zlacker

My point is that for caching to be cost-effective, it requires long lifetimes -- especially if your # of users is relatively low.

Especially for an app like Reddit with millions of subreddits. There is no monolithic "reddit"; the experience is tailored to each user based on the subs they've joined. So your % of requests that will be asking for a cached resource is lower than other high-volume websites. I think your 99-to-1 estimate is _way_ off.