POE::Resource::Clock - internal clock used for ordering the queue

       Please see POE for more information about authors and contributors.

perl v5.34.0                                       2022-03-25                          POE::Resource::Clock(3pm)

       None known.

       POE::Resource::Clock is a helper module for POE::Kernel.  It provides the features to keep an internal
       monotonic clock and a wall clock.  It also converts between this monotonic clock and the wall clock.

       The monotonic clock is used to keep an ordered queue of events.  The wall clock is used to communicate
       the time with user code ("alarm_set" in POE::Kernel, "alarm_remove" in POE::Kernel).

       There are 3 possible clock sources in order of preference: POSIX::RT::Clock, Time::HiRes and "time" in
       perlfunc.  Only "POSIX::RT::Clock" has a separate monotonic and wall clock; the other two use the same
       source for both clocks.

       Clock selection and behaviour is controlled with the following:

   USE_POSIXRT
           export POE_USE_POSIXRT=0
               or
           sub POE::Kernel::USE_POSIXRT { 0 }

       Uses the "monotonic" clock source for queue priority and the "realtime" clock source for wall clock.  Not
       used if POSIX::RT::Clock is not installed or your system does not have a "monotonic" clock.

       Defaults to true.  If you want the old POE behaviour, set this to 0.

   USE_STATIC_EPOCH
           export POE_USE_STATIC_EPOCH=0
               or
           sub POE::Kernel::USE_STATIC_EPOCH { 0 }

       The epoch of the POSIX::RT::Clock monotonic is different from that of the realtime clock.  For instance
       on Linux 2.6.18, the monotonic clock is the number of seconds since system boot.  This epoch is used to
       convert from walltime into monotonic time for "alarm" in POE::Kernel, "alarm_add" in POE::Kernel and
       "alarm_set" in POE::Kernel. If "USE_STATIC_EPOCH" is true (the default), then the epoch is calculated at
       load time.  If false, the epoch is calculated each time it is needed.

       Defaults to true.  Only relevant for if using POSIX::RT::Clock. Long-running POE servers should have this
       set to false so that system clock skew does mess up the queue.

       It is important to point out that without a static epoch, the ordering of the following two alarms is
       undefined.

           $poe_kernel->alarm_set( a1 => $time );
           $poe_kernel->alarm_set( a2 => $time );

   USE_EXACT_EPOCH
           export POE_USE_EXACT_EPOCH=1
               or
           sub POE::Kernel::USE_EXACT_EPOCH { 1 }

       There currently no way to exactly get the monotonic clock's epoch.  Instead the difference between the
       current monotonic clock value to the realtime clock's value is used.  This is obviously inexact because
       there is a slight delay between the 2 system calls.  Setting USE_EXACT_EPOCH to true will calculate an
       average of this difference over 250 ms or at least 20 samples.  What's more, the system calls are done in
       both orders (monotonic then realtime, realtime then monotonic) to try and get a more exact value.

       Defaults to false.  Only relevant if "USE_STATIC_EPOCH" is true.

   USE_HIRES
           export POE_USE_HIRES=0
               or
           sub POE::Kernel::USE_HIRES { 0 }

       Use Time::HiRes as both monotonic and wall clock source.  This was POE's previous default clock.

       Defaults to true.  Only relevant if "USE_POSIXRT" is false.  Set this to false to use "time" in perlfunc.

       This module optionally exports a few timekeeping helper functions.

   mono2wallmono2wall() converts a monotonic time to an epoch wall time.

         my $wall = mono2wall( $monotonic );

   monotimemonotime() makes a best-effort attempt to return the time from a monotonic system clock.  It may fall
       back to non-monotonic time if there are no monotonic clocks available.

         my $monotonic = monotime();

   sleepsleep() makes a best-effort attempt to sleep a particular amount of high-resolution time using a
       monotonic clock.  This feature will degrade gracefully to non-monotonic high-resolution clocks, then low-
       resolution clocks, depending on available libraries.

         sleep( 3.141 );

   timetime() is a backwards compatible alias for walltime().  Please see walltime()'s documentation for
       details.

   wall2monowall2mono() makes a best-effort attempt to convert wall time to its equivalent monotonic-clock time.  Its
       feature degrades gracefully depending on clock availability.

         my $monotonic = wall2mono( $epoch );

   walltimetime() makes a best-effort attempt to return non-monotonic wall time at the highest available resolution
       known.

         my $epoch = walltime();

       POE::Resource::Clock - internal clock used for ordering the queue

       See POE::Resource for general discussion about resources and the classes that manage them.

           sub POE::Kernel::USE_POSIXRT { 0 }
           use POE;