Block-size of 2,500,000 terms

mpirun -machinefile macs -np 1 pi
 hello, 1 nodes used to compute pi.
.............
 The block size is          :                      2500000 terms
 Total number of blocks used:                         1238 
 Total number of terms in the series: 3095000000 
 Computed value of pi    :     3.14159265349124e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009856
           difference    :     9.85553860743948e-11
 Total execution time is :    55.957 seconds.
mpirun -machinefile macs -np 2 pi
 hello, 2 nodes used to compute pi.
.............
 The block size is          :                      2500000 terms
 Total number of blocks used:                         1238 
 Total number of terms in the series: 3095000000 
 Computed value of pi    :     3.14159265349123e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009856
           difference    :     9.85647119478017e-11
 Total execution time is :    30.848 seconds.
mpirun -machinefile macs -np 4 pi
 hello, 4 nodes used to compute pi.
.............
 The block size is          :                      2500000 terms
 Total number of blocks used:                         1240 
 Total number of terms in the series: 3100000000 
 Computed value of pi    :     3.14159265349472e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009507
           difference    :     9.50732825799605e-11
 Total execution time is :    16.676 seconds.
mpirun -machinefile macs -np 8 pi
 hello, 8 nodes used to compute pi.
.............
 The block size is          :                      2500000 terms
 Total number of blocks used:                         1240 
 Total number of terms in the series: 3100000000 
 Computed value of pi    :     3.14159265349472e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009507
           difference    :     9.50737266691704e-11
 Total execution time is :     8.949 seconds.
mpirun -machinefile macs -np 16 pi
 hello, 16 nodes used to compute pi.
.............
 The block size is          :                      2500000 terms
 Total number of blocks used:                         1248 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350897e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008083
           difference    :     8.08269007279705e-11
 Total execution time is :     6.898 seconds.
mpirun -machinefile macs -np 24 pi
 hello, 24 nodes used to compute pi.
.............
 The block size is          :                      2500000 terms
 Total number of blocks used:                         1248 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350896e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008083
           difference    :     8.08286770848099e-11
 Total execution time is :     5.191 seconds.