Setting smaller buffer size for sys.stdin?

David_9908 0
Viewed 21527

I'm running memcached with the following bash command pattern:

memcached -vv 2>&1 | tee memkeywatch2010098.log 2>&1 | ~/bin/memtracer.py | tee memkeywatchCounts20100908.log

to try and track down unmatched gets to sets for keys platform wide.

The memtracer script is below and works as desired, with one minor issue. Watching the intermediate log file size, memtracer.py doesn't start getting input until memkeywatchYMD.log is about 15-18K in size. Is there a better way to read in stdin or perhaps a way to cut the buffer size down to under 1k for faster response times?

#!/usr/bin/python

import sys
from collections import defaultdict

if __name__ == "__main__":


    keys = defaultdict(int)
    GET = 1
    SET = 2
    CLIENT = 1
    SERVER = 2

    #if <
    for line in sys.stdin:
        key = None
        components = line.strip().split(" ")
        #newConn = components[0][1:3]
        direction = CLIENT if components[0].startswith("<") else SERVER

        #if lastConn != newConn:        
        #    lastConn = newConn

        if direction == CLIENT:            
            command = SET if components[1] == "set" else GET
            key = components[2]
            if command == SET:                
                keys[key] -= 1                                                                                    
        elif direction == SERVER:
            command = components[1]
            if command == "sending":
                key = components[3] 
                keys[key] += 1

        if key != None:
            print "%s:%s" % ( key, keys[key], )
buffering stdin python
6 Answers

It may be that your troubles are not with Python but with the buffering that the Linux shell injects when chaining commands with pipes. When this is the problem, the input is not buffered by line, but by 4K block.

To stop this buffering, precede the command chain with the unbuffer command from the expect package, such as:

unbuffer memcached -vv 2>&1 | unbuffer -p tee memkeywatch2010098.log 2>&1 | unbuffer -p ~/bin/memtracer.py | tee memkeywatchCounts20100908.log

The unbuffer command needs the -p option when used in the middle of a pipeline.

user_17676530
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