runtime: don't use GOGC in minimum sweep distance

Currently, the minimum sweep distance is 1MB * GOGC/100. It's been
this way since it was introduced in CL 13043 with no justification.

Since there seems to be no good reason to scale the minimum sweep
distance by GOGC, make the minimum sweep distance just 1MB.

Change-Id: I5320574a23c0eec641e346282aab08a3bbb057da
Reviewed-on: https://go-review.googlesource.com/c/go/+/66091
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -141,7 +141,7 @@ const (
 
 	// sweepMinHeapDistance is a lower bound on the heap distance
 	// (in bytes) reserved for concurrent sweeping between GC
-	// cycles. This will be scaled by gcpercent/100.
+	// cycles.
 	sweepMinHeapDistance = 1024 * 1024
 )
 
@@ -803,7 +803,7 @@ func gcSetTriggerRatio(triggerRatio float64) {
 			// that concurrent sweep has some heap growth
 			// in which to perform sweeping before we
 			// start the next GC cycle.
-			sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance*uint64(gcpercent)/100
+			sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance
 			if sweepMin > minTrigger {
 				minTrigger = sweepMin
 			}