I'd be really surprised if that results in programs in parallel mode becoming slower than sequential
I'm going to guess that you haven't played too much with executing things in parallel? In Clojure groups, people have heard beginners express surprise about parallel mode programs running slower so often, it's somewhat shaped the reaction to such questions.
few people use concurrent read-write data structures anyway (other than channels, of course)
One can still get in trouble with channels of pointers to structs.
Much more likely is that some programs become slower due to the fact that GOMAXPROCS>1 requires more locks and atomic operations. That's not false sharing,
Depending on how the locks and atomic operations are implemented and used, the slowdown with locks and atomic operations could be in large part because of false sharing. (I don't know the details for Go.)
I think you're overstating how much false sharing in particular causes parallel slowdowns. In the case of Clojure, for example, it's probably just straightforward synchronization overhead (cost of atomic ops), just as it is in Go.
False sharing is a very specific type of synchronization problem whereby data tightly packed in memory gets shared between multiple processors because the cache line size is larger than desired, even though logically the data in question are completely independent. This can happen, but it's nowhere near the most common type of synchronization-related performance problem in my experience. If your problem actually needs to share the data between multiple processors (e.g. with Go channels), then it's not "false" sharing anymore—it's "true" sharing.
I stand corrected. It's possible that other data might get accessed from another socket while going after a lock, but if every thread is just after the data for the lock, it's just sharing. Semantics got dropped somewhere, but I was still using the term.
> I'm going to guess that you haven't played too much with executing things in parallel? In Clojure groups, people have heard beginners express surprise about parallel mode programs running slower so often, it's somewhat shaped the reaction to such questions.
pcwalton is one of the lead engineers of Servo, a Mozilla Research project to develop a more parallelizable browser engine in Rust. I'd say he's played around quite a bit with executing things in parallel!
I'm going to guess that you haven't played too much with executing things in parallel? In Clojure groups, people have heard beginners express surprise about parallel mode programs running slower so often, it's somewhat shaped the reaction to such questions.
few people use concurrent read-write data structures anyway (other than channels, of course)
One can still get in trouble with channels of pointers to structs.
Much more likely is that some programs become slower due to the fact that GOMAXPROCS>1 requires more locks and atomic operations. That's not false sharing,
Depending on how the locks and atomic operations are implemented and used, the slowdown with locks and atomic operations could be in large part because of false sharing. (I don't know the details for Go.)