[systemd-devel] Improve boot-time of systemd-based device, revisited

[Harald Hoyer]

On 14.06.2015 15:17, cee1 wrote:
> Hi all,
> 
> I've recently got another chance to improve the boot-time of a
> systemd-based device. I'd like to share the experience here, and some
> thoughts and questions.
> 
> The first time I tried to improve the boot-time of systemd:
> http://lists.freedesktop.org/archives/systemd-devel/2011-March/001707.html,
> after that, we have systemd-bootchart and systemd-analyze, which help
> a lot.
> 
> It seems the biggest challenge of reducing boot-time of the ARM board
> at hand is taking are of the poor I/O performance:
> * A single fgets() call may randomly cause 200-300ms
> * A (big)service may spend 2-3s to complete its so loading - only
> ~100ms spent on CPU.
> 
> I tried to first delay services which are less important, to save the
> I/O bandwidth in the early stage, and raise the priority of important
> services to SCHED_RR/IOPRIO_CLASS_RT:
> 1. I need to find the "top I/O hunger" processes (and then delay them
> if not important), but it's not straightforward to figure it out in
> bootchart, so adding *** iotop feature *** in bootchart seems very
> useful.
> 
> 2. I think raising CPU scheduling priority works because it reduces
> chances of issuing I/O requests from other processes. Some thoughts:
> * The priority feature of I/O scheduler(CFQ) seems not work very well
> - IDLE I/O can still slow down Normal/RT I/O [1]
> * I don't know the detail of CFQ, but I wonder whether a "rate limit"
> helps - may reduce the latency between issuing I/O command and full
> filling the command?
> 
> Last, I tried some readahead(ureadahead), but not do the magic, I
> guess it's because I/O is busy in the early stage, there's simply no
> "ahead" chance.
> What readahead helps, IMHO, is a snapshot of accessed disk blocks
> during boot up, in the order of they're requested. Thus a linear
> readahead against the snapshot will always read ahead of actual
> requesting blocks.
> 
> BTW, systemd-bootchart has a option to chart entropy, how is the
> entropy involved in boot up procedure?

Well, if daemons need bytes from /dev/random (think sshd key generation), I
guess they will have to wait for enough entropy, and so does the boot process
in the end.

> 
> 
> 
> ---
> 1. http://linux-kernel.vger.kernel.narkive.com/0FC8rduf/ioprio-set-idle-class-doesn-t-work-as-its-name-suggests
> 
> 
> Regards,
> 
> - cee1
> _______________________________________________
> systemd-devel mailing list
> systemd-devel at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/systemd-devel
>