cmd/compile: limit goroutine count to parallelism

When the compiler crashes, it is not uncommon to see many hundreds
of goroutines all blocked waiting their turn to be one of the nWorkers
goroutines that is allowed to run. All these goroutine stacks are not a
terribly efficient use of memory, and they also make the crash dumps
hard to read.

Introduce a manager goroutine to hand out work to at most nWorker
goroutines, maintaining pending work in a local slice, rather than
having all those blocked goroutines hanging around waiting to run.

Change-Id: I46cb4e1afd6392805f359e14554ebc17d538bcba
Reviewed-on: https://go-review.googlesource.com/c/go/+/431956
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/gc/compile.go

@@ -126,20 +126,38 @@ func compileFunctions() {
 	}
 
 	if nWorkers := base.Flag.LowerC; nWorkers > 1 {
-		// For concurrent builds, we create a goroutine per task, but
-		// require them to hold a unique worker ID while performing work
-		// to limit parallelism.
-		workerIDs := make(chan int, nWorkers)
-		for i := 0; i < nWorkers; i++ {
-			workerIDs <- i
-		}
-
+		// For concurrent builds, we allow the work queue
+		// to grow arbitrarily large, but only nWorkers work items
+		// can be running concurrently.
+		workq := make(chan func(int))
+		done := make(chan int)
+		go func() {
+			ids := make([]int, nWorkers)
+			for i := range ids {
+				ids[i] = i
+			}
+			var pending []func(int)
+			for {
+				select {
+				case work := <-workq:
+					pending = append(pending, work)
+				case id := <-done:
+					ids = append(ids, id)
+				}
+				for len(pending) > 0 && len(ids) > 0 {
+					work := pending[len(pending)-1]
+					id := ids[len(ids)-1]
+					pending = pending[:len(pending)-1]
+					ids = ids[:len(ids)-1]
+					go func() {
+						work(id)
+						done <- id
+					}()
+				}
+			}
+		}()
 		queue = func(work func(int)) {
-			go func() {
-				worker := <-workerIDs
-				work(worker)
-				workerIDs <- worker
-			}()
+			workq <- work
 		}
 	}