Don't shift/push the samples array

- just index % 5 into it since we have a fixed size array
- rename LAST_UTILS_NUM to NUM_SAMPLES for more readability

src/core/ThreadPoolScheduler.pm

@@ -441,7 +441,7 @@ my class ThreadPoolScheduler does Scheduler {
     # it takes stock of the current situation and decides whether or not to
     # add threads.
     my constant SUPERVISION_INTERVAL = 0.01;
-    my constant LAST_UTILS_NUM       = 5;
+    my constant NUM_SAMPLES          = 5;
     method !maybe-start-supervisor(--> Nil) {
         unless $!supervisor.DEFINITE {
             $!supervisor = Thread.start(:app_lifetime, {
@@ -474,7 +474,8 @@ my class ThreadPoolScheduler does Scheduler {
                 scheduler-debug "Supervisor started";
                 my num $last-rusage-time = nqp::time_n;
                 my int $last-usage = self!getrusage-total();
-                my num @last-utils = 0e0 xx LAST_UTILS_NUM;
+                my num @last-utils = 0e0 xx NUM_SAMPLES;
+                my int $sampled;
                 my int $cpu-cores = nqp::cpucores();
                 scheduler-debug "Supervisor thinks there are $cpu-cores CPU cores";
                 loop {
@@ -496,12 +497,11 @@ my class ThreadPoolScheduler does Scheduler {
                     my num $normalized-delta = $usage-delta / $rusage-period;
                     my num $per-core = $normalized-delta / $cpu-cores;
                     my num $per-core-util =
-                      100 * ($per-core / (1000000 * LAST_UTILS_NUM));
+                      100 * ($per-core / (1000000 * NUM_SAMPLES));
 
                     # Since those values are noisy, average the last
-                    # LAST_UTILS_NUM values to get a smoothed value.
-                    @last-utils.shift;
-                    @last-utils.push: $per-core-util;
+                    # NUM_SAMPLES values to get a smoothed value.
+                    @last-utils[++$sampled % NUM_SAMPLES] = $per-core-util;
                     my $smooth-per-core-util = @last-utils.sum;
                     scheduler-debug-status "Per-core utilization (approx): $smooth-per-core-util%";