Add helpful comments for peculiar crossbeam usage

unified-scheduler-pool/src/lib.rs

@@ -404,7 +404,11 @@ impl AddressBook {
     }
 }
 
+// (this is slow needing atomic mem reads. However, this can be turned into a lot faster
+// optimizer-friendly version as shown in this crossbeam pr:
+// https://github.com/crossbeam-rs/crossbeam/pull/1047)
 fn disconnected<T>() -> Receiver<T> {
+    // drop the sender residing at .0, returning an always-disconnected receiver.
     unbounded().1
 }
 
@@ -595,6 +599,26 @@ impl<S: SpawnableScheduler<TH>, TH: TaskHandler> ThreadManager<S, TH> {
                 loop {
                     let mut is_finished = false;
                     while !is_finished {
+                        // ALL recv selectors are eager-evaluated ALWAYS by current crossbeam impl,
+                        // which isn't great and is inconsistent with `if`s in the Rust's match
+                        // arm. So, eagerly binding the result to a variable unconditionally here
+                        // makes no perf. difference...
+                        let dummy_unblocked_task_receiver =
+                            dummy_receiver(state_machine.has_unblocked_task());
+
+                        // (Assume this is biased; i.e. select_biased! in this crossbeam pr:
+                        // https://github.com/rust-lang/futures-rs/pull/1976)
+                        //
+                        // There's something special called dummy_unblocked_task_receiver here.
+                        // This odd pattern was needed to react to newly unblocked tasks from
+                        // _not-crossbeam-channel_ event sources, precisely at the specified
+                        // precedence among other selectors, while delegating the conrol flow to
+                        // select_biased!.
+                        //
+                        // In this way, hot looping is avoided and overall control flow is much
+                        // consistent. Note that unified scheduler will go
+                        // into busy looping to seek lowest latency eventually. However, not now,
+                        // to measure _actual_ cpu usage easily with the select approach.
                         select! {
                             recv(finished_task_receiver) -> executed_task => {
                                 let executed_task = executed_task.unwrap();
@@ -603,7 +627,7 @@ impl<S: SpawnableScheduler<TH>, TH: TaskHandler> ThreadManager<S, TH> {
                                 let result_with_timings = result_with_timings.as_mut().unwrap();
                                 Self::accumulate_result_with_timings(result_with_timings, executed_task);
                             },
-                            recv(dummy_receiver(state_machine.has_unblocked_task())) -> dummy => {
+                            recv(dummy_unblocked_task_receiver) -> dummy => {
                                 assert_matches!(dummy, Err(RecvError));
 
                                 let task = state_machine.schedule_unblocked_task().expect("unblocked task");