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

[Gustavo Sverzut Barbieri]

On Sun, Nov 27, 2011 at 2:44 AM, Kay Sievers <kay.sievers at vrfy.org> wrote:
> 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, do you really think slow media is the only reason? I could check
the plot right now and there is a super-strange delay from t=4.5 to
t=6.5 (before doing anything at all).

Later on there is the delay for mounts, those I agree may be related
to slow media.

But again from 13 to 22 it's basically waiting udev events... which is too much!

I'd check a fresh install to avoid problem with legacy udev rules... I
recall when I moved my gentoo from openrc to systemd I had a "hotplug"
package (http://linux-hotplug.sourceforge.net) doing nothing useful.
Then I went to notice my kernel was also configured to call
/sbin/hotplug even tho it did not exist was raising pid count, etc.

Last but not least, Kay you could write an article similar to
Lennart's blogs about the proper systemd+udev setup, like "systemd +
udev for packagers". People are doing crazy things! :-(

-- 
Gustavo Sverzut Barbieri
http://profusion.mobi embedded systems
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