python gc performance in large apps

[Robby Dermody]

Hey guys (thus begins a book of a post :),

I'm in the process of writing a commercial VoIP call monitoring and 
recording application suite in python and pyrex. Basically, this 
software sits in a VoIP callcenter-type environment (complete with agent 
phones and VoIP servers), sniffs voice data off of the network, and 
allows users to listen into calls. It can record calls as well. The 
project is about a year and 3 months in the making and lately the 
codebase has stabilized enough to where it can be used by some of our 
clients. The entire project has about 37,000 lines of python and pyrex 
code (along with 1-2K lines of unrelated java code).

Now, some disjointed rambling about the architecture of this software. 
This software has two long-running server-type components. One 
component, the "director" application, is written in pure python and 
makes use of the twisted, nevow, and kinterbasdb libraries (which I 
realize link to some C extensions). The other component, the 
"harvester", is a mixture of python and pyrex, and makes use of the 
twisted library, along with using the C libs libpcap and glib on the 
pyrex end. Basically, the director is the "master" component. A single 
director process interacts with users of the system through a web and/or 
pygtk client application interface and can coordinate 1 to n harvesters 
spread about the world. The harvester is the "heavy lifter" component 
that sniffs the network traffic and sifts out the voice and signalling 
data. It then updates the director of call status changes, and can 
provide users of the system access to the data. It records the data to 
disk as well. The scalibility of this thing is really cool: given a 
single director sitting somewhere coordinating the list of agents, 
multiple harvester can be placed anywhere there is voice traffic. A user 
that logs into the director can end up seeing the activity of all of 
these seperate voice networks presented like a single giant mesh.

Overall, I have been very pleased with python and the 3rd party 
libraries that I use (twisted, nevow, kinterbasdb and pygtk). It is a 
joy to program with, and I think the python community has done a fine 
job. However, as I have been running the software lately and profiling 
its memory usage, the one and only Big Problem I have seen is that of 
the memory usage. Ideally, the server application(s) should be able to 
run indefinitely, but from the results I'm seeing I will end up 
exhausting the memory on a 2 GB machine in 2 to 3 days of heavy load.

Now normally I would not raise up an issue like this on this list, but 
based on the conversations held on this list lately, and the work done 
by Evan Jones (http://evanjones.ca/python-memory.html), I am led to 
believe that this memory usage -- while partially due to some probably 
leaks in my program -- is largely due to the current python gc. I have 
some graphs I made to show the extent of this memory usage growth:

http://public.robbyd.fastmail.fm/iq-graph1.gif

http://public.robbyd.fastmail.fm/iq-graph-director-rss.gif

http://public.robbyd.fastmail.fm/iq-graph-harv-rss.gif

The preceding three diagrams are the result of running the 1 director 
process and 1 harvester process on the same machine for about 48 hours. 
This is the most basic configuration of this software. I was running 
this application through /usr/bin/python (CPython) on a Debian 'testing' 
box running Linux 2.4 with 2GB of memory and Python version 2.3.5. 
During that time, I gathered the resident and virtual memory size of 
each component at 120 second intervals. I then imported this data into 
MINITAB and did some plots. The first one is a graph of the resident 
(RSS) and virtual memory usage of the two applications. The second one 
is a zoomed in graph of the director's resident memory usage (complete 
with a best fit quadratic), and the 3rd one is a zoomed in graph of the 
harvester's resident memory usage.

To give you an idea of the network load these apps were undergoing 
during this sampling time, by the time 48 hours had passed, the 
harvester had gathered and parsed about 900 million packets. During the 
day there will be 50-70 agents talking. This number goes to 10-30 at night.

In the diagrams above, one can see the night-day separation clearly. At 
night, the memory usage growth seemed to all but stop, but with the 
increased call volume of the day, it started shooting off again. When I 
first started gathering this data, I was hoping for a logarithmic curve, 
but at least after 48 hours, it looks like the usage increase is almost 
linear. (Although logarithmic may still be the case after it exceeds a 
gig or two of used memory. :) I'm not sure if this is something that I 
should expect from the current gc, and when it would stop.

Now, as I stated above, I am certain that at least some of this 
increased memory usage is due to either un-collectable objects in the 
python code, or memory leaks in the pyrex code (where I make some use of 
malloc/free). I am working on finding and removing these issues, but 
from what I've seen  with the help of gc UNCOLLECTABLE traces, there are 
not many un-collectable reference issues at least. Yes, there are some 
but definitely not enough to justify growth like I am seeing. The pyrex 
side should not be leaking too much, I'm very good about freeing what I 
allocate in pyrex/C land. I will be running that linked to a memory leak 
finding library in the next few days. Past the code reviews I've done, 
what makes me think that I don't have any *wild* leaks going on at least 
with the pyrex code is that I am seeing the same type of growth patterns 
in both apps, and I don't use any pyrex with the director. Yes, the 
harvester is consuming much more memory, but it also does the majority 
of the heavy lifting.

I am alright with the app not freeing all the memory it can between high 
and low activity times, but what puzzles me is how the memory usage just 
keeps on growing and growing. Will it ever stop?

What I would like to know if others on this list have had similar 
problems with python's gc in long running, larger python applications. 
Am I crazy or is this a real problem with python's gc itself? If it's a 
python gc issue, then it's my opinion that we will need to enhance the 
gc before python can really gain leverage as a language suitable for 
"enterprise-class" applications. I have surprised many other programmers 
that I'm writing an application like this in python/pyrex that works 
just as well and even more efficiently than the C/C++/Java competitors. 
The only thing I have left to show is that the app lasts as long between 
restarts. ;)


Robby