CareerCup

http://en.wikipedia.org/wiki/Read-copy-update

RCU's updates are quite expensive. @ desi grad
RCU: Read-copy-update

@klpd
RCU is expensive only if there are frequent writes but works well with more reader and
occasional writers.

How about this
http://en.wikipedia.org/wiki/Optimistic_concurrency_control#Examples

@desi grad
now that looks fine!

but again how do you assume that there will be no frequent writes? the question doesn't say so, nah ..

Didn't the problem say

"because of the large num of readers and writers"

Depending on the nature of readers and writers and their inputs, Hashing can be introduced into the the shared storage and each hash bucket will be assigned a lock. The readers/writes should only lock concerned bucket. This will, if the hashing is done properly, ensure good level of concurrency.

We could design LRU caches for reads and writes, and a priority queues to schedule threads/processes