Slow stepping through large file

[roblourens]

@weswigham pointed this out.

Every time I step while debugging in vscode, the debug adapter gets a breakpointLocations request. We should only make this request when we need to, because it could be expensive. While debugging tests in the TS codebase, there is a case where sometimes the DA's request to the runtime takes a few seconds to complete when it is triggered at the same time as a step. The result is that each step takes about 3s, which is not a good experience. The file we are debugging is 11MB which probably accounts for part of the slowness.

[roblourens]

@weswigham here is a version of the DA that has breakpointLocations disabled, if you want to verify that this fixes it

node-debug2-1.41.3.vsix.zip

[weswigham]

Errr, it actually doesn't:

Check out line 1529:
debugadapter.txt

there's still a 3s delay between resume and the pause on the next line (and the line in question in this log is just an array.push, which shouldn't be a 3s operation). It's possible something in v8 itself is slow, but I have no idea how I'd verify that.

[weswigham]

Yeah - definitely a v8 thing - if I revert to node 8 (vs 13), perf is fine. Lemme see if I can approximately track when it regressed.

[weswigham]

Done. It's a regression in node 13.2.0 (there's no slowdown in 13.1, 13.2 and 13.3 are both slow). I haven't checked what v8 versions/changes that corresponds to, yet.

[roblourens]

I filed nodejs/node#30926, also FYI @connor4312

[connor4312]

May be related to microsoft/vscode-js-debug#138 as well (I tested on 12 and marked as CNR, could have been caused by testing on >13.2)

[roblourens]

I don't think so. There aren't many variables being sent around.

[weswigham]

Looks like this will be fixed by a new version of v8 (version 8.0 or above) in node 14+. I've checked with a canary build, specifically https://nodejs.org/download/v8-canary/v14.0.0-v8-canary2020011995087de57b/

[connor4312]

Great to hear!