diffr web: size the worker pool per browser and release the engine when idle

apps/diffr-web/src/engine/engine.ts

@@ -1,12 +1,14 @@
+import { isAndroid, isIOS } from "vs/base/common/platform.js";
+
 import { cacheKey } from "../cache.js";
 import { agentPlugins } from "../settings.js";
 /**
  * The page's handle on diffr: the wasm module is fetched and compiled once, then handed to a pool
  * of workers, so files diff in parallel without each worker paying for its own compile. The pool
  * starts at one and grows while work queues, a worker at a time: each spends a second or two
  * compiling diffr's queries, and workers starting together slow each other several times over.
- * Every worker goes after a few idle seconds, since wasm memory never shrinks: the compiled module
- * stays, so the next file's worker starts in tens of milliseconds.
+ * Every worker goes after a few idle seconds, since wasm memory never shrinks, and the compiled
+ * module with the last of them: compiling again costs the next file well under a tenth of a second.
  */
 // Gzipped by the build (vite.config.ts): static hosts cap a file at 25 MiB, and the engine is larger.
 import wasmUrl from "../wasm/diffr_web_bg.wasm.gz?url";
@@ -53,15 +55,49 @@ export interface EngineStats {
   diffMs: number;
 }
 
-const MAX_WORKERS = Math.max(
-  1,
-  Math.min(8, (navigator.hardwareConcurrency || 4) - 1),
-);
+declare global {
+  interface Navigator {
+    /** Chrome's: the device's memory in GB, rounded and capped at 8. Safari and Firefox lack it. */
+    readonly deviceMemory?: number;
+  }
+}
 
-const IDLE_MS = 3_000;
+const MB = 1024 * 1024;
+
+/**
+ * How many workers the pool grows to, and the wasm memory past which a worker is replaced once its
+ * queue drains. Each worker's memory never shrinks, so together they must fit what the browser
+ * lets the tab hold. iOS kills a tab over its budget (often well under 1 GB) rather than failing
+ * the allocation, and says nothing of its memory or cores, so it gets one lean worker.
+ */
+function workerBudget() {
+  const cores = Math.max(
+    1,
+    Math.min(8, (navigator.hardwareConcurrency || 4) - 1),
+  );
+
+  const { deviceMemory } = navigator;
+
+  if (isIOS) return { workers: 1, memoryLimit: 256 * MB };
 
-/** Past this, a worker is replaced once its queue drains. */
-const MEMORY_LIMIT = 1024 * 1024 * 1024;
+  if (isAndroid)
+    return {
+      workers:
+        deviceMemory !== undefined && deviceMemory >= 8
+          ? Math.min(2, cores)
+          : 1,
+      memoryLimit: 384 * MB,
+    };
+
+  if (deviceMemory !== undefined && deviceMemory < 8)
+    return { workers: Math.min(2, cores), memoryLimit: 512 * MB };
+
+  return { workers: cores, memoryLimit: 1024 * MB };
+}
+
+const { workers: MAX_WORKERS, memoryLimit: MEMORY_LIMIT } = workerBudget();
+
+const IDLE_MS = 3_000;
 
 const stats: EngineStats = {
   maxWorkers: MAX_WORKERS,
@@ -307,6 +343,8 @@ export class Engine {
         }
       }
 
+      // Each worker holds the module it was sent, so with none left the page holds the last.
+      if (!stats.workers) compiled = undefined;
       changed();
     }, IDLE_MS);
   }