Potential to modify ordering for split_count and tour_counter

[wang384670111]

rayon/src/iter/par_bridge.rs

Lines 113 to 115 in 990841b

	let update = self
	.split_count
	.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |c| c.checked_sub(1));

SeqCst is overly restrictive. I believe that the ordering can be appropriately modified.

In par_bridge model, I believe that split_count is just a counter across threads and does not synchronize with other locals. Therefore, using Relaxed ordering should suffice.

rayon/rayon-demo/src/tsp/solver.rs

Line 67 in 990841b

let counter = self.tour_counter.fetch_add(1, Ordering::SeqCst);

Similarly, In solver model, tour_counter is just a counter across threads, using Relaxed ordering should suffice.

(happy to make a PR if this looks reasonable)

[cuviper]

It's been this way in par_bridge since it was first added in #550, but I don't think we paid much attention to that ordering. I agree that Relaxed should be fine, but I also think it won't make much difference, as this only executes once per thread for a given par_bridge. The real contention comes when they all wait on the same Mutex<Iter>.

The tsp counter is fine to change too. You're welcome to make a PR!

[wang384670111]

Related Link for PR:
#1140