Pioneering Real-Time

Transcript

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   PIONEERING REAL-TIME ANDREW GODWIN // @andrewgodwin

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   Hi, I’m Andrew Godwin

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

   …)
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   "The Real-Time Web"

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   AJAX 1999 Comet 2006 WebSockets 2011

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   "Real-Time"

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   “A system in which input data is processed within milliseconds

   so that it is available virtually immediately
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   None
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   None
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    CPU cores run one thing at a time (yes, computers

    are basically a giant lie)
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    Program 2 Program 3 Program 1

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    Scheduling The art of running multiple things at once

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    Program 2 Program 3 Program 1 300 cycles 300 cycles

    300 cycles
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    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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    Context switching is expensive The more you have, the worse

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

    worse it gets
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    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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    "Boxcar" scheduling has… issues Don't ever write avionics software like

    this! Thanks.
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    None
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    None
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    72kb Woven core memory

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    0.085 Mhz processor

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    “ Don Eyles ...when the guidance system was first being

    conceived, the onboard software was almost an afterthought
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    None
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    Priority-based Executive Please do write avionics software like this.

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    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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    Predictable Software They knew exactly what ran and what it

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

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    None
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    Two differently phased 800Mhz clocks This is really bad, trust

    me.
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    False detections stole computer time The radar system had to

    update the guidance computer
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    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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    103%

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    None
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    SERVICER never gets to run But the system keeps trying

    to schedule it
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    Alarm 1202 The computer self-restarts, but keeps crashing

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    Buzz Aldrin works it out Turns off the display routine

    to bring the load under 100%
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    Apollo 11 was almost a failure ...but good software design

    prevented it
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    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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    “ Eugene A. Cernan, recounting Apollo 10 Don't give those

    guys an opportunity to land, 'cause they might!
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    Building for Failure Software is only as good as its

    worst outing
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    “ Don Eyles During simulations we sometimes pushed the [restart]

    button randomly
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    We often only design for the good cases After all,

    that's what we're expecting to happen
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    Hardware is always unpredictable Disks, networks, even keyboards and mice

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    Can your software survive a crash? Or does it need

    a clean shutdown to work?
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    How will you handle the unexpected? Weird input. Slow network

    connections. Emoji.
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    “ 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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    All software has consequences We may not all be designing

    spacecraft, but we affect lives.
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    doneyles.com/LM/Tales.html Tales from the Lunar Module Guidance Computer, 2004

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    Thanks. Andrew Godwin @andrewgodwin // aeracode.org