runtime: pace the scavenger according to 1% of overall CPU time

Currently the scavenger is paced to 1% of 1 CPU because it had
scalability problems. As of the last few CLs, that should be largely
resolved. This change resolves the TODO and paces the scavenger
according to 1% of overall CPU time.

This change is made separately to allow it to be more easily rolled
back.

Change-Id: I1ab4de24ba41c564960701634a128a813c55ece9
Reviewed-on: https://go-review.googlesource.com/c/go/+/358675
Trust: Michael Knyszek <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/mgcscavenge.go

@@ -272,13 +272,7 @@ func bgscavenge(c chan int) {
 
 	// idealFraction is the ideal % of overall application CPU time that we
 	// spend scavenging.
-	//
-	// TODO(mknyszek): Currently this is percent of one CPU (hence the division
-	// by gomaxprocs), but ideally this should be 1% of overall CPU time.
-	// Given a scalable memory allocator, it makes sense that the scavenger
-	// should scale with it; if you're allocating more frequently, then presumably
-	// you're also generating more work from the scavenger.
-	idealFraction := float64(scavengePercent) / 100.0 / float64(gomaxprocs)
+	idealFraction := float64(scavengePercent) / 100.0
 
 	// Input: fraction of CPU time used.
 	// Setpoint: idealFraction.