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[Robert Collins <robert.collins@xxxxxxxxxxxxx>]

If I can suggest some heuristics about things people find useful to
cache (based on squid experience ;)).

things that are:
 - very high volume 
 - expensive to calculate
 - very large [as a proxy for 'expensive'

tend to be the things that show up as 'we want to cache this'

The win on a cached object is roughly costsavings * frequency, where the
costsavings is the total cost (e.g. time, db load, whatever) to get the
full object minus the cost of checking that it is fresh (and thus can be
served from the cache).

So anything that we serve a lot of may be a big win even if the marginal
benefit is extremely low. Likewise things that are really expensive to
generate are likely worth caching even at only moderate request & hit
rates.

Things that are cheap to calculate and low volume should never get
cached.

-Rob

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