Bug 20228 – Enable use of sampling tool for profiling

• Status: RESOLVED
• Resolution: WONTFIX
• Component: JavaScriptCore

[Gavin Barraclough]

Explore the possibility to make use of sampling tool to provide pre-line accurate profiling of javascript applications.  More details in:

https://bugs.webkit.org/show_bug.cgi?id=20175

[Gavin Barraclough]

Comments from ggaren:

> (From update of attachment 22514 [details] [edit])
> There's a lot of overlap between attachment 22487 [details] [edit], which fixes a bug and does
> some cleanup, and this patch, which does those things and also adds two new
> features: per-line sampling, and sampling built in without a special binary.
> 
> I'd recommend landing attachment 22487 [details] [edit] first. Then, it will be easier discuss
> the specifics of a patch that just implements per-line sampling without a
> special binary (probably worth a separate bug report).
> 
> Generally, the DoSample opcode code in this patch looks sound. I'm surprised
> that the extra branches for callingHostFunction are not costly in the
> non-sampling case, but oh well :).
> 
> I don't really think "ENABLE_SAMPLING_TOOL_ALWAYS" captures the difference
> between these two modes well. One mode samples each opcode, to show VM
> engineers what's hot in the engine; the other mode samples each source line, to
> show webpage engineers what's hot in their JavaScript. At least, that's how I
> think of it. Do you think of per-line sampling as a feature for VM engineers?
> 
> To be fully viable, I think this second mode -- the per-line mode -- would need
> to solve three more problems:
> 
> (1) Dynamically recompile functions that were compiled before sampling started,
> so they get sampled, too. (The current user-visible profiler is able to profile
> a page without reloading it, so the per-line profiler would need to match that
> ability.)
> 
> (2) Remove the existing JavaScript profiler hooks and / or merge with them, to
> provide a consistent JavaScriptCore API to the Web Inspector. (This will also
> be a ~1% performance win.)
> 
> (3) Web Inspector UI for per-line profiles. Tim Hatcher, Kevin McCullough, and
> Adam Roben are the experts on this.
> 
> I'm going to clear the review flag, since I think attachment 22487 [details] [edit] should land
> first, and the tasks I've outlined here are pretty beefy, deserving their own
> bugs.
> 

taken from: https://bugs.webkit.org/show_bug.cgi?id=20175

[Gavin Barraclough]

May not be a bad idea to do sample based profiling in the web inspector, but I don't think this bug usefully tracks that.  Raised this bug to track some comments on a patch that is now a long way out of date (based on the interpreter).

Closing, can open a new bug if we get serious about actually doing this.