Comment 30 for bug 1626436

Thanks Joseph!

I now believe I'm seeing two different issues in 4.8 that are not in 4.4 and slow down my boot, which is very confusing.
- #1: High number of kworkers (fixed in 4.8 using the two patches and bisected by Doug Smythies already)
- #2: Unknown other issue. I've noticed that when compiling a simple cmake/ninja project using 6 compiler processes on my rather old 4-core CPU, some kernels show odd behaviour that might also slow down the boot process.

When compiling that cmake project on 4.4, 4.6-7afd16f or 4.7-rc1, issue #2 does not exist. While compilation takes place on 4.8 (no matter if kworker patches are applied or not), my mouse cursor becomes sluggish and it takes a long time to just switch to another window in Unity 7. Compilation speed is unaffected though, so maybe just some kind of task switching problem, not a CPU utilization issue.

So that makes it quite difficult to tell if any given kernel is good or bad by just looking at the boot time. As far as I can tell, this is the current list of kernels and the issues they have, where #2 was detected using the cmake compilation behaviour and #1 by the number of kworker processes after boot (should be ~35):

4.8 + patches: #2
4.8: #1, #2
4.7-rc1: #1
4.6-7afd16f: none
4.4: none

4.8 has about the same wall clock boot time as 4.7-rc1 in my case, while 4.8+patches is ~5 seconds faster, but that's hard to tell, since boot times vary by 2-3 seconds anyway. So #1 apparently dominates the slow down. I've only been able to get a "perfect" <10sec boot time from GRUB to lightdm with 4.4 and 4.6-7afd16f. So the order of boot speed is like this right now for me:

4.4|4.6-7afd16f < 4.8+patches < 4.8|4.7-rc1

Sorry for the long-winded comment, I just feel uncomfortable calling this kernel "good" or "bad" without explaining why 4.7-rc1 might actually be "good" on my system concerning issue #2. 4.6-7afd16f appears to be as "good" as 4.4 in any case.