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Python ウェブアプリケーションのためのプロファイラの実装 // Implementation of a profiler for Python web applications

Yusuke Miyazaki
September 16, 2019

Python ウェブアプリケーションのためのプロファイラの実装 // Implementation of a profiler for Python web applications

PyCon JP 2019 でのトークの発表資料です.
- GitHub: https://github.com/ymyzk/wsgi_lineprof
- YouTube: https://www.youtube.com/watch?v=ojnVMGon5d4

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Yusuke Miyazaki

September 16, 2019

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  1. Python΢ΣϒΞϓϦέʔγϣϯͷͨΊͷ
 ϓϩϑΝΠϥͷ࣮૷ Yusuke Miyazaki
 PyCon JP 2019 / 2019-09-16 Implementation

    of a profiler for Python web applications
  2. Who am I? • Yusuke Miyazaki / ٶ㟒 ༐ี /

    @ymyzk • Site Reliability Engineer @ Indeed • Personal projects • wsgi_lineprof: line-by-line profiler • mypy Playground: mypy on web browsers • tox-gh-actions:
 smooth integration of tox with GitHub Actions • See ymyzk.com for more details
  3. Have you ever used a profiler?
 Result at PyCon

    JP: about 50%
  4. Have you ever built a profiler?
 " " "

    Result at PyCon JP: only 1 person
  5. Goal of the Talk “Implementing a profiler is not so

 through understanding inside of a profiler
  6. Agenda • Introduction • What is wsgi_lineprof? • Features of

    wsgi_lineprof • How wsgi_lineprof works? • Technologies for implementing a profiler • Conclusion
  7. What is wsgi_lineprof?

  8. Line-by-Line Profiler for Web Apps Shows line-by-line execution time for

    each request
  9. How to Use Add a few lines to the existing

    WSGI application # Your existing WSGI application app: WSGIApplication # Enable profiler from wsgi_lineprof.middleware \
 import LineProfilerMiddleware app = LineProfilerMiddleware(app)
  10. Features of wsgi_lineprof • Line-by-line profiling • Finding a bottleneck

    quickly • WSGI middleware • Integration with many Python web applications • Easily pluggable • All configuration for profiling in one place
  11. Predecessors wsgi_lineprof is inspired by the predecessors • Line-by-line profiler

    for Python • rkern/line_profiler • Line-by-line profiler for Ruby web applications • kainosnoema/rack-lineprof
  12. How wsgi_lineprof works?

  13. Python Architecture Web
 Server Web

  14. Python Architecture Web
 Server Web
 App Profiler 1. Capture

  15. Python Architecture Web
 Server Web
 App Profiler 1. Capture

    2. Get information
 on execution
  16. Python Architecture Web
 Server Web
 App Profiler Timer 1. Capture

    request/response 2. Get information
 on execution 3. Measure exec. time
 per line precisely
  17. WSGI Middleware wsgi_lineprof is implemented as a WSGI middleware •

    What is WSGI middleware? • Wrapper for WSGI app to add features • Why WSGI middleware? • Keep track of HTTP requests • Support multiple web application frameworks • Easily enable/disable a middleware
  18. Implementing WSGI Middleware • WSGI application is a callable (e.g.,

    a function) • WSGI middleware can be implemented as a higher-order function WSGIApplication = Callable[
 [WSGIEnvironment, StartResponse],
 Iterable[bytes]] WSGIMiddleware = Callable[
 [WSGIApplication], WSGIApplication]
  19. Example of WSGI Middleware class MyMiddleware: def __init__(self, app): self.app

    = app
 def __call__(self, env, start_response): print(“start profiling”)
 res = self.app(env, start_response)
 print(“end profiling”) return res See wsgi_lineprof/middleware.py
  20. Tracing Line-by-Line Execution wsgi_lineprof uses PyEval_SetTrace Python/C API via Cython

    extension to track line-by-line execution • What is tracing? 1. Register a callback function to CPython 2. CPython calls the callback function on specific events like executing a line, calling a function, raising exception…
  21. How to Use Tracing on CPython CPython provides various levels

    of APIs for tracing • trace module • setprofile/settrace functions in sys module
 • PyEval_SetTrace/PyEval_SetProfile functions in Python/C API import sys sys.settrace(lambda frame, event, arg: ...) See extensions/extensions.pyx
  22. How CPython Evaluates Source Code 1. Read source code 2.

    Convert to parse tree 3. Convert to AST 4. Convert to CFG 5. Convert to byte code • Byte code retains corresponding line number 6. Evaluate byte code • Call tracing function when evaluating byte code See Python Developers’s Guide
  23. Measuring Time wsgi_lineprof calls a different timer via C extension

    depending on platforms for measuring time
 • Why measuring time is not simple? • Needs to use monotonic timer • time.time() / datetime.now() are not • Reduce overhead of getting time • Use of time.monotonic/perf_counter() incurs additional overhead
  24. Measuring Time on Various Platforms APIs used by wsgi_lineprof: •

    POSIX: clock_gettime(CLOCK_MONOTONIC) • macOS: mach_absolute_time() • Windows: QueryPerformanceCounter() • Fallback: gettimeofday() See extensions/timer.c
  25. • Cython: for implementing the core logic • linecache: for

    accessing source code efficiently • threading/queue: for writing results efficiently • wheel: for packaging with compiled extensions • Airspeed Velocity: for measuring overhead Other Technologies
  26. Conclusion

  27. Python Architecture of wsgi_lineprof Web
 Server Web
 App Profiler Timer

    1. WSGI middleware to capture
 request/response 2. Tracing API to track line-by-line execution 3. Monotonic timer to measure time
  28. Conclusion We can build a profiler by combining some basic

    technologies, though making an efficient profiler requires additional efforts 
 Future plan of wsgi_lineprof: • Show result via HTTP endpoint • Support ASGI (asyncio) / multithread • Reduce overhead wsgi_lineprof is available on GitHub