I enjoy working on embedded systems, but there’s a significant amount of work you have to do in getting the platform ready before getting to the more interesting business logic. The Raspberry Pi Zero W solves most of these problems: it’s small, has decent I/O, a decent price ($10 + $6 for storage + $4 for shipping) and runs full Linux so I can use my current language of choice.

One problem is boot speed. It’s common to power cycle an embedded project so a long boot time slows down the build/test cycle. A long boot time can also affect the safety of the system.

Because of this, I had a hack on the Raspbian boot time. The .target units are boot milestones such as basics ready, filesystem ready, and networking ready. systemd-analyze plot shows where the time went and what was running in parallel such as:

$ grep target boot-2.svg
<text class="left" x="390.802" y="94.000">remote-fs-pre.target</text>
<text class="left" x="391.128" y="114.000">cryptsetup.target</text>

As a bonus, the x attribute is the time in centi-seconds the span started which makes it easer to measure.

The biggest savings were through disabling unneeded services and switching from /etc/init.d/networking to systemd-networkd. The final boot order was:

giving:

• 2.70 s before userspace.
• 3.91 s is the earliest a no-dependency service can start. Useful for any safety response.
• 7.42 s to local-fs: filesystem is up and usable
• 7.94 s to basic
• 17.8 s to network up. Includes the wifi and DHCP time so network dependent.

Moving from ext4 to f2fs made no difference. There’s probably another 1 s to be saved through shrinking the kernel and, say, moving USB to a module so it can init later.

Update

I switched to a custom kernel with no printk() and USB as a module. I also switched from the Debian services to things like systemd-timesyncd and disabling the Debian dhcpcd. Times now are:

• 0.80 s before userspace.
• 1.70 s is the earliest a no-dependency service can start. Useful for any safety response.
• 5.11 s to local-fs: filesystem is up and usable
• 5.88 s to basic
• 7.03 s to network - although it’s hard to tell if the network is up