WordPress cache plugins were in really sad shape by the summer of 2012. WordPress 3.3 and WordPress 3.4 have changed some important parts of these plugins and they started to collapse. The first one to go was W3 Total Cache (at WordPress.org) which has been getting about 50% broken ratings since last fall. After spending months learning how to use W3 Total Cache just right (it's a complicated beast), we had to pull it off all our sites. Fortunately the much simpler WP Super Cache (at WordPress.org) remained bullet proof.
This spring and summer WP Super Cache broke too. Garbage collection does not work reliably any more, leading to people getting served week old pages. Donncha first suggested it's impossible to make WP Super Cache work for all hosting out of the box. Strange as for more than five years WP Super Cache did just that. The real answer came a bit later. WP Super Cache is a free plugin and Donncha just doesn't have time anymore.
I have hardly any time to devote to this any more. One of the dangers of becoming a father I'm afraid.
We've struggled to get garbage collection working properly to keep our clients' sites cached but reliably up to date. For the moment garbage collection still won't run reliably on at least Informed Comment. As JuanCole.com is one of the most read political sites in the world, this is something which needed to be fixed right away. It turns out there is a third WordPress cache plugin out there, Hyper Cache (at WordPress.org). We know the author of this plugin Stefano Lissa from his Newsletter Pro plugin which we use for a few clients. Lissa's code is good, he can handle sophisticated tasks while keeping the code structured enough for external teams to find and repair bugs and he answers his email.
So while HyperCache is less well-known than the big two, it seemed worth a try. We think long and hard before changing horses on core functionality like caching as we have years of experience with our core plugins. Usually our philosophy is better the devil you know is better than the one you don't.
In addition, WP Super Cache has mod_rewrite capability which means no PHP has to run (and hence presumably no CPU) to serve cached pages. We put a high value on Super Cache's capability to bypass PHP processing. But to our surprise testing last year had shown to Hyper Cache to be a bit faster than WP Super Cache. Other testing had shown Hyper Cache competitive. What is also worth noting from the other two tests is that in a comparable environment W3 Total Cache is in no way superior to either WP Super Cache and Hyper Cache, just more complicated. Complicated is bad: KISS is the best development rule ever written.
We still didn't really believe these results and thought there must be something strange in the test environment. We wanted to test against a real site and a real server that had been online.
We still had a testbed server available with Informed Comment on it and no traffic. The nice thing about this test bed is that it is a very limited 768 MB VPS with bare bones Apache and mod_php on it. I.e. we knew that if we gave it a good effort we'd be able to saturate it with proper external testing from LoadImpact. Our main dedicated servers with nginx would cost hundreds of dollars per test instead of $15/test. Based on past testing, we far prefer real web traffic than synthetic benchmarks like ApacheBench.
Here's what we found with 500 concurrent connection test. First the results for WP Super Cache.
Now from the challenger HyperCache.
Basically the results are identical. Both plugins allow the post to be downloaded at a fairly constant 1.8 seconds per load.