支持python2.7 3.5 3.6, 运用multiprocessing模块的Pool 异步进程池，分段读取文件（文件编码由chardet自动判断，需pip install chardet），并统计词频，代码如下：

# wordcounter.py
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from __future__ import print_function, division, unicode_literals
import sys, re, time, os
import operator
from collections import Counter
from functools import reduce
from multiprocessing import Pool, cpu_count
from datetime import datetime
from utils import humansize, humantime, processbar

def wrap(wcounter,  fn, p1, p2, f_size):
    return wcounter.count_multi(fn, p1, p2, f_size)
    
class WordCounter(object):
    def __init__(self, from_file, to_file=None, workers=None, coding=None,
                    max_direct_read_size=10000000):
        '''根据设定的进程数，把文件from_file分割成大小基本相同，数量等同与进程数的文件段，
        来读取并统计词频，然后把结果写入to_file中，当其为None时直接打印在终端或命令行上。
        Args:
        @from_file 要读取的文件
        @to_file 结果要写入的文件
        @workers 进程数，为0时直接把文件一次性读入内存；为1时按for line in open(xxx)
                读取；>=2时为多进程分段读取；默认为根据文件大小选择0或cpu数量的64倍
        @coding 文件的编码方式，默认为采用chardet模块读取前1万个字符才自动判断
        @max_direct_read_size 直接读取的最大值，默认为10000000（约10M）
        
        How to use:
        w = WordCounter('a.txt', 'b.txt')
        w.run()        
        '''
        if not os.path.isfile(from_file):
            raise Exception('No such file: 文件不存在')
        self.f1 = from_file
        self.filesize = os.path.getsize(from_file)
        self.f2 = to_file
        if workers is None:
            if self.filesize < int(max_direct_read_size):
                self.workers = 0
            else:
                self.workers = cpu_count() * 64 
        else:
            self.workers = int(workers)
        if coding is None:
            try:
                import chardet
            except ImportError:
                os.system('pip install chardet')
                print('-'*70)
                import chardet
            with open(from_file, 'rb') as f:    
                coding = chardet.detect(f.read(10000))['encoding']            
        self.coding = coding
        self._c = Counter()
        
    def run(self):
        start = time.time()
        if self.workers == 0:
            self.count_direct(self.f1)
        elif self.workers == 1:
            self.count_single(self.f1, self.filesize)
        else:
            pool = Pool(self.workers)
            res_list = []
            for i in range(self.workers):
                p1 = self.filesize * i // self.workers 
                p2 = self.filesize * (i+1) // self.workers 
                args = [self, self.f1, p1, p2, self.filesize]
                res = pool.apply_async(func=wrap, args=args)
                res_list.append(res)
            pool.close()
            pool.join()
            self._c.update(reduce(operator.add, [r.get() for r in res_list]))            
        if self.f2:
            with open(self.f2, 'wb') as f:
                f.write(self.result.encode(self.coding))
        else:
            print(self.result)
        cost = '{:.1f}'.format(time.time()-start)
        size = humansize(self.filesize)
        tip = '\nFile size: {}. Workers: {}. Cost time: {} seconds'     
        print(tip.format(size, self.workers, cost))
        self.cost = cost + 's'
                
    def count_single(self, from_file, f_size):
        '''单进程读取文件并统计词频'''
        start = time.time()
        with open(from_file, 'rb') as f:
            for line in f:
                self._c.update(self.parse(line))
                processbar(f.tell(), f_size, from_file, f_size, start)   

    def count_direct(self, from_file):
        '''直接把文件内容全部读进内存并统计词频'''
        start = time.time()
        with open(from_file, 'rb') as f:
            line = f.read()
        self._c.update(self.parse(line))  
                
    def count_multi(self, fn, p1, p2, f_size):  
        c = Counter()
        with open(fn, 'rb') as f:    
            if p1:  # 为防止字被截断的，分段处所在行不处理，从下一行开始正式处理
                f.seek(p1-1)
                while b'\n' not in f.read(1):
                    pass
            start = time.time()
            while 1:                           
                line = f.readline()
                c.update(self.parse(line))   
                pos = f.tell()  
                if p1 == 0: #显示进度
                    processbar(pos, p2, fn, f_size, start)
                if pos >= p2:               
                    return c      
                    
    def parse(self, line):  #解析读取的文件流
        return Counter(re.sub(r'\s+','',line.decode(self.coding)))
        
    def flush(self):  #清空统计结果
        self._c = Counter()

