Ubuntu Manpage: Devel::NYTProf - Powerful fast feature-rich Perl source code profiler

Provided by: libdevel-nytprof-perl_6.14+dfsg-1build3_amd64

NAME

       Devel::NYTProf - Powerful fast feature-rich Perl source code profiler

SYNOPSIS

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

         # 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
         nytprofcsv

       Tim Bunce has often given talks on profiling perl code, including a detailed look at how to use NYTProf
       and how to optimize your code. A video of his YAPC::NA 2014 talk can be found at
       <https://youtu.be/T7EK6RZAnEA>

DESCRIPTION

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 (<>) {
...
1;
}

After the first time around the loop, any further time spent evaluating the condition (waiting for input
in this example) would 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
<http://rt.perl.org/rt3/Ticket/Display.html?id=60954>)

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. As a result the subroutine profiler is both fast and 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".

accept()

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. Since nytprofhtml only
works with a single profile file you may want to merge multiple files using nytprofmerge.

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.

PROFILING

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

         perl -d:NYTProf some_perl.pl

       To save typing the ':NYTProf' you could set the "PERL5DB" environment variable:

         PERL5DB='use Devel::NYTProf'

       and then just perl -d would work:

         perl -d some_perl.pl

       Or you can avoid the need to add the -d option at all by using the "PERL5OPT" environment
       variable|perlrun/PERL5OPT>:

         PERL5OPT=-d:NYTProf

       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.

NYTPROF ENVIRONMENT VARIABLE

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.

addpid=1
Append the current process id to the end of the filename.

This avoids concurrent, or consecutive, processes from overwriting the same file. If a fork is detected
during profiling then the child process will automatically add the process id to the filename.

addtimestamp=1
Append the current time, as integer epoch seconds, to the end of the filename.

trace=N
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.

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

start=...
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/use/BEGIN)
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. See "RUN-TIME CONTROL OF PROFILING".

The start=init option is handy if you want to avoid profiling the loading and initialization of modules.

optimize=0
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.

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

blocks=1
Set to 1 to enable the determination of block and subroutine location per statement. This makes the
profiler about 50% slower (as of July 2008) and produces larger output files, but you gain some valuable
insight in where time is spent in the blocks within large subroutines and scripts.

stmts=0
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.

calls=N
This option is new and experimental.

With calls=1 (the default) subroutine call return events are emitted into the data stream as they happen.
With calls=2 subroutine call entry events are also emitted. With calls=0 no subroutine call events are
produced. This option depends on the "subs" option being enabled, which it is by default.

The nytprofcalls utility can be used to process this data. It too is new and experimental and so likely
to change.

The subroutine profiler normally gathers data in memory and outputs a summary when the profile data is
being finalized, usually when the program has finished. The summary contains aggregate information for
all the calls from one location to another, but the details of individual calls have been lost. The
calls option enables the recording of individual call events and thus more detailed analysis and
reporting of that data.

leave=0
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'.

findcaller=1
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.)

use_db_sub=1
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".

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.

slowops=N
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.

usecputime=1
This option has been removed. Profiling won't be enabled if set.

Use the "clock=N" option to select a high-resolution CPU time clock, if available on your system,
instead. That will give you higher resolution and work for the subroutine profiler as well.

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

compress=...
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.

clock=N
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.

sigexit=1
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 so the profile will be incomplete
and unusable. The "sigexit" option tells NYTProf to catch some signals (e.g. INT, HUP, PIPE, SEGV, BUS)
and ensure a usable profile by executing:

DB::finish_profile();
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):

sigexit=int,hup

posix_exit=1
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().

libcexit=1
Arranges for "finish_profile" to be called via the C library atexit() function. This may help some
tricky cases where the process may exit without perl executing the "END" block that NYTProf uses to call
/finish_profile().

endatexit=1
Sets the PERL_EXIT_DESTRUCT_END flag in the PL_exit_flags of the perl interpreter. This makes perl run
"END" blocks in perl_destruct() instead of perl_run() which may help in cases, like Apache, where perl is
embedded but perl_run() isn't called.

forkdepth=N
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.

nameevals=0
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.

nameanonsubs=0
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.

RUN-TIME CONTROL OF PROFILING

       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 still need to load Devel::NYTProf as early as possible,
       even if you don't call DB::enable_profile() until much later. That's because any code that's compiled
       before Devel::NYTProf is loaded will not be profiled by default. See also "use_db_sub=1".

       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).

       Always call the DB::enable_profile(), DB::disable_profile() or DB::finish_profile() function with the
       "DB::" prefix as shown because you can't import them. They're provided automatically when NYTProf is in
       use.

   disable_profile
         DB::disable_profile()

       Stops collection of profile data until DB:enable_profile() is called.

       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. Compare with "finish_profile"
       below.

   enable_profile
         DB::enable_profile($newfile)
         DB::enable_profile()

       Enables collection of profile data. If $newfile is specified 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.  If DB::enable_profile() is called without a filename argument then profile data will continue to
       be written to the current file (nytprof.out by default).

   finish_profile
         DB::finish_profile()

       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.

DATA COLLECTION AND INTERPRETATION

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 few 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.

Perl 5.10.1+ (or else 5.8.9+) is Recommended
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: <http://rt.perl.org/rt3/Ticket/Display.html?id=60954>

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/bar.pm:42]"" 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/bar.pm".

