Block-size of 100,000 terms

mpirun -machinefile macs -np 1 pi
 hello, 1 nodes used to compute pi.
....
 The block size is          :                       100000 terms
 Total number of blocks used:                        30923 
 Total number of terms in the series: 3092300000 
 Computed value of pi    :     3.14159265348986e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009994
           difference    :     9.99360594278187e-11
 Total execution time is :    56.273 seconds.
mpirun -machinefile macs -np 2 pi
 hello, 2 nodes used to compute pi.
....
 The block size is          :                       100000 terms
 Total number of blocks used:                        30924 
 Total number of terms in the series: 3092400000 
 Computed value of pi    :     3.14159265348993e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009986
           difference    :     9.98641169758230e-11
 Total execution time is :    31.934 seconds.
mpirun -machinefile macs -np 4 pi
 hello, 4 nodes used to compute pi.
....
 The block size is          :                       100000 terms
 Total number of blocks used:                        30924 
 Total number of terms in the series: 3092400000 
 Computed value of pi    :     3.14159265348994e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009985
           difference    :     9.98539029239964e-11
 Total execution time is :    17.883 seconds.
mpirun -machinefile macs -np 8 pi
 hello, 8 nodes used to compute pi.
....
 The block size is          :                       100000 terms
 Total number of blocks used:                        30928 
 Total number of terms in the series: 3092800000 
 Computed value of pi    :     3.14159265349020e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009959
           difference    :     9.95896698441356e-11
 Total execution time is :    28.063 seconds.
mpirun -machinefile macs -np 16 pi
 hello, 16 nodes used to compute pi.
....
 The block size is          :                       100000 terms
 Total number of blocks used:                        30928 
 Total number of terms in the series: 3092800000 
 Computed value of pi    :     3.14159265349020e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009960
           difference    :     9.95958870930735e-11
 Total execution time is :    31.746 seconds.
mpirun -machinefile macs -np 24 pi
 hello, 24 nodes used to compute pi.
....
 The block size is          :                       100000 terms
 Total number of blocks used:                        30936 
 Total number of terms in the series: 3093600000 
 Computed value of pi    :     3.14159265349076e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009903
           difference    :     9.90323378857738e-11
 Total execution time is :    23.367 seconds.