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=head1 NAME

perldtrace - Perl's support for DTrace

=head1 SYNOPSIS

    # dtrace -Zn 'perl::sub-entry, perl::sub-return { trace(copyinstr(arg0)) }'
    dtrace: description 'perl::sub-entry, perl::sub-return ' matched 10 probes

    # perl -E 'sub outer { inner(@_) } sub inner { say shift } outer("hello")'
    hello

    (dtrace output)
    CPU     ID                    FUNCTION:NAME
      0  75915       Perl_pp_entersub:sub-entry   BEGIN
      0  75915       Perl_pp_entersub:sub-entry   import
      0  75922      Perl_pp_leavesub:sub-return   import
      0  75922      Perl_pp_leavesub:sub-return   BEGIN
      0  75915       Perl_pp_entersub:sub-entry   outer
      0  75915       Perl_pp_entersub:sub-entry   inner
      0  75922      Perl_pp_leavesub:sub-return   inner
      0  75922      Perl_pp_leavesub:sub-return   outer

=head1 DESCRIPTION

DTrace is a framework for comprehensive system- and application-level
tracing. Perl is a DTrace I<provider>, meaning it exposes several
I<probes> for instrumentation. You can use these in conjunction
with kernel-level probes, as well as probes from other providers
such as MySQL, in order to diagnose software defects, or even just
your application's bottlenecks.

Perl must be compiled with the C<-Dusedtrace> option in order to
make use of the provided probes. While DTrace aims to have no
overhead when its instrumentation is not active, Perl's support
itself cannot uphold that guarantee, so it is built without DTrace
probes under most systems. One notable exception is that Mac OS X
ships a F</usr/bin/perl> with DTrace support enabled.

=head1 HISTORY

=over 4

=item 5.10.1

Perl's initial DTrace support was added, providing C<sub-entry> and
C<sub-return> probes.

=item 5.14.0

The C<sub-entry> and C<sub-return> probes gain a fourth argument: the
package name of the function.

=item 5.16.0

The C<phase-change> probe was added.

=back

=head1 PROBES

=over 4

=item sub-entry(SUBNAME, FILE, LINE, PACKAGE)

Traces the entry of any subroutine. Note that all of the variables
refer to the subroutine that is being invoked; there is currently
no way to get ahold of any information about the subroutine's
I<caller> from a DTrace action.

    :*perl*::sub-entry {
        printf("%s::%s entered at %s line %d\n",
               copyinstr(arg3), copyinstr(arg0), copyinstr(arg1), arg0);
    }

=item sub-return(SUBNAME, FILE, LINE, PACKAGE)

Traces the exit of any subroutine. Note that all of the variables
refer to the subroutine that is returning; there is currently no
way to get ahold of any information about the subroutine's I<caller>
from a DTrace action.

    :*perl*::sub-return {
        printf("%s::%s returned at %s line %d\n",
               copyinstr(arg3), copyinstr(arg0), copyinstr(arg1), arg0);
    }

=item phase-change(NEWPHASE, OLDPHASE)

Traces changes to Perl's interpreter state. You can internalize this
as tracing changes to Perl's C<${^GLOBAL_PHASE}> variable, especially
since the values for C<NEWPHASE> and C<OLDPHASE> are the strings that
C<${^GLOBAL_PHASE}> reports.

    :*perl*::phase-change {
        printf("Phase changed from %s to %s\n",
            copyinstr(arg1), copyinstr(arg0));
    }

=back

=head1 EXAMPLES

=over 4

=item Most frequently called functions

    # dtrace -qZn 'sub-entry { @[strjoin(strjoin(copyinstr(arg3),"::"),copyinstr(arg0))] = count() } END {trunc(@, 10)}'

    Class::MOP::Attribute::slots                                    400
    Try::Tiny::catch                                                411
    Try::Tiny::try                                                  411
    Class::MOP::Instance::inline_slot_access                        451
    Class::MOP::Class::Immutable::Trait:::around                    472
    Class::MOP::Mixin::AttributeCore::has_initializer               496
    Class::MOP::Method::Wrapped::__ANON__                           544
    Class::MOP::Package::_package_stash                             737
    Class::MOP::Class::initialize                                  1128
    Class::MOP::get_metaclass_by_name                              1204

=item Trace function calls

    # dtrace -qFZn 'sub-entry, sub-return { trace(copyinstr(arg0)) }'

    0  -> Perl_pp_entersub                        BEGIN
    0  <- Perl_pp_leavesub                        BEGIN
    0  -> Perl_pp_entersub                        BEGIN
    0    -> Perl_pp_entersub                      import
    0    <- Perl_pp_leavesub                      import
    0  <- Perl_pp_leavesub                        BEGIN
    0  -> Perl_pp_entersub                        BEGIN
    0    -> Perl_pp_entersub                      dress
    0    <- Perl_pp_leavesub                      dress
    0    -> Perl_pp_entersub                      dirty
    0    <- Perl_pp_leavesub                      dirty
    0    -> Perl_pp_entersub                      whiten
    0    <- Perl_pp_leavesub                      whiten
    0  <- Perl_dounwind                           BEGIN

=item Function calls during interpreter cleanup

    # dtrace -Zn 'phase-change /copyinstr(arg0) == "END"/ { self->ending = 1 } sub-entry /self->ending/ { trace(copyinstr(arg0)) }'

    CPU     ID                    FUNCTION:NAME
      1  77214       Perl_pp_entersub:sub-entry   END
      1  77214       Perl_pp_entersub:sub-entry   END
      1  77214       Perl_pp_entersub:sub-entry   cleanup
      1  77214       Perl_pp_entersub:sub-entry   _force_writable
      1  77214       Perl_pp_entersub:sub-entry   _force_writable

=item System calls at compile time

    # dtrace -qZn 'phase-change /copyinstr(arg0) == "START"/ { self->interesting = 1 } phase-change /copyinstr(arg0) == "RUN"/ { self->interesting = 0 } syscall::: /self->interesting/ { @[probefunc] = count() } END { trunc(@, 3) }'

    lseek                                                           310
    read                                                            374
    stat64                                                         1056

=back

=head1 REFERENCES

=over 4

=item DTrace User Guide

L<http://download.oracle.com/docs/cd/E19082-01/819-3620/index.html>

=item DTrace: Dynamic Tracing in Oracle Solaris, Mac OS X and FreeBSD

L<http://www.amazon.com/DTrace-Dynamic-Tracing-Solaris-FreeBSD/dp/0132091518/>

=back

=head1 AUTHORS

Shawn M Moore C<sartak@gmail.com>

=cut