Nested string evals can give rise to file names like

(eval 1047)[(eval 93)[/foo/bar.pm:42]: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/bar.pm:42]:12]

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

Timing

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

MAKING NYTPROF FASTER

       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".  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 call stacks or flamegraph then disable it via calls=0.  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 calls=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) or --no-flame options.

REPORTS

       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:

   nytprofhtml
       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.

   nytprofcg
       Translates a profile into a format that can be loaded into KCachegrind <http://kcachegrind.github.io/>

   nytprofcalls
       Reads a profile and processes the calls events it contains.

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

LIMITATIONS

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.

Coro
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.

FCGI::Engine
Using open('-|') in code running under FCGI::Engine causes a panic in nytprofcalls. See
https://github.com/timbunce/devel-nytprof/issues/20 for more information.

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 "TIE"whatever 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.

CLOCKS

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

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

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 http://preview.tinyurl.com/5wawnn so
CLOCK_MONOTONIC is preferred.

CLOCK_VIRTUAL

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

CLOCK_PROF

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

CLOCK_PROCESS_CPUTIME_ID

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

CLOCK_THREAD_CPUTIME_ID

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,

gettimeofday

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.

mach_absolute_time

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

Time::HiRes

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:

http://www.opengroup.org/onlinepubs/000095399/functions/clock_getres.html
http://linux.die.net/man/3/clock_gettime

Why 'realtime' can appear to go backwards:

http://preview.tinyurl.com/5wawnn

The PostgreSQL pg_test_timing utility documentation has a good summary of timing issues:

http://www.postgresql.org/docs/9.2/static/pgtesttiming.html

CAVEATS

SMP Systems
On systems with multiple processors, which includes most modern machines, (from Linux docs though
applicable to most SMP systems):

The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are realized on
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 http://en.wikipedia.org/wiki/Processor_affinity)

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.

Windows

THIS SECTION DOESN'T MATCH THE CODE

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.

BUGS

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

TROUBLESHOOTING

"Profile data incomplete, ..." or "Profile format error: ..."
This error message means the file doesn't contain all the expected data or the data has been corrupted in
some way. 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, like
SIGTERM, or a SIGINT from pressing Ctrl-C, then the "sigexit=1" option 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".

You'll also get this error if the code trying to read the profile is itself being profiled. That's most
likely to happen if you enable profiling via the "PERL5OPT" environment variable and have forgotten to
unset it.

If you've encountered this error message, and you're sure you've understood the concerns described above,
and you're sure they don't apply in your case, then please open an issue. Be sure to include sufficient
information so I can see how you've addressed these likely causes.

Some source files don't have profile information
This is usually due to NYTProf being initialized after some perl files have already been compiled.

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

You could also try using the "use_db_sub=1" option.

Eval ... has unknown invoking fid
When using the statement profiler you may see a warning message like this:

Eval '(eval 2)' (fid 9, flags:viastmt,savesrc) has unknown invoking fid 10

Notice that the eval file id (fid 9 in this case) is lower than the file id that invoked the eval (fid 10
in this case). This is a known problem caused by the way perl works and how the profiler assigns and
outputs the file ids. The invoking fid is known but gets assigned a fid and output after the fid for the
eval, and that causes the warning when the file is read.

Warning: %d subroutine calls had negative time
There are two likely causes for this: clock instability, or accumulated timing errors.

Clock instability, if present on your system, is most likely to be noticeable on very small/fast
subroutines that are called very few times.

Accumulated timing errors can arise because the subroutine profiler uses floating point values (NVs) to
store the times. They are most likely to be noticed on subroutines that are called a few times but which
make a large number of calls to very fast subroutines (such as opcodes). In this case the accumulated
time apparently spent making those calls can be greater than the time spent in the calling subroutine.

If you rerun nytprofhtml (etc.) with the "trace=N" option set >0 you'll see trace messages like "%s call
has negative time: incl %fs, excl %fs" for each affected subroutine.

Try profiling with the "slowops=N" option set to 0 to disable the profiling of opcodes. Since opcodes
often execute in a few microseconds they are a common cause of this warning.

You could also try recompiling perl to use 'long doubles' for the NV floating point type (use Configure
-Duselongdouble). If you try this please let me know. I'd also happily take a patch to use long doubles,
if available, by default.

panic: buffer overflow ...
You have unusually long subroutine names in your code. You'll need to rebuild Devel::NYTProf with the
NYTP_MAX_SUB_NAME_LEN environment variable set to a value longer than the longest subroutine names in
your code.

AUTHORS AND CONTRIBUTORS

       Tim Bunce (<http://blog.timbunce.org>) 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.

COPYRIGHT AND LICENSE

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.

Background
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 forked 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 Tim 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 was sponsored by The New York Times Co. <http://open.nytimes.com>. Tim's work was partly
sponsored by Shopzilla <http://www.shopzilla.com> during 2008 but hasn't been sponsored since then.

For the record, Tim has never worked for the New York Times nor has he received any kind of sponsorship
or support from them in relation to NYTProf. The name of this module is simply result of the history
outlined above, which can be summarised as: Adam forked an existing module when he added his enhancements
and Tim didn't.