threads

Python threads VS processes

I was revisiting Jeff Knupp’s great article from 2012, Python’s hardest problem. It talks about the Global Interpreter Lock, or GIL, in python. It basically explains how the GIL works, and why it’s such an important problem for python coders.

Probably the most notable consequence of the GIL is that python cannot do “pure” multi-threaded operations, in the sense that only one thread can execute at any time. The GIL prevents strange things from happening when you can have more than one thread write to the same chunk of memory. Knupp also wrote a follow-up to that article, Python’s hardest problem, revisited, where he advises people who want to do many things at the same time (parallelism) to use the multiprocessing module.

It’s great advice, I’ve used multiprocessing in the wild. It needs a bit more effort to communicate data between the processes (typically using queues), but it’s well worth the security that separate processes afford you. In essence, every process can then run it’s own thread without sharing any memory.

As I was reading down the article, I noticed he didn’t have any examples! So I started playing around, just for fun. Let’s make a very simple  program that appends integers from 0 to 999998 and discards the list, 50 times.

Version 1: simple single-threaded

import time
 
nb_repeat = 50
 
 
def a_complex_operation(*args):
    a = []
    for x in range(999999):
        a.append(x)
    return None
 
 
t1 = time.time()
for _ in range(nb_repeat):
    a_complex_operation()
print time.time()-t1

Running time: 4.82960796356 seconds

Version 2: with processes

from multiprocessing import Pool
nb_repeat = 50
 
def a_complex_operation(*args):
    a = []
    for x in range(999999):
        a.append(x)
    return None
pool = Pool(processes=nb_repeat)
results = pool.map(a_complex_operation, [None for _ in range(nb_repeat)])

Running time: 2.74916887283 seconds! Almost half the initial time.

Version 3: threaded version

from threading import Thread
import time
 
nb_repeat = 50
 
def a_complex_operation(*args):
    a = []
    for x in range(999999):
        a.append(x)
    return None
 
t1 = time.time()
threads = []
for _ in range(nb_repeat):
    threads.append(Thread(target=a_complex_operation))
 
[x.start() for x in threads]
[x.join() for x in threads]
print time.time()-t1

Running time: 14.0888431072 seconds!

Not extremely surprising, but quite interesting still. As expected, the version with processes is the fastest. But threading our program does not only not improve its running time, it actually slows it down quite a bit. This is probably due to the overhead involved in switching context between 50 threads. The multiprocess version is a nice little optimization, but it’s fair to say that the normal, single-threaded version is running pretty quickly too. When in doubt, keep it simple!

Parallel S3 uploads using Boto and threads in python

A typical setup

Uploading multiple files to S3 can take a while if you do it sequentially, that is, waiting for every operation to be done before starting another one. S3 latency can also vary, and you don’t want one slow upload to back up everything else. Here’s a typical setup for uploading files – it’s using Boto for python :

AWS_KEY = "your_aws_key"
AWS_SECRET = "your_aws_secret"
 
from boto.s3.connection import S3Connection
 
filenames = ['1.json', '2.json', '3.json', '4.json', '5.json', '6.json', '7.json', '8.json', '9.json', '10.json']
conn = S3Connection(aws_access_key_id=AWS_KEY, aws_secret_access_key=AWS_SECRET)
for fname in filenames:
    bucket = conn.get_bucket("parallel_upload_tests")
    key = bucket.new_key(fname).set_contents_from_string('some content')
    print "uploaded file %s" % fname

Nothing fancy, this works fine, and it reuses the same S3Connection object. If I print the execution time though, it’s around 1.3 seconds.

How to speed this up

A) Using the multiprocessing module’s ThreadPool (concurrency)

Python has a multiprocessing module, which allows you to “side-step the Global Interpreter Lock by using subprocesses instead of threads”. What this means is that if you have a multi-processor machine, you can leverage them to your advantage. Here’s an example using a ThreadPool:

AWS_KEY = "your_aws_key"
AWS_SECRET = "your_aws_secret"
 
from boto.s3.connection import S3Connection
from multiprocessing.pool import ThreadPool
 
filenames = ['1.json', '2.json', '3.json', '4.json', '5.json', '6.json', '7.json', '8.json', '9.json', '10.json']
conn = S3Connection(aws_access_key_id=AWS_KEY, aws_secret_access_key=AWS_SECRET)
 
def upload(myfile):
        bucket = conn.get_bucket("parallel_upload_tests")
        key = bucket.new_key(myfile).set_contents_from_string('some content')
        return myfile
 
pool = ThreadPool(processes=10)
pool.map(upload, filenames)

Execution time? 0.3 seconds! That’s about 4X faster than our previous example. I’m running this example on a 4-CPU ThinkPad. Note that there’s an overhead cost of starting a 10 process ThreadPool as opposed to just using the same process over and over. Also note that we’re also reusing our S3Connection here, since we’re using subprocesses and not threads per se.

B) Using threads (parallelism)

This solution will effectively spawn new threads of control, which can be quite expensive. Also important to note, we can’t reuse our S3 connection here since Boto’s library isn’t thread-safe, apparently:

AWS_KEY = "your_aws_key"
AWS_SECRET = "your_aws_secret"
 
from boto.s3.connection import S3Connection
import threading
 
filenames = ['1.json', '2.json', '3.json', '4.json', '5.json', '6.json', '7.json', '8.json', '9.json', '10.json']
def upload(myfile):
    conn = S3Connection(aws_access_key_id=AWS_KEY, aws_secret_access_key=AWS_SECRET)
    bucket = conn.get_bucket("parallel_upload_tests")
    key = bucket.new_key(myfile).set_contents_from_string('some content')
    return myfile
 
for fname in filenames:
    t = threading.Thread(target = upload, args=(fname,)).start()

Execution time? 0.018 seconds, about 72X faster than our original script. Not bad at all – but don’t forget, we’re creating 10 threads here, uploading the files in parallel. Your threads will automatically die when the uploads finish, “when its run() method terminate” according to the docs. Please keep in mind that if have tons of files to upload at once, this might not be the best approach – on this topic, here’s a good discussion on How Many Threads is Too Many?