Summary:

I think I know why OpenGuides has a habit of occasionally taking over servers. If you've experienced problems with occasional ridiculously high load averages, take a look at the Action paragraph below.

Action:

In the short term, I suggest that anyone who's been having trouble with OpenGuides taking over their machines should comment the per-user RSS feed links out of userstats.tt - copy it to your custom_templates directory and either delete everything between [% IF username %] and the next [% END %] statement (inclusive), or change [% IF username %] to [% IF username AND 0 %] - both will have the same effect.

Details:

I suspect that I have at last found out the thing that makes OpenGuides take people's servers over. It's the per-user RSS feeds, which are usually linked from e.g. http://cambridge.openguides.org/wiki/?username=Kake;action=userstats

The problematic feeds are those which omit minor edits, and they're only a problem on large guides (Cambridge is fine, RGL is not). The reason for this is that they effectively select _everything_ from the database and then winnow it down. When a spider is hitting one of these feeds every second... I think you can see where this is going, and it's not pretty.

The reason this has gone unnoticed for so long is that (a) nobody really uses these feeds anyway, so they only get accessed by spiders etc, and spiders aren't going to email us and say "hey, this link is very slow", and (b) these feeds are the only thing in OpenGuides that use "last n changes" as a selection criterion - Recent Changes uses "last n days", which is much more efficient.

The root problem is with the _find_recent_changed_by_criteria method in Wiki::Toolkit::Store::Database, specifically the "metadata_wasnt" parameter in combination with the "limit" one. I am working on a rewrite of this method to make it more efficient, but this is going to require some thinking and hence some time.

Kake