tracing: service uncached fs stats

Author: JoshuaKGoldberg

README.md

@@ -94,8 +94,8 @@ Comparing equivalent code paths:
 
 > 📌 Filed on TypeScript as [⚡ Performance: Project service spends excess time cleaning client files when called synchronously](https://github.com/microsoft/TypeScript/issues/59335).
 
-This trace shows the cost of the TypeScript project service calling `cleanupProjectsAndScriptInfos`.
-It also was run a common shape of linting: 1024 files with the "even" (triangle-shaped) imports layout.
+This comparison shows the cost of the TypeScript project service calling `cleanupProjectsAndScriptInfos`.
+It also was run on a common shape of linting: 1024 files with the "even" (triangle-shaped) imports layout.
 
 See `traces/service-file-cleanup/`:
 
@@ -107,7 +107,7 @@ They were generated with:
 ```shell
 cd files-1024-layout-even-singlerun-true-types-service
 node --cpu-prof --cpu-prof-interval=100 --cpu-prof-name=baseline.cpuprofile ../../node_modules/eslint/bin/eslint.js
-# clear ../../node_modules/typescript/lib/js > cleanupProjectsAndScriptInfos
+# clear ../../node_modules/typescript/lib/typescript.js > cleanupProjectsAndScriptInfos
 node --cpu-prof --cpu-prof-interval=100 --cpu-prof-name=skipping.cpuprofile ../../node_modules/eslint/bin/eslint.js
 ```
 
@@ -118,6 +118,61 @@ Hyperfine measurements show a ~15-20% improvement in lint time:
 | Baseline | 3.215 s ± 0.041 s | 4.483 s   |
 | Skipping | 2.501 s ± 0.017 s | 3.758 s   |
 
+### Comparison: Project Service Uncached File System Stats
+
+This comparison shows the cost uncached `fs.statSync` calls inside the project service.
+It also was run on a common shape of linting: 1024 files with the "even" (triangle-shaped) imports layout.
+
+See `traces/service-uncached-stats/`:
+
+- `baseline.cpuprofile`: Baseline measurement with no changes
+- `caching.cpuprofile`: Adding a caching `Map` to TypeScript's `statSync`
+
+They were generated with:
+
+```shell
+cd files-1024-layout-even-singlerun-true-types-service
+node --cpu-prof --cpu-prof-interval=100 --cpu-prof-name=baseline.cpuprofile ../../node_modules/eslint/bin/eslint.js
+# edit ../../node_modules/typescript/lib/typescript.js > statSync (see diff below)
+node --cpu-prof --cpu-prof-interval=100 --cpu-prof-name=caching.cpuprofile ../../node_modules/eslint/bin/eslint.js
+```
+
+<details>
+<summary><code>diff</code> patch to switch to the <em>Caching</em> variant...</summary>
+
+```diff
+diff --git a/node_modules/typescript/lib/typescript.js b/node_modules/typescript/lib/typescript.js
+index 4baad59..44639d5 100644
+--- a/node_modules/typescript/lib/typescript.js
++++ b/node_modules/typescript/lib/typescript.js
+@@ -8546,9 +8546,15 @@ var sys = (() => {
+         }
+       }
+     };
++    const statCache = new Map();
+     return nodeSystem;
+     function statSync(path) {
+-      return _fs.statSync(path, { throwIfNoEntry: false });
++      if (statCache.has(path)) {
++        return statCache.get(path);
++      }
++      const result = _fs.statSync(path, { throwIfNoEntry: false });
++      statCache.set(path, result);
++      return result;
+     }
+     function enableCPUProfiler(path, cb) {
+       if (activeSession) {
+```
+
+</details>
+
+Hyperfine measurements show a ~7-12% improvement in lint time:
+
+| Variant  | Measurement       | User Time |
+| -------- | ----------------- | --------- |
+| Baseline | 3.112 s ± 0.033 s | 4.382     |
+| Caching  | 2.740 s ± 0.030 s | 4.032     |
+
 ## Contributors
 
 <!-- spellchecker: disable -->