perl calls or returns from a subroutine.

The individual statement timings for a subroutine usually add up to slightly less than the exclusive time for the subroutine. That's because the handling of the subroutine call and return overheads is included in the exclusive time for the subroutine. The difference may only be a new microseconds but that may become noticeable for subroutines that are called hundreds of thousands of times.

The statement profiler keeps track how much time was spent on overheads, like writing statement profile data to disk. The subroutine profiler subtracts the overheads that have accumulated between entering and leaving the subroutine in order to give a more accurate profile. The statement profiler is generally very fast because most writes get buffered for zip compression so the profiler overhead per statement tends to be very small, often a single 'tick'. The result is that the accumulated overhead is quite noisy. This becomes more significant for subroutines that are called frequently and are also fast. This may be another, smaller, contribution to the discrepancy between statement time and exclusive times.

If Headline Subroutine Timings Don't Match the Called Subs

Overall subroutine times are reported with a headline like spent 10s (2+8) within .... In this example, 10 seconds were spent inside the subroutine (the "inclusive time") and, of that, 8 seconds were spent in subroutines called by this one. That leaves 2 seconds as the time spent in the subroutine code itself (the "exclusive time", sometimes also called the "self time").

The report shows the source code of the subroutine. Lines that make calls to other subroutines are annotated with details of the time spent in those calls.

Sometimes the sum of the times for calls made by the lines of code in the subroutine is less than the inclusive-exclusive time reported in the headline (10-2 = 8 seconds in the example above).

What's happening here is that calls to other subroutines are being made but NYTProf isn't able to determine the calling location correctly so the calls don't appear in the report in the correct place.

Using an old version of perl is one cause (see below). Another is calling subroutines that exit via goto &sub; - most frequently encountered in AUTOLOAD subs and code using the Memoize module.

In general the overall subroutine timing is accurate and should be trusted more than the sum of statement or nested sub call timings.

These versions of perl yield much more detailed information about calls to BEGIN, CHECK, INIT, and END blocks, the code handling tied or overloaded variables, and callbacks from XS code.

Perl 5.12 will hopefully also fix an inaccuracy in the timing of the last statement and the condition clause of some kinds of loops:

eval $string

Perl treats each execution of a string eval (eval $string; not eval { ... }) as a distinct file, so NYTProf does as well. The 'files' are given names with this structure:

        (eval $sequence)[$filename:$line]

for example "(eval 93)[/foo/]" would be the name given to the 93rd execution of a string eval by that process and, in this case, the 93rd eval happened to be one at line 42 of "/foo/".

Nested string evals can give rise to file names like

        (eval 1047)[(eval 93)[/foo/]:17]

Merging Evals

Some applications execute a great many string eval statements. If NYTProf generated a report page for each one it would not only slow report generation but also make the overall report less useful by scattering performance data too widely. On the other hand, being able to see the actual source code executed by an eval, along with the timing details, is often very useful.

To try to balance these conflicting needs, NYTProf currently merges uninteresting string eval siblings.

What does that mean? Well, for each source code line that executed any string evals, NYTProf first gathers the corresponding eval 'files' for that line (known as the 'siblings') into groups keyed by distinct source code.

Then, for each of those groups of siblings, NYTProf will 'merge' a group that shares the same source code and doesn't execute any string evals itself. Merging means to pick one sibling as the survivor and merge and delete all the data from the others into it.

If there are a large number of sibling groups then the data for all of them are merged into one regardless.

The report annotations will indicate when evals have been merged together.

Merging Anonymous Subroutines

Anonymous subroutines defined within string evals have names like this:

        main::__ANON__[(eval 75)[/foo/]:12]

That anonymous subroutine was defined on line 12 of the source code executed by the string eval on line 42 of /foo/ That was the 75th string eval executed by the program.

Anonymous subroutines defined on the same line of sibling evals that get merged are also merged. That is, the profile information is merged into one and the others are discarded.


