bench_alpha_lib.php

<?php
/* Shared helper for the Zend/bench_*_alpha.php scripts.
 *
 * Order-alternated microbenchmark harness. Each bench() call warms up, then
 * runs an EVEN number of order-alternated repeats of `iters` invocations of
 * $base and $alpha, and reports the BEST (minimum) time for each arm. It prints
 * a single line the Zend/alpha_guard.sh harness parses:
 *
 *   <label> iters=<N> base=<ns> ns  alpha=<ns> ns  ratio=<base/alpha>x  speedup=<%>
 *
 * ratio > 1.0 means the _alpha variant is faster.
 *
 * Why min-of-N (not median), and why an EVEN repeat count: on a thermally
 * constrained host (e.g. a Raspberry Pi), the arm that runs SECOND in a pass
 * runs on a warmer SoC and is biased slower. With an odd repeat count the
 * order alternation is unbalanced (one arm runs second more often), which
 * systematically penalises that arm. Using an EVEN count (equal first/second
 * splits) plus the per-arm MINIMUM (the coolest pass, least throttled)
 * cancels that bias: each arm's reported time is its least-throttled run.
 * require this from a bench script with:  require __DIR__ . '/bench_alpha_lib.php';
 */

function bench(string $label, callable $base, callable $alpha, int $iters): void {
    for ($i = 0; $i < 50; $i++) { $base(); $alpha(); }
    $repeats = 8;
    $baseTimes = []; $alphaTimes = [];
    for ($r = 0; $r < $repeats; $r++) {
        if (($r & 1) === 0) {
            $t0 = hrtime(true); for ($i = 0; $i < $iters; $i++) $base(); $tb = hrtime(true) - $t0;
            $t0 = hrtime(true); for ($i = 0; $i < $iters; $i++) $alpha(); $ta = hrtime(true) - $t0;
        } else {
            $t0 = hrtime(true); for ($i = 0; $i < $iters; $i++) $alpha(); $ta = hrtime(true) - $t0;
            $t0 = hrtime(true); for ($i = 0; $i < $iters; $i++) $base(); $tb = hrtime(true) - $t0;
        }
        $baseTimes[] = $tb; $alphaTimes[] = $ta;
    }
    sort($baseTimes); sort($alphaTimes);
    /* min per arm: the least-throttled (coolest) pass, which removes the
     * second-arm-warms-the-SoC bias. [0] is the smallest after sort(). */
    $bm = $baseTimes[0];
    $am = $alphaTimes[0];
    $ratio = ($am > 0) ? ($bm / $am) : 0;
    $speedup = ($bm > 0) ? (100 * (1 - $am / $bm)) : 0;
    printf("%-52s iters=%-6d base=%10.0f ns  alpha=%10.0f ns  ratio=%5.2fx  speedup=%+6.1f%%\n",
        $label, $iters, $bm, $am, $ratio, $speedup);
}