[systemd-devel] Bootchart speeding up boot time

[Martin Townsend]

Hi,

Thanks for your reply.  I wouldn't really call this system stripped down,
it has an nginx webserver, DHCP server, postgresql-server, sftp server, a
few mono (C#) daemons running, loads quite a few kernel modules during
boot, dbus, sshd, avahi, and a bunch of other stuff I can't quite
remember.  I would imagine glibc will be a tiny portion of what gets loaded
during boot.
I have another arm system which has a similar boot time with systemd, it's
only a single cortex A9 core, it's running a newer 4.1 kernel with a new
version of systemd as it's built with the Jethro version of Yocto so
probably a newer version of glibc and this doesn't speed up when using
bootchart and in fact slows down slightly (which is what I would expect).
So my current thinking is that it's either be down to the fact that it's a
dual core and only one core is being used during boot unless a fork/execl
occurs? Or it's down to the newer kernel/systemd/glibc or some other
component.

Is there anyway of seeing what the CPU usage for each core is for systemd
on boot without using bootchart then I can rule in/out the first idea.

Many Thanks,
Martin.


On Mon, Feb 22, 2016 at 6:52 PM, Kok, Auke-jan H <auke-jan.h.kok at intel.com>
wrote:

> On Fri, Feb 19, 2016 at 7:15 AM, Martin Townsend
> <mtownsend1973 at gmail.com> wrote:
> > Hi,
> >
> > I'm new to systemd and have just enabled it for my Xilinx based dual core
> > cortex A-9 platform.  The linux system is built using Yocto (Fido branch)
> > which is using version 219 of systemd.
> >
> > The main reason for moving over to systemd was to see if we could improve
> > boot times and the good news was that by just moving over to systemd we
> > halved the boot time.  So I read that I could analyse the boot times in
> > detail using bootchart so I set init=/..../bootchart in my kernel command
> > line and was really suprised to see my boot time halved again.  Thinking
> > some weird caching must have occurred on the first boot I reverted back
> to
> > normal systemd boot and boot time jumped back to normal (around 17/18
> > seconds), putting bootchart back in again reduced it to ~9/10 seconds.
> >
> > So I created my own init using bootchart as a template that just slept
> for
> > 20 seconds using nanosleep and this also had the same effect of speeding
> up
> > the boot time.
> >
> > So the only difference I can see is that the kernel is not starting
> > /sbin/init -> /lib/systemd/systemd directly but via another program that
> is
> > performing a fork and then in the parent an execl to run
> > /lib/systemd/systemd.  What I would really like to understand is why it
> runs
> > faster when started this way?
>
>
> systemd-bootchart is a dynamically linked binary. In order for it to
> run, it needs to dynamically link and load much of glibc into memory.
>
> If your system is really stripped down, then the portion of data
> that's loaded from disk that is coming from glibc is relatively large,
> as compared to the rest of the system. In an absolute minimal system,
> I expect it to be well over 75% of the total data loaded from disk.
>
> It seems in your system, glibc is about 50% of the stuff that needs to
> be paged in from disk, hence, by starting systemd-bootchart before
> systemd, you've "removed" 50% of the total data to be loaded from the
> vision of bootchart, since, bootchart cannot start logging data until
> it's loaded all those glibc bits.
>
> Ultimately, your system isn't likely booting faster, you're just
> forcing it to load glibc before systemd starts.
>
> systemd-analyze may actually be a much better way of looking at the
> problem: it reports CLOCK_MONOTONIC timestamps for the various parts
> involved, including, possibly, firmware, kernel time, etc.. In
> conjunction with bootchart, this should give a full picture.
>
> Auke
>
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