[systemd-devel] Reduce loading time (size) of init program (systemd)

[Auke Kok]

On 9/16/19 7:14 AM, Paul Menzel wrote:
> In his talk*Linux kernel fastboot on the way*  at this years Linux
> Plumbers Conference [1][2], Feng Tang (CCed) has the notes below on
> the slide for userspace/systemd.
>
>> • Systemd is ~1.5MB - the loading time for emmc is 100ms> • Can we use a small lightweight “init” program, which starts
>>    target programs in parallel and readahead to preload libraries
>>    and executables?
> Are you already aware of these issues? If yes, have you ideas or
> suggestions how to solve this? Maybe something to discuss at this
> weeks All System Go! 2019.

Chiming in here

The benefits of doing that often don't outweigh the cost. Readahead is 
*difficult* and very much impossible to get right for a generic 
situation, so in many cases it has stopped being used. The only place 
e.g. ClearLinux uses it now is for targeted use cases where it makes 
sense (in our case, the desktop, because GNOME is ... monstrous). For 
typical cloud and bare metal situations and even VM instances, readahead 
doesn't appear to be useful at all.

Despite loading time for eMMC being 100ms,  I've not seen the actual 
loading time of systemd be an issue. The worst thing at boot that can 
happen is that some process starts writing a lot (journal rotating, for 
instance), and is far more detrimental to boot time from my experience.

I like Feng's slides, but, it's not the first time that things were 
proposed that didn't pan out as much as we thought it did.

Auke


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