threads accessing same cache line

Question

I came across a suggestion for threads not to access same cache lines and I really cant understand why, also while doing a search on that topic I came around with this questions: Multiple threads and CPU cache where one of the answers suggested:

you just want to avoid two threads from simultaneously trying to access data that is located on the same cache line

The way I see it, the cache stores pages of memory for quick access from the process, And as it says here:http://en.wikipedia.org/wiki/Thread_%28computing%29#How_threads_differ_from_processes

threads share their address space

It shouldn't be a problem for two threads to access same cache line since if a page is in the cache and a thread trying to access the memory will get a cache hit regardless of the other thread.

I heard the argument about avoiding threads from accessing same cache line on several different occasions so it cant be a myth. What am I missing here?

Answer 1

In most (probably all but I don't have an exhaustive hardware knowledge) multicore CPUs, the cache will lock the currently accessed line when one core tries to write into the corresponding memory. So other cores trying to access the same cache line will be held in wait.

You can share the same data among threads as long as it's read only (or infrequently updated), but if you keep writing into it, the hidden access serialization will yield performances equivalent to running all threads on the same core (actually a bit worse due to cache locking delays).