Profilers Are Lying Hobbitses

on YouTube ↗52m

" summary

Claim: common Java profilers mislead because of sampling mechanics and JIT transformations. VisualVM-style profilers show safepoint bias that pins samples to arbitrary loop entry lines; Mission Control (JFR) uses JFR cycles-based sampling and can hide missed samples and native work; async/async-profiler exposes native and GC cost (example: GC taking ~50% of samples), while instruction-level profiles misplace blame near expensive ops like division and suffer from inlining and compiled-version mixing. Use multiple profilers, enable debug info, check sample counts and per-thread breakdowns before optimizing.

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" chapters (11)

1. 0:00Introduction to profilers
2. 2:15How sampling profilers work
3. 5:26Safepoint bias in profilers
4. 11:28Java Mission Control overview
5. 18:14Flame graphs and JFR comparison
6. 24:37Instruction-level profiling
7. 32:10Accuracy of instruction profilers
8. 37:33Mapping assembly to code
9. 41:08Inlining and profiling issues
10. 45:51Thread utilization in profiles
11. 48:27Using profilers effectively

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