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Cross-compilation: Revisited ​

Date: 2018/05/22

Context ​

Previous research concluded that cross compiling ARMv7 Docker images was not feasible.

Re-evaluating used references indicated that a solution was not yet considered.

Revisiting ​

As a point of reference, the last attempt made during the cross compilation research was to enable the QEMU compiler, and then use an ARMv7 image.

Enabling QEMU:

bash
docker run --rm --privileged multiarch/qemu-user-static:register --reset

Docker image inheritance declaration:

docker
FROM arm32v7/python:3.6
...

This resulted in the following error:

bash
Sending build context to Docker daemon  17.92kB
Step 1/6 : FROM arm32v7/python:3.6
3.6: Pulling from arm32v7/python
no matching manifest for linux/amd64 in the manifest list entries

At this point a Docker issue was dug up that indicated that pulling ARMv7 images on an AMD64 architecture is not yet supported.

Alternative approach ​

After some consideration, the hypothesis was made that one approach was not yet considered: using a Docker base image explicitly intended for cross compilation.

Enabling QEMU remained the same:

bash
docker run --rm --privileged multiarch/qemu-user-static:register --reset

The base image declaration was changed to:

docker
FROM resin/raspberry-pi-python:3.6
...

This worked. The generated image can't be run on an AMD64 machine, but works perfectly on a Raspberry Pi.

It does require that each service maintains two similar Docker files: one for AMD/desktop, and one for ARM/Raspberry.

Build time ​

The excessive build time for the ARM history image remained an issue (>1 hour). This was caused by having to recompile a large C/C++ dependency of a dependency - one we didn't use.

An issue was submitted to the relevant project, and resolved in short order by the author. This reduced the history service build time from more than an hour to less than a minute.