python多进程分块读取文件-优快云博客

本文介绍了一个使用Python多进程实现的大文件分块读取方法。通过将文件读取任务分配给多个进程并行处理，有效提高了文件读取效率。每个进程负责读取文件的一部分，并将数据写入到临时文件中。

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# -*- coding: GBK -*-
# filename: multiprocessreadfile.py

import urlparse
import datetime
import os
from multiprocessing import Process,Queue,Array,RLock

"""
多进程分块读取文件
"""

WORKERS = 4
BLOCKSIZE = 100000000 #字节，接近100MB
FILE_SIZE = 0

def getFilesize(file):
    """
        获取要读取文件的大小
    """
    global FILE_SIZE
    fstream = open(file,'r')
    fstream.seek(0,os.SEEK_END)
    FILE_SIZE = fstream.tell()
    fstream.close()

def process_found(pid,array,file,rlock):
    global FILE_SIZE
    global JOB
    global PREFIX
    """
        进程处理
        Args:
            pid:进程编号
            array:进程间共享队列，用于标记各进程所读的文件块结束位置
            file:所读文件名称
        各个进程先从array中获取当前最大的值为起始位置startpossition
        结束的位置endpossition (startpossition+BLOCKSIZE) if (startpossition+BLOCKSIZE)<FILE_SIZE else FILE_SIZE
        if startpossition==FILE_SIZE则进程结束
        if startpossition==0则从0开始读取
        if startpossition!=0为防止行被block截断的情况，先读一行不处理，从下一行开始正式处理
        if 当前位置 <=endpossition 就readline
        否则越过边界，就从新查找array中的最大值
    """
    fstream = open(file,'r')
    
    while True:
        rlock.acquire()
        print 'pid%s'%pid,','.join([str(v) for v in array])
        startpossition = max(array)            
        endpossition = array[pid] = (startpossition+BLOCKSIZE) if (startpossition+BLOCKSIZE)<FILE_SIZE else FILE_SIZE
        rlock.release()
        
        if startpossition == FILE_SIZE:#end of the file
            print 'pid%s end'%(pid)
            break
        elif startpossition !=0:
            fstream.seek(startpossition)
            fstream.readline()
        pos = ss = fstream.tell()
        ostream = open('/data/download/tmp_pid'+str(pid)+'_jobs'+str(endpossition),'w')
        while pos<endpossition:
            #处理line
            line = fstream.readline()                        
            ostream.write(line)
            pos = fstream.tell()

        print 'pid:%s,startposition:%s,endposition:%s,pos:%s'%(pid,ss,pos,pos)
        ostream.flush()
        ostream.close()
        ee = fstream.tell()        

    fstream.close()

def main():
    global FILE_SIZE
    print datetime.datetime.now().strftime("%Y/%d/%m %H:%M:%S") 
    
#    file = "/data/pds/download/scmcc_log/tmp_format_2011004.log"
    file = "zdmm1_10.txt"
    getFilesize(file)
    print FILE_SIZE
    
    rlock = RLock()
    array = Array('l',WORKERS,lock=rlock)
    threads=[]
    for i in range(WORKERS):
        p=Process(target=process_found, args=[i,array,file,rlock])
        threads.append(p)

    for i in range(WORKERS):
        threads[i].start()
    
    for i in range(WORKERS):
        threads[i].join()

    print datetime.datetime.now().strftime("%Y/%d/%m %H:%M:%S") 

if __name__ == '__main__':
    main()

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