java - Are volatile variable 'reads' as fast as normal reads?

Question

I know that writing to a volatile variable flushes it from the memory of all the cpus, however I want to know if reads to a volatile variable are as fast as normal reads?

Can volatile variables ever be placed in the cpu cache or is it always fetched from the main memory?

Answer 1

You should really check out this article: http://brooker.co.za/blog/2012/09/10/volatile.html. The blog article argues volatile reads can be a lot slower (also for x86) than non-volatile reads on x86.

• Test 1 is a parallel read and write to a non-volatile variable. There is no visibility mechanism and the results of the reads are potentially stale.
• Test 2 is a parallel read and write to a volatile variable. This does not address the OP's question specifically. However worth noting that a contended volatile can be very slow.
• Test 3 is a read to a volatile in a tight loop. Demonstrated is that the semantics of what it means to be volatile indicate that the value can change with each loop iteration. Thus the JVM can not optimize the read and hoist it out of the loop. In Test 1, it is likely the value was read and stored once, thus there is no actual "read" occurring.

Credit to Marc Booker for running these tests.