[ruby-core:86265] idle lazy sweeping + stack depth tracking

Hey all, I wonder if these ideas have been proposed, yet:

1) idle lazy sweeping

Typical Ruby processes spend a fair amount of time waiting for
I/O.  With select/poll (and better, kqueue/epoll with green-threads),
we can use a zero-timeout first, before using real timeout:

    while (!rc && is_lazy_sweeping() && most_threads_idle() && timeout != 0) {
	rc = poll(..., 0); /* no waiting, just a quick check */
	if (rc == 0) {
	    lazy_sweep_step(); /* call gc_sweep_continue */
	}
    }
    if (!rc) { /* normal execution path */
	rc = poll(..., timeout);
    }

Maximum effectiveness would be gained by making all pipe/socket
non-blocking by default (like 1.8) so it's easier to track I/O wait.


2) stack depth tracking for marking

Building on above, but use an online algorithm to track average
stack depth and bias the GC to start marking when poll (or
similar function) sleeps when stack is shallow.

Having the GC favor marking when the stack is shallow should
reduce live objects and mark time.

The online algorithm to track average stack depth will add
some overhead, unfortunately.  So I have more confidence in 1.



The basic idea of both is to make sure the Ruby VM can always
do cleanup work without introducing internal background threads
and causing contention.

I won't have time to implement, yet, maybe mid-April; but these
will be MUCH easier after IO updates for green threads in
https://bugs.ruby-lang.org/issues/13618 gets accepted because
calling poll/ppoll/select repeatedly with the same FD sets is
expensive and epoll/kqueue avoids that cost.

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