Slow performance in incremental mode with no changes

[JelleZijlstra]

Bug Report

🔎 Search Terms

Performance

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about performance

⏯ Playground Link

n/a

💻 Code

n/a

🙁 Actual behavior

I have a huge, closed-source TypeScript project (7600+ files, 2M+ lines of TypeScript).

Typechecking is slow, about 30 s even with incremental mode on. We'd like to get this faster and discovered that even with incremental mode on and with no changes in the code, TS visits and parses every single file in the project.

Here is a screenshot from the awesome perf tracing tool (from an incremental run with no changes since the previous run):

This was from typescript@next (which seems a little faster than the current release, thanks for that!). Of 28 s total, 22 s was spent in findSourceFile.

🙂 Expected behavior

Incremental mode should cache enough information that it's not necessary to re-parse each file on the next run.

[DanielRosenwasser]

Do you have this issue with findSourceFile on non-incremental compiles?

[JelleZijlstra]

When I turn off incremental: true and tsBuildInfoFile in my tsconfig, I get the above trace. findSourceFile is still equally slow, but in addition it spends a lot of time actually type checking the code.

[samuliasmala]

I'm experiencing the same. I was expecting the incremental flag to reduce the compilation time to watch mode incremental compilation time, but the reality is different. I have smaller codebase so npx tsc with incremental: false takes 8 seconds. First incremental: true run takes the same 8 seconds but after that it's reduced to 4 seconds.

However, if I run npx tsc --watch it takes the same amount of time to start as without the --watch flag (8 s or 4 s) but after that the compilation takes less than a second.

[10:37:59 PM] Starting compilation in watch mode...

[10:38:03 PM] Found 0 errors. Watching for file changes.

[10:38:31 PM] File change detected. Starting incremental compilation...

[10:38:31 PM] Found 0 errors. Watching for file changes.

Steps to reproduce

Some output removed for clarity.

$ node -v
v16.10.0

$ npm -v
7.24.0

$ git clone [email protected]:microsoft/TypeScript-Node-Starter.git
$ cd TypeScript-Node-Starter/
$ npm install

$ time npx tsc
real    0m6.447s

$ time npx tsc
real    0m6.365s

$ time npx tsc --incremental
real    0m6.444s

$ time npx tsc --incremental
real    0m2.441s

$ time npx tsc --incremental --watch

Again the first compilation of the watch command takes that 2 s but after that it is less than a second.