Locks up? It doesn't serve any other requests? What OS?

Serving a diff needs more processing than normal (producing the diff, probably rendering an old revision, which is always uncached...) but lockup web server??

Ubuntu x64 10.10

It won't serve any more pages, until the diff is loaded.. If you try and browse away, it won't finish loading, and I have to restart the apache instance

Can you get another page from a different browser (ie. not logged in)?
It could be the php session locking.

(In reply to comment #4)

Can you get another page from a different browser (ie. not logged in)?
It could be the php session locking.

You look to be right.

If i open another tab in the same instance/session, it waits until it's finished loading the diff, then does that.

Another tab works fine though

Assuming it is "Another browser works fine though"

Not a MediaWiki problem then. You can skip php session locking by placing the sessions in memcached or a non-lockable fs (nfs). That means that the last change wins.

I changed to memcached a little back as Brion suggested it'd help speed things up

It still happens with that...

I noticed the other day I hadn't set sessions in memcached... But the rest of the cache was

Haven't tested it, but I'd guess that will help fix it