Block-size of 5,000,000 terms

mpirun -machinefile macs -np 1 pi
 hello, 1 nodes used to compute pi.
.......
 The block size is          :                      5000000 terms
 Total number of blocks used:                          619 
 Total number of terms in the series: 3095000000 
 Computed value of pi    :     3.14159265349072e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009907
           difference    :     9.90700854686111e-11
 Total execution time is :    55.930 seconds.
mpirun -machinefile macs -np 2 pi
 hello, 2 nodes used to compute pi.
.......
 The block size is          :                      5000000 terms
 Total number of blocks used:                          620 
 Total number of terms in the series: 3100000000 
 Computed value of pi    :     3.14159265349421e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009558
           difference    :     9.55786561007699e-11
 Total execution time is :    29.578 seconds.
mpirun -machinefile macs -np 4 pi
 hello, 4 nodes used to compute pi.
.......
 The block size is          :                      5000000 terms
 Total number of blocks used:                          620 
 Total number of terms in the series: 3100000000 
 Computed value of pi    :     3.14159265349421e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009558
           difference    :     9.55791001899797e-11
 Total execution time is :    15.352 seconds.
mpirun -machinefile macs -np 8 pi
 hello, 8 nodes used to compute pi.
.......
 The block size is          :                      5000000 terms
 Total number of blocks used:                          624 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350846e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008133
           difference    :     8.13322742487799e-11
 Total execution time is :     8.023 seconds.
mpirun -machinefile macs -np 16 pi
 hello, 16 nodes used to compute pi.
.......
 The block size is          :                      5000000 terms
 Total number of blocks used:                          624 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350846e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008133
           difference    :     8.13340506056193e-11
 Total execution time is :     7.113 seconds.
mpirun -machinefile macs -np 24 pi
 hello, 24 nodes used to compute pi.
.......
 The block size is          :                      5000000 terms
 Total number of blocks used:                          624 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350846e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008133
           difference    :     8.13340506056193e-11
 Total execution time is :     5.480 seconds.