import subprocess, time

for i in range(8):
    MB = 1024**2
    LENGTH =2**i*MB
    data = b"X"*LENGTH

    p = subprocess.Popen(['cat'],
                         stdin=subprocess.PIPE,
                         stdout=subprocess.PIPE)
    time.sleep(1)
    t = time.time()
    x = p.communicate(data)[0]
    t = time.time() - t

    assert x == data
    print('amount = %.0f MB; time taken = %.2f secs; rate = %.2f MB/s' % (
            1.0*LENGTH/MB, t, 1.0*LENGTH/MB/t))
    del x, data