Pioneering Real-Time

My keynote from PyCon Balkan 2019

Transcript

1. PIONEERING
   REAL-TIME
   ANDREW GODWIN // @andrewgodwin
   

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2. Hi, I’m
   Andrew Godwin
   

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3. async def view(request):
   data = await api_call(pk=5)
   return render(request, "a.html", …)
   

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4. "The Real-Time Web"
   

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5. AJAX
   1999
   Comet
   2006
   WebSockets
   2011
   

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6. "Real-Time"
   

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7. “A system in which input data is
   processed within milliseconds
   so that it is available
   virtually immediately
   

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10. CPU cores run one thing at a time
    (yes, computers are basically a giant lie)
    

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11. Program 2
    Program 3
    Program 1
    

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12. Scheduling
    The art of running multiple things at once
    

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13. Program 2
    Program 3
    Program 1 300 cycles
    300 cycles
    300 cycles
    

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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
    

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15. Context switching is expensive
    The more you have, the worse it gets
    

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16. Python's threading has this problem!
    The more you have, the worse it gets
    

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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
    

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18. "Boxcar" scheduling has… issues
    Don't ever write avionics software like this! Thanks.
    

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21. 72kb
    Woven core memory
    

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22. 0.085 Mhz
    processor
    

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23. “
    Don Eyles
    ...when the guidance system
    was first being conceived, the
    onboard software was
    almost an afterthought
    

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26. Priority-based Executive
    Please do write avionics software like this.
    

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27. Job Priorities
    Higher numbers beat lower
    Limited job "RAM"
    Only 7 running at once
    Interrupts from hardware
    Descent radar, keyboard, etc.
    

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28. Predictable Software
    They knew exactly what ran and what it took
    

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29. ...Unpredictable Hardware
    It's never easy, especially in space.
    

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31. Two differently phased 800Mhz clocks
    This is really bad, trust me.
    

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32. False detections stole computer time
    The radar system had to update the guidance computer
    

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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
    

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34. 103%
    

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36. SERVICER never gets to run
    But the system keeps trying to schedule it
    

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37. Alarm 1202
    The computer self-restarts, but keeps crashing
    

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38. Buzz Aldrin works it out
    Turns off the display routine to bring the load under 100%
    

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39. Apollo 11 was almost a failure
    ...but good software design prevented it
    

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40. Apollo 9
    First flight test of Lunar Module
    Apollo 10
    Second test and partial descent of Lunar Module
    Apollo 11
    First Moon landing
    

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41. “
    Eugene A. Cernan, recounting Apollo 10
    Don't give those guys an
    opportunity to land,
    'cause they might!
    

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42. Building for Failure
    Software is only as good as its worst outing
    

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43. “
    Don Eyles
    During simulations we
    sometimes pushed the
    [restart] button randomly
    

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44. We often only design for the good cases
    After all, that's what we're expecting to happen
    

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45. Hardware is always unpredictable
    Disks, networks, even keyboards and mice
    

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46. Can your software survive a crash?
    Or does it need a clean shutdown to work?
    

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47. How will you handle the unexpected?
    Weird input. Slow network connections. Emoji.
    

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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.
    

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49. All software has consequences
    We may not all be designing spacecraft, but we affect lives.
    

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50. doneyles.com/LM/Tales.html
    Tales from the Lunar Module Guidance Computer, 2004
    

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51. Thanks.
    Andrew Godwin
    @andrewgodwin // aeracode.org
    

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