如何通过Python多进程分块读取超大文件？

# -*- coding: GBK -*-
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
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"
 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()