Care should be taken when interpreting the report annotations associated with a string eval statement. Normally the report annotations embedded into the source code related to timings from the subroutine profiler. This isn't (currently) true of annotations for string eval statements.

This makes a significant different if the eval defines any subroutines that get called after the eval has returned. Because the time shown for a string eval is based on the statement times it will include time spent executing statements within the subs defined by the eval.

In future NYTProf may involve the subroutine profiler in timings evals and so be able to avoid this issue.

Calls from XSUBs and Opcodes

Calls record the current filename and line number of the perl code at the time the call was made. That's fine and accurate for calls from perl code. For calls that originate from C code however, such as an XSUB or an opcode, the filename and line number recorded are still those of the last perl statement executed.

For example, a line that calls an xsub will appear in reports to also have also called any subroutines that that xsub called. This can be construed as a feature.

As an extreme example, the first time a regular expression that uses character classes is executed on a unicode string you'll find profile data like this:

      # spent 1ms within main::BEGIN@4 which was called
      #    once (1ms+0s) by main::CORE:subst at line 0
  4   s/ (?: [A-Z] | [\d] )+ (?= [\s] ) //x;
      # spent  38.8ms making 1 call to main::CORE:subst
      # spent  25.4ms making 2 calls to utf8::SWASHNEW, avg 12.7ms/call
      # spent  12.4ms making 1 call to utf8::AUTOLOAD


You can reduce the cost of profiling by adjusting some options. The trade-off is reduced detail and/or accuracy in reports.

If you don't need statement-level profiling then you can disable it via "stmts=0". If you do want it but don't mind loosing block-level timings then set "blocks=0". To further boost statement-level profiling performance try "leave=0" but note that will apportion timings for some kinds of statements less accurate).

If you don't need subroutine profiling then you can disable it via "subs=0". If you do need it but don't need timings for perl opcodes then set "slowops=0".

Generally speaking, setting blocks=0 and slowops=0 will give you a useful boost with the least loss of detail.

Another approach is to only enable NYTProf in the sections of code that interest you. See "RUN-TIME CONTROL OF PROFILING" for more details.

To speed up nytprofhtml try using the --minimal (-m) option.


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 some scripts which turn the raw profile data into more useful formats:


Creates attractive, richly annotated, and fully cross-linked html reports (including statistics, source code and color highlighting). This is the main report generation tool for NYTProf.


Creates comma delimited profile reports. Old and limited.


Translates a profile into a format that can be loaded into KCachegrind


Reads multiple profile data files and writes out a new file containing the merged profile data.


Threads and Multiplicity

Devel::NYTProf is not currently thread safe or multiplicity safe. If you'd be interested in helping to fix that then please get in touch with us. Meanwhile, profiling is disabled when a thread is created, and NYTProf tries to ignore any activity from perl interpreters other than the first one that loaded it.


The Devel::NYTProf subroutine profiler gets confused by the stack gymnastics performed by the Coro module and aborts. When profiling applications that use Coro you should disable the subroutine profiler using the "subs=0" option.

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 some implicit calls and callbacks

For perl versions prior to 5.8.9 and 5.10.1, some implicit subroutine calls can't be seen by the subroutine profiler. Technically this affects calls made via the various perl call_*() internal APIs.

For example, BEGIN/CHECK/INIT/END blocks, the TIEwhatever subroutine called by tie(), all calls made via operator overloading, and callbacks from XS code, are not seen.

The effect is that time in those subroutines is accumulated by the subs that triggered the call to them. So time spent in calls invoked by perl to handle overloading are accumulated by the subroutines that trigger overloading (so it is measured, but the cost is dispersed across possibly many calling locations).

Although the calls aren't seen by the subroutine profiler, the individual statements executed by the code in the called subs are profiled by the statement profiler.

