Pioneering Real-Time

My keynote from PyCon Balkan 2019

Transcript

1. PIONEERING REAL-TIME ANDREW GODWIN // @andrewgodwin
2. Hi, I’m Andrew Godwin
3. async def view(request): data = await api_call(pk=5) return render(request, "a.html", …)
4. "The Real-Time Web"
5. AJAX 1999 Comet 2006 WebSockets 2011
6. "Real-Time"
7. “A system in which input data is processed within milliseconds so that it is available virtually immediately
10. CPU cores run one thing at a time (yes, computers are basically a giant lie)
11. Program 2 Program 3 Program 1
12. Scheduling The art of running multiple things at once
13. Program 2 Program 3 Program 1 300 cycles 300 cycles 300 cycles
14. Program 2 Program 3 Program 1 300 cycles Restore Save Restore Save Restore Save 30 cycles 30 cycles 300 cycles 30 cycles 30 cycles 300 cycles 30 cycles 30 cycles
15. Context switching is expensive The more you have, the worse it gets
16. Python's threading has this problem! The more you have, the worse it gets
17. Active Threads Idle Threads Runtime 1 0 1.3 seconds 1 1 2.3 seconds 1 5 5.4 seconds 1 30 39 seconds 1 100 15 minutes
18. "Boxcar" scheduling has… issues Don't ever write avionics software like this! Thanks.
21. 72kb Woven core memory
22. 0.085 Mhz processor
23. “ Don Eyles ...when the guidance system was first being conceived, the onboard software was almost an afterthought
26. Priority-based Executive Please do write avionics software like this.
27. Job Priorities Higher numbers beat lower Limited job "RAM" Only 7 running at once Interrupts from hardware Descent radar, keyboard, etc.
28. Predictable Software They knew exactly what ran and what it took
29. ...Unpredictable Hardware It's never easy, especially in space.
31. Two differently phased 800Mhz clocks This is really bad, trust me.
32. False detections stole computer time The radar system had to update the guidance computer
33. Descent Phase: 85% load Capsule orientation and thrust calculations Hardware Bug: 13% Caused by the descent radar Display Routine: 5% Astronauts trying to see how far to go
34. 103%
36. SERVICER never gets to run But the system keeps trying to schedule it
37. Alarm 1202 The computer self-restarts, but keeps crashing
38. Buzz Aldrin works it out Turns off the display routine to bring the load under 100%
39. Apollo 11 was almost a failure ...but good software design prevented it
40. Apollo 9 First flight test of Lunar Module Apollo 10 Second test and partial descent of Lunar Module Apollo 11 First Moon landing
41. “ Eugene A. Cernan, recounting Apollo 10 Don't give those guys an opportunity to land, 'cause they might!
42. Building for Failure Software is only as good as its worst outing
43. “ Don Eyles During simulations we sometimes pushed the [restart] button randomly
44. We often only design for the good cases After all, that's what we're expecting to happen
45. Hardware is always unpredictable Disks, networks, even keyboards and mice
46. Can your software survive a crash? Or does it need a clean shutdown to work?
47. How will you handle the unexpected? Weird input. Slow network connections. Emoji.
48. “ Don Eyles When Hal Laning designed the Executive and Waitlist system, he made it up from whole cloth with no examples to guide him. The design [...] still represents the state of the art.
49. All software has consequences We may not all be designing spacecraft, but we affect lives.
50. doneyles.com/LM/Tales.html Tales from the Lunar Module Guidance Computer, 2004
51. Thanks. Andrew Godwin @andrewgodwin // aeracode.org