get_ncpu()

 my $ncpu = MCE::Util::get_ncpu();

Specifying 'auto' for max_workers calls MCE::Util::get_ncpu automatically. MCE 1.521 sets an upper-limit when specifying 'auto'. The reason is mainly to safeguard apps from spawning 100 workers on a box having 100 cores. This is important for apps which are IO-bound.

 use MCE;

 ## 'Auto' is the total # of logical cores (lcores) (8 maximum, MCE 1.521).
 ## The computed value will not exceed the # of logical cores on the box.

 my $mce = MCE->new(

 max_workers => 'auto',       ##  1 on HW with 1-lcores;  2 on  2-lcores
 max_workers =>  16,          ## 16 on HW with 4-lcores; 16 on 32-lcores

 max_workers => 'auto',       ##  4 on HW with 4-lcores;  8 on 16-lcores
 max_workers => 'auto*1.5',   ##  4 on HW with 4-lcores; 12 on 16-lcores
 max_workers => 'auto*2.0',   ##  4 on HW with 4-lcores; 16 on 16-lcores
 max_workers => 'auto/2.0',   ##  2 on HW with 4-lcores;  4 on 16-lcores
 max_workers => 'auto+3',     ##  4 on HW with 4-lcores; 11 on 16-lcores
 max_workers => 'auto-1',     ##  3 on HW with 4-lcores;  7 on 16-lcores

 max_workers => MCE::Util::get_ncpu,   ## run on all lcores
 );

In summary:

 1. Auto has an upper-limit of 8 in MCE 1.521 (# of lcores, 8 maximum)
 2. Math may be applied with auto (*/+-) to change the upper limit
 3. The computed value for auto will not exceed the total # of lcores
 4. One can specify max_workers explicitly to a hard value
 5. MCE::Util::get_ncpu returns the actual # of lcores