[systemd-devel] Understanding systemd-analyze's plots

[Kay Sievers]

On Sat, Nov 26, 2011 at 22:52, Stefan Majewsky
<stefan.majewsky at googlemail.com> wrote:
> my openSUSE 12.1 system boots in about 30 seconds, and I wanted to cut
> that time down a bit, so I took a look at systemd-analyze's blame and
> plot output.
>
> But I do not really know how to interpret the results which I see in
> the plot [1]. The startup sequence takes 20.5 seconds in userspace, of
> which only the last 3 seconds seem to be spent on what I consider "the
> interesting stuff": starting all sorts of services and finally
> bringing up KDM.
>
> The rest of the time seems to be spent activating the hardware,
> various mounts and udev. (According to the LED on my notebook's case,
> the disk is busy all the time.) To put my confusion into questions:
>
> 1. Why does the system need 6 seconds (from t=6.3s to t=12.3s on the
> plot) to activate some tmpfs mounts?
>
> 2. Why is localnet.service activating for a whole 7 seconds? I looked
> into it, it's only a SysV init script that sets hostname and
> domainname from the config in /etc, yet it's number 1 in
> systemd-analyze blame.
>
> 3. Why does it look like about nothing happens between t=13s and t=22s?
>
> It might be that openSUSE's unit files (or SysV leftovers) are not yet
> optimized for the early boot: For example, I seem to have saved some
> seconds by masking lvm.service (I don't use LVM at all). But that
> won't explain why systemd is actually slower on this stage of boot vs.
> the old SysV init some distro versions ago.
>
> Can someone enlighten me?

These numbers just look like a slower disk. The timing graph is not
really useful if things wait for I/O. The old SUSE boot was in some
cases better optimized for slower rotating media than systemd is. How
fast is the disk? Try hdparm -t /dev/sda

The filesystem is ext4? This is an updated or newly installed system?
The filesystem is formatted a while back? We've seen strange ext4
performance numbers on older filesystems, that just went away after
reformatting. We have no real idea what's causing this, maybe some
weird fragmentation issue.

Kay

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