Python wheels ● Wheels are built distributions, and can be uploaded to PyPI alongside source distributions ● This saves users from building themselves ● Wheels are architecture-specific – e.g. win32, win64, macosx, linux32, linux64 ● A recent addition allowed “manylinux” wheels to be uploaded ● Raspberry Pi is not the “manylinux” architecture, it’s ARM
ARM wheels ● Technically... – Pi 3 is ARMv8 – Pi 2 is ARMv7 – Pi 1 / Zero are ARMv6 ● But... – Wheels built on Pi 2/3 are tagged “armv7l” – Wheels built on Pi 1 / Zero are tagged “armv6l” ● And… – They are actually ARMv6 wheels, and they’re all the same ● So… – A wheel built on a Pi 3 will work on a Pi 2, as is – A wheel built on a Pi 3 will work on a Pi 1 / Zero (if renamed to “armv6l”)
piwheels ● I could build everything on PyPI and host my own repository – “pip wheel numpy” works on a Pi – You can distribute the wheel and it’s a super fast install ● Can I host my own package repository? – Apparently, yes! – At minimum, an Apache directory listing will do the trick
piwheels v1 ● Pi 3 in my living room ● Build the latest version of every package (106k packages at the time) ● Log output into postgres database ● Host a package repository on the same Pi
piwheels v1: the results ● It took 10 days to complete the build run ● 76% build success rate ● Repository (was) live at piwheels.bennuttall.com ● “pip install numpy i http://piwheels.bennuttall.com” works and takes 6 seconds :) ● Proof of concept: it works, it’s probably useful
Planning piwheels v2 ● Build every version of every package ● Keep up with new releases automatically ● Host a package repository as before ● Create a test suite ● Provide installation instructions & developer documentation so people can contribute
Planning piwheels v2 ● Now 113k packages on PyPI ● But now I’m building every version of every package ● 750k package versions to build ● At the previous rate, this will take 70 days on one Pi… – Maybe I could use … more than one Pi?
Why don’t you just cross-compile? ● It’s not all about speed ● Reliability ● Compatability ● Familiarity ● Ease of use ● I can scale up Pis easily ● Eating my own dog food
Adding more Pis ● Provisioned a second Pi – No web server or database – Connected to the database on first Pi – rsync files to first Pi ● Provisioned a third Pi ● Installed “terminator” ● Provisioned Pis 4 and 5 ● This is easy. I’ll be done in no time!
Make it scale! ● Pull request: – Query optimisations – Queuing system with zeromq ● Re-deployed the code ● Original Pi is now “master” running database and web server only ● Other Pis are now “builders” using master’s database and rsync- ing files to master
20 Raspberry Pis ● ~3k packages per hour ● ~72k per day (10%) ● Now also logging which Pi built each package ● Pis seem to be holding up ● Dropped rsync in favour of sshfs ● It’s going well! I’ll be done in no time!
People do stupid things ● Random files created in my home directory ● Random stuff appended to my .bashrc ● Some people run “git clone” in their setup.py ● Inadvertently importing numpy
The results ● Total packages processed: 113, 649 ● Package versions built: 570, 648 / 752, 817 (76%) ● Total cumulative time spent building: 156 days, 18 hours (including duplicates) – In real time this was 26 days: ● 16 days with 1 Pi building ● 10 days with up to 19 Pis building ● 250GB disk space used by wheels
Supporting multiple Python versions ● We built everything on Raspbian Jessie (Python 3.4) ● Raspbian Stretch is now released (Python 3.5) ● We also want to support Python 3.6 (and 2.7) ● Pure Python packages are built for a whole major Python version (i.e. any Python 3.x) ● Compiled packages are built for a specific minor Python version (e.g. Python 3.4) and need rebuilding for other ABIs
Build procedure ● Keep package list and version list up-to-date in database ● Form a build queue for unattempted package versions ● Attempt to build everything in the queue ● If a wheel is tagged with “linux_armv7l”, create a symlink from “linux_armv6l” for Raspberry Pi 1/Zero users ● If a wheel is tagged with an ABI other than “none”, trigger it to be rebuilt for other ABIs ● Stretch builder will pick this up and attempt to build for Python 3.5 ● Same principle for Python 3.6 and 2.7
How to get it ● Raspbian Jessie (old stable): – Manually configure pip to use piwheels (/etc/pip.conf) ● Raspbian Stretch (new stable): – pip now pre-configured to use piwheels as an additional index – “sudo apt upgrade” will bring in the config :)
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