[systemd-devel] Does systemd have points to spend many times to complete its process?

[David Strauss]

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On Thu, Jul 25, 2013 at 4:58 AM, Tony Seo <tonyseo7 at gmail.com> wrote:
> 1. what is "-.mount" ?
> when I first saw "-.mout", I was confused how to configure it.

You configure it from the /etc/fstab entry for the root file system,
which then generates that unit during boot-up. The unit is only
present for the sake of consistency and dependency management.

> 2. I doubt that there are critical points which make systemd more delayed
> for several reasons.
>
> I have tried to test my systemd for optimization in  perspective of time
> consume.
>
> So, I changed the order of unit dependency and sequence among units.
>
> But I found that I may not reduce the time to activate getty.target,
> multi-user.target and graphical.target.
>
> And I doubt that there are critical points or span of stage to make the
> systemd more delayed.
>
> I want to know whether I'm wrong or not.
>
> what do you think about it?

systemd eagerly starts any unit wanted or required, and it does so
with maximum parallelism. In other words, it keeps the boot hierarchy
as flat as possible to minimize startup time overhead caused by
waiting on dependencies.

systemd introduces a couple mechanisms to increase parallelism: (1)
the ability to want or require other units without waiting for them to
finish starting (no assumed After=) and (2) socket activation.

> If you know some tip to improve a systemd in optimize perspective, could you
> give some tips to make my systemd more fast?

You should use systemd's boot chart [1] and analysis [2] tools.

> 3. I need a clear knowledge about transaction stage.
>
> I read a manpage which introduces  a few contents of transaction, but I
> would like to get more information about it.

A systemd transaction is simply the idea that the operation is only
successful if the main operation and its dependencies succeed. If
service A requires service B, you start service A, and the
not-yet-running service B fails to start, the transaction fails.

[1] http://www.freedesktop.org/software/systemd/man/systemd-bootchart.html
[2] http://www.freedesktop.org/software/systemd/man/systemd-analyze.html