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 ϓϩϑΝΠϥͷ࣮૷ Yusuke Miyazaki
 PyCon JP 2019 / 2019-09-16 Implementation of a profiler for Python web applications

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

Have you ever used a profiler?
 
 Result at PyCon JP: about 50%

Have you ever built a profiler?
 
 " " " Result at PyCon JP: only 1 person

Goal of the Talk “Implementing a profiler is not so difficult”
 through understanding inside of a profiler

Agenda • Introduction • What is wsgi_lineprof? • Features of wsgi_lineprof • How wsgi_lineprof works? • Technologies for implementing a profiler • Conclusion

What is wsgi_lineprof?

Line-by-Line Profiler for Web Apps Shows line-by-line execution time for each request

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)

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

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

How wsgi_lineprof works?

Python Architecture Web
 Server Web
 App

Python Architecture Web
 Server Web
 App Profiler 1. Capture
 request/response

Python Architecture Web
 Server Web
 App Profiler 1. Capture
 request/response 2. Get information
 on execution

Python Architecture Web
 Server Web
 App Profiler Timer 1. Capture
 request/response 2. Get information
 on execution 3. Measure exec. time
 per line precisely

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

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]

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

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…

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

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

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

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

• 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

Conclusion

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

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