From 657576ae7c9da142bdbbe43e64d425fa292124fe Mon Sep 17 00:00:00 2001
From: Brandon Cheng <brandon.cheng@protonmail.com>
Date: Fri, 14 Aug 2020 14:13:19 -0400
Subject: [PATCH] Set stackTraceLimit to 0 in fileSystemEntryExists

The exception thrown by Node.js's fs.statSync function contains a stack
trace that can be expensive to compute. Since this exception isn't used
by fileSystemEntryExists, we can safely set Error.stackTraceLimit to 0
without a change in behavior.

---

A significant performance improvement was noticed with this change while
profiling tsserver on packages within a proprietary monorepo.
Specifically, my team saw high self time percentages for Node.js's
uvException and handleErrorFromBinding internal functions. These
functions are executed within fs.statSync when it fails to find the
given path.

https://user-images.githubusercontent.com/906558/90183227-220cb800-dd81-11ea-8d61-f41f89481f46.png

fs.statSync: https://github.com/nodejs/node/blob/v14.4.0/lib/fs.js#L1030-L1037
handleErrorFromBinding: https://github.com/nodejs/node/blob/v14.4.0/lib/internal/fs/utils.js#L254-L269
uvException: https://github.com/nodejs/node/blob/v14.4.0/lib/internal/errors.js#L390-L443

 ## Measurements

After adding Error.stackTraceLimit = 0, we saw:

- For a large configured project with 12,565 files, tsserver reached the
  projectLoadingFinish event 48.78% faster. (~46.786s vs ~31.447s)
- For a medium project with 7,064 files, tsserver was 25.75% faster.
  (~20.897s vs ~16.618s)
- For a small project with 796 files, tsserver was only a negligible
  3.00% faster. (~3.545s vs ~3.442)

Measurements were taken on macOS 10.15.6, Node.js 14.4.0, and a recent
master commit of TypeScript (610fa28d). The average of 3 runs before and
after this change were taken.

I would normally include .cpuprofile and isolate-*-*-*.log files, but
can't post them publicly in this case. If there's any other summaries
the TypeScript team would be curious about I can report them.

 ## fs.statSync Misses

Within our monorepo, the fs.statSync misses were mostly searches for
alternative file extensions of module imports.

- For node_modules imports, a lot of .ts/.tsx lookups failed until the
  .d.ts file was found.
- Within projects with a lot of JSX files, .ts files were looked for
  before finding the .tsx version.
- In the medium scale project mentioned above, a total of 38,515
  non-existent files were queried during createProgram.
---
 src/compiler/sys.ts | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 1662a7ef31096..efa51678f4da0 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -1662,6 +1662,11 @@ namespace ts {
             }
 
             function fileSystemEntryExists(path: string, entryKind: FileSystemEntryKind): boolean {
+                // Since the error thrown by fs.statSync isn't used, we can avoid collecting a stack trace to improve
+                // the CPU time performance.
+                const originalStackTraceLimit = Error.stackTraceLimit;
+                Error.stackTraceLimit = 0;
+
                 try {
                     const stat = _fs.statSync(path);
                     switch (entryKind) {
@@ -1673,6 +1678,9 @@ namespace ts {
                 catch (e) {
                     return false;
                 }
+                finally {
+                    Error.stackTraceLimit = originalStackTraceLimit;
+                }
             }
 
             function fileExists(path: string): boolean {