`_remote_debugging`: `RemoteUnwinder(..., jit=True)`

Feature or enhancement

Proposal:

:)

The idea here is to expose JIT internals a bit more in the Tachyon profiler.

The questions that could be answered trivially:

• Is JIT actually used? How often?
• Which executor is hot?
• What change made the largest difference (diff_flamegraph), in terms of % spent in JIT? Per line?

A bit harder:

• Which UOPs actually eat time?
• Which stensils are good?
• What guards or deopts are bad?
• What is the trace shape?

The very minimal change that I'm thinking about here is just adding --jit flag (active for --jsonl and maybe for --live but it's a catnip), and adding JitInfo to ThreadInfo.

That's how it could look:

JitInfo(
  executor_id,
  flags
)

This will requires extending debug offsets, obviously.

We could potentially expose native_pc, native_offset, uop_index, exit_index etc. The question would be how to expose metadata for it without balooning JitInfo, and how far we can get without blocking and native unwinding. :)

I'm pretty much open to any other ideas.

There will be definitely some performance impact (more VM reads), but hidden behind the --enable-experimental-jit flag should be bearable, and we could be more playful, going back and forth.

If you think it makes sense, I'd be willing to take a shot, but this will take multiple PRs.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Start by tracing _remote_debugging, RemoteUnwinder(..., jit=True), and the existing ThreadInfo flow. Compare the proposed --jit behavior for --jsonl and possibly --live, then inspect how debug offsets are handled. Done would require a settled scope for exposing JitInfo and its metadata behind --enable-experimental-jit, with performance impact considered.