Can we avoid to call try_advance in some senarios? [crossbeam-epoch]

[Yriuns]

Recently I'm working on a epoch-based memory reclamation library and learned a lot of from crossbeam-epoch. Thank you for your great work.

But I noticed that we will call global().collect() every PINNINGS_BETWEEN_COLLECT 1st pins.
In the senario which defer_destroy happens rarely, there is nothing to collect in most time. But try_advance issues a SeqCst ordering and a traverse of all threads, which is heavy. I wonder if we can add something like:

if self.queue.empty() {
    return
}

at the beginning of collect(), to reduce the overhead?

crossbeam/crossbeam-epoch/src/internal.rs

Lines 201 to 219 in ce31c18

	pub(crate) fn collect(&self, guard: &Guard) {
	let global_epoch = self.try_advance(guard);
	let steps = if cfg!(crossbeam_sanitize) {
	usize::max_value()
	} else {
	Self::COLLECT_STEPS
	};
	for _ in 0..steps {
	match self.queue.try_pop_if(
	|sealed_bag: &SealedBag| sealed_bag.is_expired(global_epoch),
	guard,
	) {
	None => break,
	Some(sealed_bag) => drop(sealed_bag),
	}
	}
	}

[loyd]

I observed the same situation for my case (99.99% reads), especially with many threads that's related to #852

Another approach is implemented in the scc crate, where scan is started ONLY (@wvwwvwwv, is it correct?) when:

• the current thread has garbage
• OR some other thread terminated with some garbage
• OR user code in another thread explicitly called suspend().

@taiki-e, what do you think about it?

[wvwwvwwv]

@loyd yes, correct.