[bawolff]

Varnish caching really only helps if the majority of your traffic is logged out requests. Its the sort of thing that is really useful at a high scale but matters much less at a low scale.

Application level caching (memcached/redis/apcu) is super important even at a small scale.

Most of the time (unless complex extensions are involved or your wiki pages are very simple) mediawiki should be io-bound on converting wikitext -> html (which is why caching that process is important). Normally if db is healthy, db requests shouldn't be the bottle neck (unless you have extensions like smw or cargo installed)

[jchw]

Most of MediaWiki seems to avoid too much trouble with contention in the database, but I was seeing it prior to enabling application-level caching. It seemed to be a combination of factors primarily driven by expensive tasks in the background. Particularly complex pages can cause some of those background tasks to become rather explosive.