From c40ae87fe54d74d2a8b1bed1cdfbb4aa823c7c3d Mon Sep 17 00:00:00 2001
From: not-matthias <matthias@codspeed.io>
Date: Fri, 23 Oct 2026 18:06:15 +0200
Subject: [PATCH] fix(core): seed the V8 RNG and string hashes in walltime mode

Walltime runs left Math.random() and V8's hash seed random per process, so benchmarks using random inputs or hash-heavy structures varied between runs for reasons unrelated to the code under test. Pass the analysis-mode seed flags (--hash-seed=1, --random-seed=1, and --no-randomize-hashes on Node < 18) in walltime too, via one shared helper. Disabled mode stays unseeded. crypto randomness and key iteration order are unchanged.
---
 docs/introspection.md              | 11 ++++++++++-
 packages/core/src/introspection.ts | 17 ++++++++++++-----
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/docs/introspection.md b/docs/introspection.md
index 6bd9fd2b..c5d2ca21 100644
--- a/docs/introspection.md
+++ b/docs/introspection.md
@@ -8,7 +8,16 @@ benchmark is measured. TurboFan remains disabled with `--no-opt`, and Maglev is
 explicitly disabled on Node.js 24 and newer.
 
 This changes the measurement baseline compared with earlier releases. It does not
-isolate inline-cache feedback from other benchmarks. Walltime flags are unchanged.
+isolate inline-cache feedback from other benchmarks.
+
+## Seed flags
+
+Simulation, memory, and walltime mode pass `--hash-seed=1` and `--random-seed=1` (plus
+`--no-randomize-hashes` on Node.js < 18). `Math.random()` returns the same
+sequence on every run, including in worker threads, and hash tables get the same
+internal layout, so hash collisions do not change between runs. Key iteration
+order is unaffected, since it follows insertion order. `crypto` randomness stays
+random. Walltime mode uses no other analysis flags.
 
 ## Testing introspection locally
 
diff --git a/packages/core/src/introspection.ts b/packages/core/src/introspection.ts
index 18c29ba0..8906c1d9 100644
--- a/packages/core/src/introspection.ts
+++ b/packages/core/src/introspection.ts
@@ -7,6 +7,15 @@ const CUSTOM_INTROSPECTION_EXIT_CODE = 0;
 
 const V8_LOG_FILENAME_PATTERN = "codspeed-v8-%p.log";
 
+// Fixed seeds so Math.random() and hash table layout match on every run
+const getSeedFlags = (nodeVersionMajor: number) => {
+  const flags = ["--hash-seed=1", "--random-seed=1"];
+  if (nodeVersionMajor < 18) {
+    flags.push("--no-randomize-hashes");
+  }
+  return flags;
+};
+
 export const getV8Flags = () => {
   const nodeVersionMajor = parseInt(process.version.slice(1).split(".")[0]);
   const instrumentMode = getInstrumentMode();
@@ -17,8 +26,7 @@ export const getV8Flags = () => {
     case "analysis": {
       flags.push(
         ...[
-          "--hash-seed=1",
-          "--random-seed=1",
+          ...getSeedFlags(nodeVersionMajor),
           "--no-opt",
           "--always-sparkplug",
           "--predictable",
@@ -28,9 +36,6 @@ export const getV8Flags = () => {
           "--max-old-space-size=4096",
         ],
       );
-      if (nodeVersionMajor < 18) {
-        flags.push("--no-randomize-hashes");
-      }
       if (nodeVersionMajor < 20) {
         flags.push("--no-scavenge-task");
       } else {
@@ -48,6 +53,8 @@ export const getV8Flags = () => {
     }
 
     case "walltime": {
+      flags.push(...getSeedFlags(nodeVersionMajor));
+
       // Emit the V8 jitdump
       flags.push("--perf-prof");