Block-size of 10,000,000 terms

mpirun -machinefile macs -np 1 pi
 hello, 1 nodes used to compute pi.
....
 The block size is          :                     10000000 terms
 Total number of blocks used:                          310 
 Total number of terms in the series: 3100000000 
 Computed value of pi    :     3.14159265349314e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009666
           difference    :     9.66560165238661e-11
 Total execution time is :    56.000 seconds.
mpirun -machinefile macs -np 2 pi
 hello, 2 nodes used to compute pi.
....
 The block size is          :                     10000000 terms
 Total number of blocks used:                          310 
 Total number of terms in the series: 3100000000 
 Computed value of pi    :     3.14159265349314e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000009666
           difference    :     9.66564606130760e-11
 Total execution time is :    28.828 seconds.
mpirun -machinefile macs -np 4 pi
 hello, 4 nodes used to compute pi.
....
 The block size is          :                     10000000 terms
 Total number of blocks used:                          312 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350738e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008241
           difference    :     8.24096346718761e-11
 Total execution time is :    14.883 seconds.
mpirun -machinefile macs -np 8 pi
 hello, 8 nodes used to compute pi.
....
 The block size is          :                     10000000 terms
 Total number of blocks used:                          312 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350738e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008241
           difference    :     8.24114110287155e-11
 Total execution time is :     7.617 seconds.
mpirun -machinefile macs -np 16 pi
 hello, 16 nodes used to compute pi.
....
 The block size is          :                     10000000 terms
 Total number of blocks used:                          320 
 Total number of terms in the series: 3200000000 
 Computed value of pi    :     3.14159265356956e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000002023
           difference    :     2.02300398655098e-11
 Total execution time is :     7.043 seconds.
mpirun -machinefile macs -np 24 pi
 hello, 24 nodes used to compute pi.
...
 The block size is          :                     10000000 terms
 Total number of blocks used:                          312 
 Total number of terms in the series: 3120000000 
 Computed value of pi    :     3.14159265350738e+00
 Machine  value of pi    :     3.14159265358979e+00
           difference    :     0.00000000008241
           difference    :     8.24100787610860e-11
 Total execution time is :     4.848 seconds.