On Tue, 15 Sep 2020 09:41:24 +0100
The Natural Philosopher  wrote:

>On 14/09/2020 18:49, Scott Alfter wrote:
>> In article ,
>> ray carter   wrote:
>>> On Thu, 10 Sep 2020 12:47:40 +0100, The Natural Philosopher wrote:
>>>> I just want to run one high priority daemon and one lower priority
>>>> demon, with shared memory between them...is there something simpler than
>>>> Linux? More like e.g. DOS?
>>>
>>> You could also do LFS (Linux From Scratch) - you'd build up from nothing
>>> instead of try to remove. Absolutely nothing in there that you don't want.
>>
>> There's also Gentoo Linux, which you could think of as a more automated LFS.
>> You can even get a proper 64-bit userland under Gentoo on the RPi 3 & 4;
>> it's been around for probably at least a year now while Raspbian is just now
>> getting around to a 64-bit beta.
>>
>> It's also systemd-free, unless you select otherwise.
>
>I've been looking at all the options and trying to understand the boot
>process and the kernel threads.
>First of all thank you everyone, because ist been an interesting
>extensions of the knowledge base.
>
>It seems that the pi boots (via the GPU)  some stock boot loader that
>tarts around with the hardware using instructions found in config.txt
>and then boots a default kernel (on my Pi zero its simply kernel.img) -
>or one specified in config.txt in the FAT formatted boot partition. A
>few parameters may be  passed to that: device drivers are specified in
>config.txt and cmdline.txt has a few more boot options like where to
>find the root filesystem.
>
>Now this is the bit that I am not sure about - the kernel having booted
>from the FAT partition then invokes - I assume - /sbin/init, which in my
>case is linked to systemd.
>
>I understand that in some sort of way it needn't be, or it could be
>specified to NOT be /sbin/init....(where?)
>
>So effectively one can intercept the boot process before systemd gets is
>ugly evil claws into the nice shiny Linux, and run something else -
>busybox?
>
>So in principle one would lose all the daemons spun up by systemd as
>default - logging, hotplugging and so on would all simply never get
>started ... and start only those required - network layer perhaps.
>I wonder what apache needs to start it? Never mind. Don't need it, just
>need a very simple web server for control purposes, could even write one
>that lived in user space so didn't pre-empt valuable real time
>background task cycles.
>
>And that brings me to the next question.
>
>Assuming I am just running a kernel, plus networking of some sort, I
>still have all the kernel threads which don't seem to take up CPU time,
>or do they?
>
>Under systemd on my system (it uses NFS, hence the related daemons,
>these would not be needed in the system I want to build) I have..
>
>ps -eadf | grep "1  0" | awk '{print $8}'
>
>/lib/systemd/systemd-journald
>/usr/sbin/blkmapd
>/lib/systemd/systemd-udevd
>/lib/systemd/systemd-timesyncd
>/usr/sbin/rpc.idmapd
>/usr/bin/dbus-daemon
>/usr/sbin/cron
>/usr/sbin/rsyslogd
>/lib/systemd/systemd-logind
>/usr/lib/rtkit/rtkit-daemon
>avahi-daemon:
>/sbin/wpa_supplicant
>/usr/sbin/thd
>wpa_supplicant
>/sbin/dhcpcd
>/sbin/rpcbind
>/usr/sbin/sshd
>/usr/sbin/rpc.mountd
>/sbin/agetty
>/sbin/agetty
>/usr/sbin/apache2
>/usr/sbin/exim4
>/usr/lib/policykit-1/polkitd
>/lib/systemd/systemd
>grep
>
>The ONLY one of those I might need (this is a pi zero W)  is
>wpa-supplicant and children. Or the equivalent on an ethernet equipped Pi...
>
>So my next questions are:
>
>1/. How much latency do the kernel thread processes conceivably
>introduce? There are an awful lot of them. If these are insurmountable
>that really rules out Linux altogether.
>
>2/. How interruptible are they by interrupts coming from the Pi's GPIO
>pins? I want my handler to have top priority for its run - whose length
>is a bit indeterminate till I know how fast the pi executes floating point.
>
>3/. What have I forgotten that systemD does that I might actually need?
>
>For the purposes of setting the context for the last question, the Pi
>would be sitting there waiting for a low priority webserver request,
>executing  low priority foreground code in an interruptible loop, and
>responding to time critical interrupts on a GPIO pin.
>
>Disk access would not be involved in the initial prototype.
>
>By using suitable buffering I might be able to cope with 50ms max time
>spent by the pi doing housekeeping.
>

You might like to think about devuan as a base rather than raspbian. It uses
SysV for init, so is far more accessible. I use this for a standalone
soft-synth. It is dramatically quicker to start and I can run at lower latency.
The current release is beowulf and runs on the Pi4, as well as the earlier
ones.


--
W J G

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