Python Performance vs. C++ in a Complex System

[Doug Fort]

Our website load testing system system, http://www.stressmy.com is very much
like an agent-based simulation.  My experience in running  on Linux is that the
system bogs down on a few hundred threads.  (This is the 2.2 kernel, 2.4 is
supposed to be better).  We recently started using select() in a single
threaded model for this reason.  What platform are you running on?  How many
threads did your prototype manage?

Courageous wrote:

> I've been recently working on a agent-based simulation system which is
> intended to run with massive numbers of threads in the 15,000-30,000 range.
>
> In my initial prototype of the system, I selected Python with Continuations
> as a base, and crafted the simulation core in about two man-weeks. This
> core performed well enough at the time, but certain that I could do better,
> I decided to use the Python core as a proof-of-concept, and rewrote the
> core in C++. This took approximately eight man-weeks of my time and was
> quite labor intensive.
>
> Having completed both cores, and with the C++ core HIGHLY OPTIMIZED,
> I was finally able to perform a performance test of the the C++ system
> versus the Python system. To my surprise, the C++ core only beat Python
> by about 30%. Given the obvious inequities in coding time in both efforts,
> plus whatever future coding time inequities I might project onto users of
> either core by implication of the programming language, I was quite
> surprised by these results.
>
> In retrospect, it's the lack of a true continuation model in C++ which is to
> blame. The C++ implementation suffers from extensive context-switching
> burden, having to frequently swap out all the registers and manage the
> stacks for all the threads, whereas Python-with-Continuations does not.
>
> This was an interesting outcome, which I'm still exploring.
>
> It may very well be that the C++ system shows better gains when the
> performance tests involve non-trivial code in the threads.
>
> However, it's still remarkable that a Python implementation of a relatively
> complex system like this one (an massively-parallel agent simulator),
> could come as close to C++ so easily.
>
> One caveat: to those who know simulation engines, the priority heap
> at the heart of the Python system was an optimized C-extension.
>
> Joe Kraska
> BBN Technologies
> San Diego CA

--
Doug Fort
Senior Meat Manager
Downright Software LLC


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