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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. 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
  2. Have you ever built a profiler?
 " " "

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

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

    wsgi_lineprof • How wsgi_lineprof works? • Technologies for implementing a profiler • Conclusion
  5. 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)
  6. 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
  7. 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
  8. Python Architecture Web
 Server Web
 App Profiler Timer 1. Capture

    request/response 2. Get information
 on execution 3. Measure exec. time
 per line precisely
  9. 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
  10. 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]
  11. 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
  12. 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…
  13. 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
  14. 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
  15. 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
  16. 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
  17. • 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
  18. 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
  19. 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