graph: Increase the rate at which the load manager reduces the kill rate

It now takes up to 5s instead of 10s to completely resolve an overload and
stop killing queries. On the way down, we take roughly two steps at once

Author: lutter

graph/src/data/graphql/effort.rs

@@ -394,7 +394,7 @@ impl LoadManager {
             if overloaded {
                 kill_rate = (kill_rate + KILL_RATE_STEP * (1.0 - kill_rate)).min(1.0);
             } else {
-                kill_rate = (kill_rate - KILL_RATE_STEP).max(0.0);
+                kill_rate = (kill_rate - 2.0 * KILL_RATE_STEP).max(0.0);
             }
             let event = {
                 let mut state = self.kill_state.write().unwrap();