Very slow debug stepping in large file after Node 13.2

[roblourens]

We noticed that debugging tests in the TypeScript repo has become extremely slow after v13.2. The file we are stepping through is extremely large, which is probably part of it. Here are steps to reproduce:

• Clone and build https://github.com/microsoft/TypeScript
• Run node --inspect-brk node_modules/mocha/bin/_mocha -u bdd --no-timeouts --colors built/local/run.js -f tests/cases/compiler --skip-percent 0
• Attach with devtools
• Hit a breakpoint anywhere in parser.ts
• Step
• Notice that every step takes several seconds to pause on the next line
• Using any version of node before 13.2, stepping is very fast

For slightly easier setup, you can debug in VS Code with the "Mocha Tests" launch config

[addaleax]

I can’t reproduce this using the steps above; can you fill out the bug report template here, including OS information?

Also, my main suspect in the release would be the V8 update to 7.9.

[roblourens]

• Version: v13.3.0
• Platform: Linux robfast 5.0.0-1027-azure docs: added a the #29~18.04.1-Ubuntu SMP Mon Nov 25 21:18:57 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

[roblourens]

I wasn't immediately sure how to test with the same code in Chrome, but if you think this would be a V8 issue, I can try to repro it there and report upstream.

[weswigham]

I also originally reported this to @roblourens using node 13.2, on Windows 10 (so it does repro on other platforms/versions); and have tested that it does not repro on node 13.1. I, too, have come to the conclusion that it's likely a problem in a v8 upgrade, since pretty much the whole inspector API and debug protocol is just reexposed from v8. It saw some notable change recently to better support wasm debugging.

[roblourens]

Filed https://bugs.chromium.org/p/chromium/issues/detail?id=1034191&q=&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified

[targos]

Fixed in V8 8.0.