    @property
    def counter(self):  #返回统计结果的Counter类       
        return self._c
                    
    @property
    def result(self):  #返回统计结果的字符串型式，等同于要写入结果文件的内容
        ss = ['{}: {}'.format(i, j) for i, j in self._c.most_common()]
        return '\n'.join(ss)
        
def main():
    if len(sys.argv) < 2:
        print('Usage: python wordcounter.py from_file to_file')
        exit(1)
    from_file, to_file = sys.argv[1:3]
    args = {'coding' : None, 'workers': None, 'max_direct_read_size':10000000}
    for i in sys.argv:
        for k in args:
            if re.search(r'{}=(.+)'.format(k), i):
                args[k] = re.findall(r'{}=(.+)'.format(k), i)[0]

    w = WordCounter(from_file, to_file, **args)
    w.run()
    
if __name__ == '__main__':
    main()        

# utils.py
#coding=utf-8
from __future__ import print_function, division, unicode_literals
import os
import time

def humansize(size):
    """将文件的大小转成带单位的形式
    >>> humansize(1024) == '1 KB'
    True
    >>> humansize(1000) == '1000 B'
    True
    >>> humansize(1024*1024) == '1 M'
    True
    >>> humansize(1024*1024*1024*2) == '2 G'
    True
    """
    units = ['B', 'KB', 'M', 'G', 'T']    
    for unit in units:
        if size < 1024:
            break
        size = size // 1024
    return '{} {}'.format(size, unit)

def humantime(seconds):
    """将秒数转成00:00:00的形式
    >>> humantime(3600) == '01:00:00'
    True
    >>> humantime(360) == '06:00'
    True
    >>> humantime(6) == '00:06'
    True
    """
    h = m = ''
    seconds = int(seconds)
    if seconds >= 3600:
        h = '{:02}:'.format(seconds // 3600)
        seconds = seconds % 3600
    if 1 or seconds >= 60:
        m = '{:02}:'.format(seconds // 60)
        seconds = seconds % 60
    return '{}{}{:02}'.format(h, m, seconds)
        
def processbar(pos, p2, fn, f_size, start):
    '''打印进度条
    just like:
    a.txt, 50.00% [=====     ] 1/2 [00:01<00:01]
    '''
    percent = min(pos * 10000 // p2, 10000)
    done = '=' * (percent//1000)
    half = '-' if percent // 100 % 10 > 5 else ''
    tobe = ' ' * (10 - percent//1000 - len(half))
    tip = '{}{}, '.format('\33[?25l', os.path.basename(fn))  #隐藏光标          
    past = time.time()-start
    remain = past/(percent+0.01)*(10000-percent)      
    print('\r{}{:.1f}% [{}{}{}] {:,}/{:,} [{}<{}]'.format(tip, 
            percent/100, done, half, tobe, 
            min(pos*int(f_size//p2+0.5), f_size), f_size, 
            humantime(past), humantime(remain)),
        end='')
    if percent == 10000:
        print('\33[?25h', end='')     # 显示光标  

if __name__ == '__main__':
    import doctest
    doctest.testmod()

github地址：https://github.com/waketzheng/wordcounter
可以直接：

git clone https://github.com/waketzheng/wordcounter

• 运行结果：

[willie@localhost linuxtools]$ python wordcounter.py test/var/20000thousandlines.txt tmp2.txt 
20000thousandlines.txt, 100.0% [==========] 115,000,000/115,000,000 [06:57<00:00]
File size: 109 M. Workers: 128. Cost time: 417.8 seconds