dashboard: pick 4 TryBot helpers for -longtest SlowBots

Based on the data we have so far, 4 helpers seems like a good number
to pick. It provides a balance between resource use and diminishing
returns. We may need to adjust it as future work for issue 42661,
but a first step will be to reduce overhead in accessing historical
build timing data.

Fixes golang/go#37439.
Updates golang/go#42661.

Change-Id: I1bfddf85f2dba8b42dc7b8f8749bbf45b614e93d
Reviewed-on: https://go-review.googlesource.com/c/build/+/279513
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index dfc0a30..04afa8d 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -1873,10 +1873,7 @@
 		env: []string{
 			"GO_TEST_TIMEOUT_SCALE=5", // give them lots of time
 		},
-		// TODO(golang.org/issue/37439): Decide whether it's worth it
-		// to go for 3~9 extra helpers for -longtest SlowBot requests
-		// based on how well having more helpers scales. Try 3, 5, 9.
-		numTryTestHelpers: 9,
+		numTryTestHelpers: 4, // Target time is < 15 min for golang.org/issue/42661.
 	})
 	addBuilder(BuildConfig{
 		Name:     "linux-386-longtest",
@@ -1900,10 +1897,7 @@
 			"GOHOSTARCH=386",
 			"GO_TEST_TIMEOUT_SCALE=5", // give them lots of time
 		},
-		// TODO(golang.org/issue/37439): Decide whether it's worth it
-		// to go for 3~9 extra helpers for -longtest SlowBot requests
-		// based on how well having more helpers scales. Try 3, 5, 9.
-		numTryTestHelpers: 5,
+		numTryTestHelpers: 4, // Target time is < 15 min for golang.org/issue/42661.
 	})
 	addBuilder(BuildConfig{
 		Name:     "linux-arm",
@@ -2240,10 +2234,7 @@
 		env: []string{
 			"GO_TEST_TIMEOUT_SCALE=5", // give them lots of time
 		},
-		// TODO(golang.org/issue/37439): Decide whether it's worth it
-		// to go for 3~9 extra helpers for -longtest SlowBot requests
-		// based on how well having more helpers scales. Try 3, 5, 9.
-		numTryTestHelpers: 3,
+		numTryTestHelpers: 4, // Target time is < 15 min for golang.org/issue/42661.
 	})
 	addBuilder(BuildConfig{
 		Name:     "windows-amd64-race",