apc, eaccelerator & varnish

these 3 caching apps appear to be the most promoted online for webservers..
apc is closely associated with PHP and i had installed it and was using it without even noticing!

eaccelerator is often given a thumbs up, yet when i read the notes about it on github and on the official pages there was no mention of nginx, which is what i am using.. however i have seen people saying they succesfully use it with nginx.

i installed varnish yesterday and briefly explored the possibilities it offers, though didn't get it to work as the tutorial information contains contradictions regarding port numbers - most likely i will come back to it again soon.

so i am wondering if anyone here has used all 3 and maybe compared their effect with elgg? that would help a lot.
mostly i want to cache images and any other data that can be cached that is not already cached.. and i am also considering using cloudflare or another similar service; so knowing how all that fits together would help..

if i were to use cloudflare with a caching app, plus the elgg caches.. and there is also an option to setup a cache directly in nginx.. how can i trace in a useful way what is happening across all layers of software and data and ensure all the settings are optimal..? obviously it can be done, yet the idea appears to be intimidatingly complex prior to doing it. and since i also have the advice of google and pingdom tools' page analysis devices to go by - that leaves a vast array of changes to go through and test..
so once again, any tips are welcomed!

Performance and Scalability

Performance and Scalability

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