does false sharing occur when data is read in openmp?

Question

If I have a C++ program with OpenMP parallelization, where different threads constantly use some small shared array only for reading data from it, does false sharing occur in this case? in other words, is false sharing related only to memory write operations, or it can also happen with memory read operations.

Answer 1

Typically used cache coherence protocols, such as MESI (modified, exclusive, shared, invalid), have a specific state for cache lines called "shared". Cache lines are in this state if they are read by multiple processors. Each processor then has a copy of the cache line and can safely read from it without false-sharing. On a write, all processors are informed to invalidate the cache line which is the main cause for false-sharing

Answer 2

False sharing is a performance issue because it causes additional movement of a cache line which takes time. When two variables which are not really shared reside in the same line and separate threads update each of them, the line has to bounce around the machine which increases the latency of each access. In this case if the variables were in separate lines each thread would keep a locally modified copy of "its" line and no data movement would be required.

However, if you are not updating a line, then no data movement is necessary and there is no performance impact from the sharing beyond the fact that you might have been able to have data each thread does need in there, rather than data it doesn't. That is a small, second order, effect. though. So unless you know you are cache capacity limited ignore it!