If you have huge amount of money, you can just keep throwing hardware at scaling problems. For the rest of us, though, performance tuning is a must.
Note – Incidentally, if anyone with monstrous gobs of cash is actually reading this book, please
consider a substantial donation to the Django project. We accept uncut diamonds and gold ingots, too.
Unfortunately, performance tuning is much more of an art than a science, and it is even more difficult to write about than scaling. If you’re serious about deploying a large-scale Django application, you should spend a great deal of time learning how to tune each piece of your stack.
The following sections, though, present a few Django-specific tuning tips we’ve discovered over the years.
There’s No Such Thing As Too Much RAM – As of this writing, the really expensive RAM costs only about $200 per gigabyte — pennies compared to the time spent tuning elsewhere. Buy as much RAM as you can possibly afford, and then buy a little bit more.
Faster processors won’t improve performance all that much; most Web servers spend up to 90% of their time waiting on disk I/O. As soon as you start swapping, performance will just die. Faster disks might help slightly, but they’re much more expensive than RAM, such that it doesn’t really matter.
If you have multiple servers, the first place to put your RAM is in the database server. If you can afford it, get enough RAM to get fit your entire database into memory. This shouldn’t be too hard. LJWorld.com’s database — including over half a million newspaper articles dating back to 1989 — is under 2GB. Next, max out the RAM on your Web server. The ideal situation is one where neither server swaps — ever. If you get to that point, you should be able to withstand most normal traffic.
Turn Off Keep-Alive
Keep-Alive is a feature of HTTP that allows multiple HTTP requests to be served over a single TCP connection, avoiding the TCP setup/teardown overhead. This looks good at first glance, but it can kill the performance of a Django site. If you’re properly serving media from a separate server, each user browsing your site will only request a page from your Django server every ten seconds or so. This leaves HTTP servers waiting around for the next keep-alive request, and an idle HTTP server just consumes RAM that an active one should be using.
Use Memcached
Although Django supports a number of different cache back-ends, none of them even come close to being as fast as memcached. If you have a high-traffic site, don’t even bother with the other back-ends — go straight to memcached.
Use Memcached Often
Of course, selecting memcached does you no good if you don’t actually use it. Chapter 13 is your best friend here: learn how to use Django’s cache framework, and use it everywhere possible. Aggressive, preemptive caching is usually the only thing that will keep a site up under major traffic.
