fix(bench): lower bench time budget when n is specified (#28454)

Closes #28430

Author: dsherret

cli/js/40_bench.js

@@ -241,7 +241,7 @@ const allMaxLength = 10_000_000;
 let all = new Array(allMaxLength);
 const lowPrecisionThresholdInNs = 1e4;
 
-async function benchMeasure(timeBudget, fn, desc, context) {
+async function benchMeasure(fn, desc, context) {
   let n = 0;
   let avg = 0;
   let wavg = 0;
@@ -251,7 +251,7 @@ async function benchMeasure(timeBudget, fn, desc, context) {
 
   // warmup step
   let c = 0;
-  let iterations = desc.warmup > 0 ? desc.warmup : 20;
+  let iterations = desc.warmup >= 0 ? desc.warmup : 20;
   let budget = 10 * 1e6;
 
   if (!desc.async) {
@@ -298,7 +298,7 @@ async function benchMeasure(timeBudget, fn, desc, context) {
 
   // measure step
   iterations = desc.n > 0 ? desc.n : 10;
-  budget = timeBudget * 1e6;
+  budget = desc.n > 0 ? 10 * 1e6 : 500 * 1e6;
 
   if (wavg > lowPrecisionThresholdInNs) {
     if (!desc.async) {
@@ -475,10 +475,8 @@ function wrapBenchmark(desc) {
         });
       }
 
-      const benchTimeInMs = 500;
       const context = createBenchContext(desc);
       const stats = await benchMeasure(
-        benchTimeInMs,
         fn,
         desc,
         context,

cli/tsc/dts/lib.deno.ns.d.ts

@@ -1242,9 +1242,16 @@ declare namespace Deno {
     /** If at least one bench has `only` set to true, only run benches that have
      * `only` set to `true` and fail the bench suite. */
     only?: boolean;
-    /** Number of iterations to perform. */
+    /** Number of iterations to perform.
+     * @remarks When the benchmark is very fast, this will only be used as a
+     * suggestion in order to get a more accurate measurement.
+     */
     n?: number;
-    /** Number of warmups to do before running the benchmark. */
+    /** Number of warmups to do before running the benchmark.
+     * @remarks A warmup will always be performed even if this is `0` in order to
+     * determine the speed of the benchmark in order to improve the measurement. When
+     * the benchmark is very fast, this will be used as a suggestion.
+     */
     warmup?: number;
     /** Ensure the bench case does not prematurely cause the process to exit,
      * for example via a call to {@linkcode Deno.exit}.