1Devel::NYTProf::Core(3)User Contributed Perl DocumentatioDnevel::NYTProf::Core(3)
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NAME

6       Devel::NYTProf::Core - load internals of Devel::NYTProf
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DESCRIPTION

9       This module is not meant to be used directly.  See Devel::NYTProf,
10       Devel::NYTProf::Data, and Devel::NYTProf::Reader.
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12       While it's not meant to be used directly, it is a handy place to
13       document some internals.
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SUBROUTINE PROFILER

16       The subroutine profiler intercepts the "entersub" opcode which perl
17       uses to invoke a subroutine, both XS subs (henceforth xsubs) and pure
18       perl subs.
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20       The following sections outline the way the subroutine profiler works:
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22   Before the subroutine call
23       The profiler records the current time, the current value of
24       cumulative_subr_secs (as initial_subr_secs), and the current
25       cumulative_overhead_ticks (as initial_overhead_ticks).
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27       The statement profiler measures time at the start and end of processing
28       for each statement (so time spent in the profiler, writing to the file
29       for example, is excluded.) It accumulates the measured overhead into
30       the cumulative_overhead_ticks variable.
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32       In a similar way, the subroutine profiler measures the exclusive time
33       spent in subroutines and accumulates it into the cumulative_subr_secs
34       global.
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36   Make the subroutine call
37       The call is made by executing the original perl internal code for the
38       "entersub" opcode.
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40       Calling a perl subroutine
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42       If the sub being called is a perl sub then when the entersub opcode
43       returns, back into the subroutine profiler, the subroutine has been
44       'entered' but the first opcode of the subroutine hasn't been executed
45       yet.  Crucially though, a new scope has been entered by the entersub
46       opcode.
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48       The subroutine profiler then pushes a destructor onto the context
49       stack.  The destructor is effectively just inside the sub, like a
50       "local", and so will be triggered when the subroutine exits by any
51       means. Also, because it was the first thing push onto the context
52       stack, it will be triggered after any activity caused by the
53       subroutines scope exiting.
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55       When the destructor is invoked it calls a function which completes the
56       measurement of the time spent in the sub (see below).
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58       In this way the profiling of perl subroutines is very accurate and
59       robust.
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61       Calling an xsub
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63       If the sub being called is an xsub, then control doesn't return from
64       the entersub opcode until the xsub has returned. The profiler detects
65       this and calls the function which completes the measurement of the time
66       spent in the xsub.
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68       So far so good, but there's a problem. What if the xsub doesn't return
69       normally but throws an exception instead?
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71       In that case (currently) the profiler acts as if the xsub was never
72       called.  Time spent inside the xsub will be allocated to the calling
73       sub.
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75   Completing the measurement
76       The function which completes the timing of a subroutine call does the
77       following:
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79       It calculates the time spent in the statement profiler:
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81           overhead_ticks  = cumulative_overhead_ticks - initial_overhead_ticks
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83       and subtracts that from the total time spent 'inside' the subroutine:
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85           incl_subr_sec = (time now - time call was made) - overhead_ticks
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87       That gives us an accurate inclusive time. To get the exclusive time it
88       calculates the time spent in subroutines called from the subroutine
89       call we're measuring:
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91           called_sub_secs = cumulative_subr_secs - initial_subr_secs
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93       and subtracts that from the incl_subr_sec:
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95           excl_subr_sec = incl_subr_sec - called_sub_secs
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97       To make that easier to follow, consider a call to a sub that calls no
98       others.  In that case cumulative_subr_secs remains unchanged during the
99       call, so called_sub_secs is zero, and excl_subr_sec is the same as
100       incl_subr_sec.
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102       Finally, it adds the exclusive time to the cumulative exclusive time:
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104           cumulative_subr_secs += excl_subr_sec
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AUTHOR

107       Tim Bunce, <http://www.tim.bunce.name> and <http://blog.timbunce.org>
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110         Copyright (C) 2008, 2009 by Tim Bunce.
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112       This library is free software; you can redistribute it and/or modify it
113       under the same terms as Perl itself, either Perl version 5.8.8 or, at
114       your option, any later version of Perl 5 you may have available.
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118perl v5.32.1                      2021-04-07           Devel::NYTProf::Core(3)
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