I am trying to do some rough math:
Assuming 100ms per page, that’s about 10 requests per second.
With 8 mongrels per cpu (based on suggestions on the web), that’s 80
requests per second.
I’d say 100ms should be the upper end, you’d want to be aiming for much faster than that but anyway.
This seems like a very low request/second per machine.
How are people getting 1000 or so requests per second? Is my math
High performance sites generally tend to use a lot of caching. Rails has a lot of caching mechanisms built in - page caching, partial caching, generic caching.
What the average overhead by mongrel for cpu and ram?
No idea, I use Passenger. The overhead for Passenger is hard to determine, but my clients are happy with the performance so that’s what counts.
The best bet might be to see what percentage of your hits are to the homepage and each internal page type. Then setup a JMeter profile to hit pages in that ratio and see what you get. You can then try tweaking and caching. But find out your numbers before worrying about it - don’t guess at performance, measure!