#line directives

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

Freed values in @_ may be mutated

Perl has a class of bugs related to the fact that values placed in the stack are not reference counted. Consider this example:

  @a = (1..9);  sub s { undef @a; print $_ for @_ }  s(@a);

The undef @a frees the values that @_ refers to. Perl can sometimes detect when a freed value is accessed and treats it as an undef. However, if the freed value is assigned some new value then @_ is effectively corrupted.

NYTProf allocates new values while it's profiling, in order to record program activity, and so may appear to corrupt @_ in this (rare) situation. If this happens, NYTProf is simply exposing an existing problem in the code.

Lvalue subroutines aren't profiled when using use_db_sub=1

Currently 'lvalue' subroutines (subs that can be assigned to, like foo() = 42) are not profiled when using use_db_sub=1.


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_MONOTONIC represents the amount of time since an unspecified point in the past (typically system start-up time). It increments uniformly independent of adjustments to 'wallclock time'. NYTProf will use this clock by default, if available.


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.

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).

Surprisingly, it can also go backwards, for reasons explained in so CLOCK_MONOTONIC is preferred.


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.

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


This is the traditional high resolution time of day interface for most unix-like systems. With this clock NYTProf outputs timings as a count of 1 microsecond ticks.


On Mac OS X the mach_absolute_time() function is used. With this clock NYTProf outputs timings as a count of 100 nanosecond ticks.


On systems which don't support other clocks, 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:


SMP Systems

On systems with multiple processors, which includes most modern machines, (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.

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.


On Windows NYTProf uses Time::HiRes which uses the windows QueryPerformanceCounter() API with some extra logic to adjust for the current clock speed and try to resync the raw counter to wallclock time every so often (every 30 seconds or if the timer drifts by more than 0.5 of a seconds). This extra logic may lead to occasional spurious results.

(It would be great if someone could contribute a patch to NYTProf to use QueryPerformanceCounter() directly and avoid the overheads and resyncing behaviour of Time::HiRes.)

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 lose 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.


Possibly. All complex software has bugs. Let me know if you find one.


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

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.


"Profile data incomplete, ..."

This error message means the file doesn't contain all the expected data. That may be because it was truncated (perhaps the filesystem was full) or, more commonly, because the all the expected data hasn't been written.

NYTProf writes some important data to the data file when finishing profiling. If you read the file before the profiling has finished you'll get this error.

If the process being profiled is still running you'll need to wait until it exits cleanly (runs END blocks or "finish_profile" is called explicitly).

If the process being profiled has exited then it's likely that it met with a sudden and unnatural death that didn't give NYTProf a chance to finish the profile. If the sudden death was due to a signal then "sigexit=1" may help. If the sudden death was due to calling POSIX::_exit($status) then you'll need to call "finish_profile" before calling POSIX::_exit.

Some files don't have profile information

This is usually due to NYTProf being loaded after the other files, for example

If you can't alter the command line to add "-d:NYTProf" you could try using the PERL5OPT environment variable. See "PROFILING".


Tim Bunce ( and leads the project and has done most of the development work thus far.

Nicholas Clark contributed zip compression and nytprofmerge. Chia-liang Kao contributed nytprofcg. Peter (Stig) Edwards contributed the VMS port. Jan Dubois contributed the Windows port. Gisle Aas contributed the Devel::NYTProf::ReadStream module. Steve Peters contributed greater perl version portability and use of POSIX high-resolution clocks. Other contributors are noted in the Changes file.

Many thanks to Adam Kaplan who created NYTProf initially by forking Devel::FastProf adding reporting from Devel::Cover and a test suite. For more details see "HISTORY" below.


  Copyright (C) 2008 by Adam Kaplan and The New York Times Company.
  Copyright (C) 2008-2010 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 from Adam)...

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, many optimizations, and nytprofmerge. Jan Dubois contributed Windows support.

Adam's work is sponsored by The New York Times Co. Tim's work was partly sponsored by Shopzilla during 2008.