I suspect they pull down the data for whether a cron needs to be run in an initializing query that also pulls down a ton of other data (site name, for example). So in theory, they're only doing a few time comparisons of extra work per request.
Also - I haven't looked too deep into WP's implementation, but are they actually doing it on page load? I've seen it in other php software (I believe phpBB) as a 1px image. This kinda solves the caching issues others are talking about...your server might still get hung up, and that picture might never load, but if you're cached properly I think it won't stop anyone from reading.
Ignoring the bugs though...I've had my shared hostgator plan handle a couple of top page HN post with no additional caching installed, which makes me think it's probably not a problem for most people running WP. (I just double checked to make sure that's actually true, and it is.)
And if a plugin has registered a long-running function, you're kinda screwed.
> Also - I haven't looked too deep into WP's implementation, but are they actually doing it on page load?
Yep.
> I've had my shared hostgator plan handle a couple of top page HN post with no additional caching installed, which makes me think it's probably not a problem for most people running WP.
HostGator install WP Supercache by default.
On page load (that is, when WP actually runs), WP fires off a non-blocking HTTP request to itself. This runs at maximum once per minute, and shouldn't actually run if you're using Super Cache or similar and hit a cache page.
I remember doing something similar for a stats script that would wipe old data every time it was viewed by an admin.
remove_action('init', 'wp_cron');
> If your site gets a large influx in traffic the server runs wp-cron for every page hit and further slows things down.
Which involves a single database hit and a `foreach` loop. Or, if you're using an external object cache (memcached, APC, etc), no DB hits.
Probably not as dire a situation as it seems. This...
> If your site gets no traffic then it doesn't run at all.
...is a huge problem.
>...is a huge problem.
Why?
If you are relying on things to happen at an exact time, them you should know not to use wp-cron. However, for the scope of internal wordpress tasks that don't need to happen every load, I don't see any problem with it happening when wordpress is prodded eventually, and the time it was scheduled to run at has passed.
1. On hosts that don't support cron, tell users to use an external service to do it, and in the meantime their scheduled posts and other functionality breaks.
2. Implement a workaround like this.
Yes, it's not the greatest solution, but the point is that it works.
In addition, "every single page that is rendered also checks if it should fire off a cron job" is incorrect. A lock is used to avoid running more often than once per minute. If you're at the sort of load where this is an issue, you should be running proper cron anyway.
And it only checks if it needs to run a job at page load, it doesn't actually run it everytime, which isn't that big of a deal to check.
Wordpress has a very high focus on readable code which does seem to affect it's speed. But it is easy to use, setup and hack, which is why it is popular. And as Yahoo and wordpress.com clearly shows, it does scale.
Essentially, given the constraints of the LAMP architecture, wp-cron is the only way to do it.
You overlooked that wrapping WP in layers of caching restricts wp-cron's supply of triggering events. It's a mess.
Especially since LAMP is not the universal web app target any more. We can and should architect from the OS up.
On the topic of new stacks from the OS on up, Docker[1] (and lxc in general) and CoreOS[2] are really cool. It's a much more elegant and long term solution to packaged, yet controlled, environments.
[2]: http://coreos.com/
This is of course fixable, but still interesting given the context.
If you don't have a native cron installation (eg. shared hosting or Windows) you can config WP to act as a cron daemon (actually might be enabled by default). The catch though is since everything is WP is triggered by HTTP requests the way wp-cron works is to check if there is a job to run every time a WP page is hit. Yes you understood that correctly, every single page that is rendered also checks if it should fire off a cron job. The jobs get executed async in the background so it doesn't hold up the ongoing request but it has two interesting properties.
1. If your site gets a large influx in traffic the server runs wp-cron for every page hit and further slows things down.
2. If your site gets no traffic then it doesn't run at all.
Funny eh?
[1]: http://core.trac.wordpress.org/browser/tags/3.6/wp-cron.php
[2]: http://wp.tutsplus.com/articles/insights-into-wp-cron-an-int...