Finding a kernel (and a distro) that works with 5k iMacs

[fox]

When I tried to disable the other (i.e. non-gpu) service items extending the boot time, it only helped minimally. But more important was that my internet connection wasn't functioning. By that I mean that networkmanager showed that my wifi was connected to my router, but when I tried to access anything with Firefox, it said it couldn't find the site. I re-enabled those services and that will be the end of my experiment for now. (Unless someone comes up with another good idea worth trying.) I at least have the knowledge that everything works well on the 4.15 kernel and that I will eventually be able to try amdgpu-pro drivers once AMD updates them for Ubuntu 20.04.

[Jason]

Not being able to connect is probably why NetworkManager was taking so long in the service startup but I might be wrong. Why'd you re-enable all the services that sped up your boot time by disabling them? Did you lose funtionality?

[buster]

If you haven't lost patience with your search, you might go after some 5.? in the Ubuntu repositories. They should be there because my Kubuntu 19.10 uses 5.3.0-46-generic.

Not saying that one would work, but there may be a 5.? that does work, and handles video and boot. Of course the long term security of these kernels might be shorter, but it would be interesting to know if there is a cut-off point in kernel evolution.

Of course it would be interesting if you did the work and reported back, not if we had to do it. I suspect you do all this because you've been cooped up too long Mike.  Time to get the bike out. 

[fox]

The bike is out. Yesterday I cycled on the TransCanada Trail to the crossing near Keene. Nice ride.

Strangely enough, there are no 5.x kernels in the 20.04 repositories except for 5.4 I'm guessing that they removed the non-LTS kernels because they don't want to maintain them. The only other kernel I could find was 4.15, which is the 18.04 LTS kernel. Otherwise I would have tried that. I tried to do the equivalent by trying other distros with different kernels, but I realize that when I'm doing that I'm changing more than one variable.

[fox]

Not being able to connect is probably why NetworkManager was taking so long in the service startup but I might be wrong. Why'd you re-enable all the services that sped up your boot time by disabling them? Did you lose funtionality?

Only the network and I'm not sure which of them messed up my wifi connection since I didn't disable network-manager. Unless I come up with another plan, I'm now resigned to using the 4.15 kernel until a new kernel works or the AMD proprietary drivers are modified for Ubuntu 20.04. Or until there is a way I change something in the boot process to make 5.4 load much faster.

[fox]

Well surprise, surprise. I found the long boot culprit(s). The main one was fractional scaling. When I turned that off, it cut the boot time to a little over a minute, and the long delay after logging in was gone. When I then analyzed "blame", systemd-timesyncd.service was adding almost 40 sec to the boot time. Disabling that got my boot time down to 25 sec! All this happened after the latest update, which in itself cut the boot time to 2 minutes.

Fair enough on the fractional scaling; it is an experimental feature which just happens to work perfectly in the older kernel. Hopefully they will fix it before 20.04 is officially released. I was able to compensate for the lack of fractional scaling by doubling the font multiplier, and this gives me a desktop I can easily work with. Unfortunately, the shutdown is still relatively long, over a minute, but no need to wait for the shutdown to finish before walking away from the computer.

Anyway, I'm a happier camper.

[Jason]

A minute still seems like a long time, but certainly a lot better than 2 minutes! Good job figuring it out. Buster knew you would, I wasn't so sure.

[fox]

There was one more piece I hadn't realized until after I posted this. Changing the resolution also causes the slow boot. The default resolution for my so-called 5k iMac is 3840 x 2160. I had been adjusting this to 2560 x 1440 instead of scaling fonts or scaling the display because it works better with certain programs like Word 2010 and Mendeley Desktop. This results in slow boot on the 5.4 kernel; basically any display change from the default causes the problem. So it has to be the interaction of the open source amdgpu driver with the newer kernel (since the driver works fine on the older kernel). I now have a solution, but the best for now is to stick with the old kernel until the fix the driver or until the propietary amdgpu-pro driver is available for Ubuntu 20.04.

But wait. This doesn't explain why I was getting a slow boot in all those other live distros I tried, since I wasn't adjusting or scaling the display or fonts. The latest Ubuntu update must have fixed the problem to the extent that fast boot was possible as long as I didn't mess with resolution or scaling. Those distros must not have gotten the same updates.