Question

I am guessing that even reading from shared data in openmp causes some parallel overheads, as depending on processor architecture (if different cores have their own cache…) it may be necessary to refresh the cache to ensure that no other cpu has modified the data before reading.

Am I right in thinking this?

If so, is there a way to tell openmp (on the intel compiler fwiw) that some of the shared data is constant, so such cache refreshing isn’t necessary?

If the answer is c++ const is there an easy way to turn non-const data into const data, without actually reallocating the memory, once the program has passed a certain point at runtime?

UPDATE

Ah, ok. I now remember where I got my impression that const was a good thing in this context: http://www.akkadia.org/drepper/cpumemory.pdf , section 6.4.1. It’s to do with false sharing, where readonly variables which share cache lines with readwrite variables incur the penalty of the cache line being marked exclusive by the readwrite variable. The linked document recommends, for example with gcc, to mark those vars as __attribute__((section(something.else))) to ensure they get stored elsewhere.

As it happens this is not relevant in my own situation – large arrays and stl containers of data in which the read/write granularity will span many cache lines and which are allocated from different memory pools in any case. So these will naturally be located on different cache lines. No problem!

Answer 1

Sharing read-only data among multiple cores has no overhead. The same copy of the data will be in the cache for all the cores that are using it.

The only time you will incur overhead is when one of the cores writes to the shared data. In that situation, you will incur a (potentially large) overhead because the write will invalidate all the other copies of that cache line and force them to grab it from memory or from a different cache.

In other words, the cache is only “refreshed” when somebody changes the data. There’s no periodic “refreshing”.

So the answer to your question is: You do not have to do anything. There’s no way to tell OpenMP or the hardware that the data is constant and const will have no effect aside from syntax.