from multiprocessing import Pool
import time
FILEIN = "input.txt"
FILEOUT = "output.txt"
fileIn = open(FILEIN)
fileOut = open(FILEOUT,"a+")

def process_line(line):
    line = line[::-1]
    return line

def process_lines_list(lines):
    pool = Pool(processes=4)
    linesOut = pool.map(process_line, lines)
    fileOut.write("".join(linesOut))
    del linesOut

LINESTOREAD = 500000
lines = []
start = time.time()
for line in fileIn:
    lines.append(line)
    if(len(lines)==LINESTOREAD):
        process_lines_list(lines)
        del lines
        lines = []

if(lines):
        process_lines_list(lines)
        del lines
        lines = []
fileOut.close()
print("Run time",time.time()-start)