October 03, 2019
My keynote from PyCon Balkan 2019
October 03, 2019
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PIONEERING REAL-TIME ANDREW GODWIN // @andrewgodwin
Hi, I’m Andrew Godwin
async def view(request): data = await api_call(pk=5) return render(request, "a.html",
"The Real-Time Web"
AJAX 1999 Comet 2006 WebSockets 2011
“A system in which input data is processed within milliseconds
so that it is available virtually immediately
CPU cores run one thing at a time (yes, computers
are basically a giant lie)
Program 2 Program 3 Program 1
Scheduling The art of running multiple things at once
Program 2 Program 3 Program 1 300 cycles 300 cycles
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
Context switching is expensive The more you have, the worse
Python's threading has this problem! The more you have, the
worse it gets
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
"Boxcar" scheduling has… issues Don't ever write avionics software like
72kb Woven core memory
0.085 Mhz processor
“ Don Eyles ...when the guidance system was first being
conceived, the onboard software was almost an afterthought
Priority-based Executive Please do write avionics software like this.
Job Priorities Higher numbers beat lower Limited job "RAM" Only
7 running at once Interrupts from hardware Descent radar, keyboard, etc.
Predictable Software They knew exactly what ran and what it
...Unpredictable Hardware It's never easy, especially in space.
Two differently phased 800Mhz clocks This is really bad, trust
False detections stole computer time The radar system had to
update the guidance computer
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
SERVICER never gets to run But the system keeps trying
to schedule it
Alarm 1202 The computer self-restarts, but keeps crashing
Buzz Aldrin works it out Turns off the display routine
to bring the load under 100%
Apollo 11 was almost a failure ...but good software design
Apollo 9 First flight test of Lunar Module Apollo 10
Second test and partial descent of Lunar Module Apollo 11 First Moon landing
“ Eugene A. Cernan, recounting Apollo 10 Don't give those
guys an opportunity to land, 'cause they might!
Building for Failure Software is only as good as its
“ Don Eyles During simulations we sometimes pushed the [restart]
We often only design for the good cases After all,
that's what we're expecting to happen
Hardware is always unpredictable Disks, networks, even keyboards and mice
Can your software survive a crash? Or does it need
a clean shutdown to work?
How will you handle the unexpected? Weird input. Slow network
“ 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.
All software has consequences We may not all be designing
spacecraft, but we affect lives.
doneyles.com/LM/Tales.html Tales from the Lunar Module Guidance Computer, 2004
Thanks. Andrew Godwin @andrewgodwin // aeracode.org