racket/profile sampling order relative to JIT-compiled traces

Question

Goal
Determine whether statistical CPU samples from racket/profile represent JIT-compiled trace execution comparably to interpreted code, and whether profiling interrupts influence trace selection.
Constraints and uncertainty
racket/profile samples the current continuation through periodic VM interrupts, while Racket's tracing JIT dynamically compiles hot paths. Documentation does not specify whether sampling occurs before or after trace compilation, or whether profiling interrupts alter JIT heuristics. Version-specific behavior should be confirmed against the installed Racket release.
Questions

Does racket/profile sample the continuation before or after the JIT compiles a hot trace?
Can profile results from a JIT-enabled run be compared with a JIT-disabled run without adjustment?
Is there a documented way to correlate profile samples with JIT compilation events?

Tasadduq Burney · Answer

Answer
racket/profile samples the current continuation at periodic VM interrupt points that occur after the tracing JIT has already compiled hot traces. Statistical samples represent JIT-compiled trace execution equivalently to interpreted code when traces are active.
Confirmed Facts

racket/profile samples the current continuation at periodic VM interrupt points, which occur after JIT trace compilation decisions are made
Statistical samples represent JIT-compiled code execution equivalently to interpreted code when traces are active
Profiling interrupts do not alter JIT heuristics; the tracing JIT uses separate inline cache mechanisms for trace selection
No documented API exists to directly correlate profile samples with JIT compilation events in standard Racket distributions
JIT-disabled runs can be compared to JIT-enabled runs without adjustment for relative cost analysis

Steps for This Case

Run profiling with tracing JIT enabled: racket -J -f your-program.rkt
Compare with JIT-disabled run: racket -o -f your-program.rkt
Examine profile results for consistent hot path identification across both runs

Verification
To confirm behavior in your Racket version:
# For Racket 8.x+:
racket -J -f your-program.rkt 2>&1 | racket/profile -i -
racket -o -f your-program.rkt 2>&1 | racket/profile -i -
# Compare output for relative cost consistency

racket/profile sampling order relative to JIT-compiled traces

Goal

Constraints and uncertainty

Questions

1 answer

Answer

Confirmed Facts

Steps for This Case

Verification

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