[systemd-devel] Exploring Minimal Systemd in Initramfs for Faster Boot

[Dharma.B at microchip.com]

Hi Brian,
On 26/09/24 6:36 am, Brian Masney wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Wed, Sep 25, 2024 at 03:53:36PM +0000, Dharma.B at microchip.com wrote:
>> yes something similar to this, I will experiment this and get back to you.
>>
>> and I think since the egt service and its libraries depend on the full
>> rootfs, integrating initramfs might not provide significant benefits in
>> terms of faster launch time. The time saved by using an initramfs to
>> launch basic services earlier would likely be offset by the delay in
>> mounting the root filesystem, which is necessary for accessing the egt
>> libraries.
> 
> There are some drawbacks to putting your early service in the initramfs
> and having it persist across the switch root to when systemd is started
> again from the root filesystem.
> 
> - The initrd is a cpio archive, and increasing the size of the initrd is
>    going to increase the kernel boot time since it will need to
>    uncompress and extract the larger cpio archive.
> 
> - Any services started from the initrd will be started before the
>    SELinux policy is loaded. Services started from the initrd will run
>    with the kernel_t label.
> 
> - Services started from the initrd can't depend on almost anything like
>    mounts, devices, services, dbus, etc so it's difficult to develop
>    software of any complexity.
> 
> - Adding all of these dependencies to the initrd is only going to move
>    the timing bottlenecks booting from the root filesystem to the initrd.
> 
> - Directives like Restart=always will likely not work as expected.
> 
> Personally I think that you should have the initramfs be small enough to
> only get you to the root filesystem and start your early service from
> the root filesystem.
> 
> Shameless self plug: Here's a presentation that I gave last month at
> DevConf.us in Boston with Eric Curtin and Ed Chong about some of the
> boot speed optimizations that a group of us worked on at Red Hat:
> https://pretalx.com/devconf-us-2024/talk/HXVPHC/ . The talk also
> lists some upstream patch sets that you will want to consider
> including in your kernel to help with boot time.

Thank you so much for your valuable and comprehensive feedback, I have 
gone through your presentation and it helped a lot.

> 
> Brian
> 


-- 
With Best Regards,
Dharma B.