I haven't looked into the details, but it sort of makes sense that evaluating a breakpoint condition would require reparsing. Conditional breakpoints are basically a local eval(), which can bind to variables from any enclosing scope by name. But if those scopes didn't contain the word "eval" during the original parse, then they had no reason to keep full variable data around. There's no way to reparse every enclosing scope without at least lazy-parsing through the rest of the code in the file. And lazy-parsing the rest of the file could cause this exact problem, because presumably we don't save full local-eval data for the other functions which might also have conditional breakpoints.
________________________________ From: [email protected] <[email protected]> on behalf of Gunes Acar <[email protected]> Sent: Tuesday, October 6, 2020 4:27 AM To: [email protected] <[email protected]> Subject: [EXTERNAL] Re: [v8-dev] Re: Breakpoints trigger repeated full-parse of the debugged script Thank you, Leszek and Dan for the suggestions. I tried passing<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Fgunesacar%2F2daf6cca049b10c832536591f69b58a8%23file-breakpoint_perf-js-L77&data=04%7C01%7Cseth.brenith%40microsoft.com%7Ca7e89172bb6644c6c01408d869eacc4b%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637375805933671070%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=dzGy5XoFPLbBzlHfGI7rZ3OUZXDYual8mibiaEUn6ho%3D&reserved=0> --no-enable-lazy-source-positions, but I still get full script parses for each hit breakpoint. I put together the small repro<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Fgunesacar%2F2daf6cca049b10c832536591f69b58a8&data=04%7C01%7Cseth.brenith%40microsoft.com%7Ca7e89172bb6644c6c01408d869eacc4b%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637375805933681022%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=zx%2BJY37H2gp8zH4971RK3AAnaJV2whXSTsaKWnKy7Nk%3D&reserved=0> that Dan suggested. It uses Puppeteer, although passing `headless=false` didn't change things either. The script will print instructions<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Fgunesacar%2F2daf6cca049b10c832536591f69b58a8%23file-breakpoint_perf-js-L175-L177&data=04%7C01%7Cseth.brenith%40microsoft.com%7Ca7e89172bb6644c6c01408d869eacc4b%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637375805933681022%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=Ubhe8tD3PavmL%2F%2FkfH3gskQwZlOAGv9Qyw9sXPYSmx4%3D&reserved=0> to count the number of full parses in the logs. Let me know if I can make it easier for you to reproduce the issue. Thanks again, Gunes On Tue, Oct 6, 2020 at 10:23 AM Dan Elphick <[email protected]<mailto:[email protected]>> wrote: If it is down to lazy source positions, then what should happen is that it would reparse each function at the point that you set the breakpoint as otherwise it wouldn't know where to set it. In that case you would get 1000 function reparses. It shouldn't need to reparse every function each time, because a) it only needs the source positions for the function it's setting the break point on and b) it doesn't need to parse the entire program to do that. As Leszek says, disabling the feature via the flag would tell you if it's responsible. The code in your gist is just the code to be debugged, but not the code that sets the break points. A repro with just 10 functions should be sufficient to show that it's doing 100 reparses and we can go from there. Thanks, Dan On Tue, 6 Oct 2020 at 09:08, Leszek Swirski <[email protected]<mailto:[email protected]>> wrote: This might be lazy source positions forcing a reparse, have you tried running with --no-enable-lazy-source-positions? On Tue, Oct 6, 2020 at 3:23 AM Gunes Acar <[email protected]<mailto:[email protected]>> wrote: The missing screenshot of the trace can be seen at: Github: https://gist.github.com/gunesacar/b8f82266e93fbfda2bf06c573af3caea#gistcomment-3478635<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Fgunesacar%2Fb8f82266e93fbfda2bf06c573af3caea%23gistcomment-3478635&data=04%7C01%7Cseth.brenith%40microsoft.com%7Ca7e89172bb6644c6c01408d869eacc4b%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637375805933690981%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=rZ2L%2FyZpm6jrpQkkIMlyl2DBnltBALX0NpjVWUze3KY%3D&reserved=0> On Tuesday, October 6, 2020 at 3:15:53 AM UTC+2 Gunes Acar wrote: Hi all, For an academic research project, we use Chrome Devtools Protocol to set breakpoints at function returns, and collect function call metadata (stack frames, arguments, timestamps) using a condition script(*). Although the condition script always returns `false` to avoid pausing the debugger, we still observe a significant overhead per "hit" breakpoint. To identify the root cause, we set up tracing and found that V8.ParseProgram is called for each "hit" breakpoint. We then logged function events using "--js-flags=--log-function-events", which showed that all functions in the debugged script is fully parsed (i.e., `function,full-parse,...` event) for each "hit" breakpoint. [X] For example, assume the debugged script contains 10K function definitions, of which only a 100 is called. We observe ~10K*100=~1M function (full-)parse events in the logs, which slows down the process and makes it unfeasible on heavy websites. 1) Is it possible to prevent repeated parsing of the debugged script for each evaluated breakpoint? 2) Could there be an easier and faster way of logging all function calls with the associated stack frames and arguments? Pointers to the relevant code locations would be much appreciated if modifying Chrome/V8 source code is the only way to do that. The trace and test files can be found in this Gist<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Fgunesacar%2Fb8f82266e93fbfda2bf06c573af3caea%23file-trace-json&data=04%7C01%7Cseth.brenith%40microsoft.com%7Ca7e89172bb6644c6c01408d869eacc4b%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637375805933690981%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=bKo0KZMWMvSK1CDP959M2V0bDfGe0J%2Feg1zZFwb4%2Fc8%3D&reserved=0>. Thanks so much and stay safe, Gunes PS: None of our breakpoints were actually hit, since the condition script always returns false. 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