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https://github.com/neilw/systemd-dining

Dining philosophers using systemd and the Unix Programming Environment
https://github.com/neilw/systemd-dining

dining-philosophers dining-philosophers-problem linux systemd

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Dining philosophers using systemd and the Unix Programming Environment

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README 

          
# Dining Philosophers using systemd.



The dining philosophers problem is a classic in Computer Science that

illustrates the major issues with concurrency.



Most people immediately reach for their favourite programming language

to attack this problem but here we're going to do something different.



The Linux operating system is a multi-user concurrent system which is

designed to have separate people using it all at the same time. Most of

this remains unused these days.



So I thought it would be fun to bring back the multi-user power of Unix

via a set of NPCs fighting over some rather fiddly pasta, while driving

the characters using the user level systemd functions.



All while allowing the real world adminstrator to watch each user via

the logging systems.



We're going to use classic Unix tools from the archives and give the

Unix Programming Environment a work out it hasn't seen for a good while.



## Design overview



We'll be using a variant of the Chandry/Misra "hygiene" solution to

the Dining Philosophers problem.



Each philosopher is a separate user ID on the server and sits in its

own unix group. It is also a member of the `philosophers` group which

has access to the dining room at /home/share/dining-room.



The philosophers run autonomously and take their seats once the

dining-room is open (writable by group `philosophers`). Seats are

dynamically allocated on a first come first served basis with each

philosopher grabbing the first seat they can by creating a file in the

dining room with their seat number on it that nobody else can write to.



Once a philosopher has taken a seat they will pick up a fork by creating

a fork file with their name on it and the number of the *next* seat

position. The philosopher in seat #1 will exercise their head of the

table privilege and take fork #1 as well. This sets the fork graph up to be

acyclical - ensuring the system will make progress.



The philosophers now enter the thinking state, contemplating a proposition

from their project file, and set a time in the future when they will

become hungry.



Once a philosopher reaches the hungry target, they check if they have

both forks. If not they email the relevant dining neighbour(s) a "fork

request" asking for the shared fork.



A sendmail `.forward` file runs a mail processor for each email received.



For each fork request related to a sender

- if we are thinking and we have the fork, we delete the fork and send a fork response back.

- otherwise we ignore the request



For each fork response received a fork is created. Forks can only be

acquired as a result of a fork response and thereby with permission

of the neighbour. This both maintains the acyclical nature of the fork

graph and avoids starvation (systemically and metaphorically).



Once a philosopher has two forks they enter the eating state and

set a time in the future when they will start thinking again.



A philosopher will check whether to start eating as soon as they become

hungry and thereafter when they receive a fork response.



Before they enter the thinking state, the forks are deleted and they

are transferred to neighbours by sending them a fork response.



## Installation



[Follow the instructions to install the container on a new server](SETUP.md)



## Run the simulation



Create the philosophers



    root@philosophers:~# xargs -L 1 create_philosopher < philosophers



Open the dining room



    root@philosophers:~# open_dining_room



## Stop the simulation



Close the dining room



    root@philosophers:~# close_dining_room



Remove the philosophers



    root@philosophers:~# awk '{print $1}' philosophers | xargs remove_philosopher



## Interesting things to look at



List the philosophers in the simulation



    agent1:~$ npcs

    kant

    heidegger

    hegel

    hume

    schlegel

    wittgenstein

    nietzsche

    socrates

    mill

    plato

    aristotle

    hobbes

    descartes



Check a philosopher's activity



    agent1:~$ npcjournal hegel

    Nov 14 09:15:33 srv-5o1ic select-seat[7048]: The Dining Room is open. Looking for a seat.

    Nov 14 09:15:33 srv-5o1ic select-seat[7048]: I have seat #6

    Nov 14 09:15:33 srv-5o1ic select-seat[7048]: Acquired fork #7



Follow a philosopher's activity in real time



    agent1:~$ npcjournal -f hegel

    Nov 19 14:10:30 mail-tasks[1072039]: Received fork response from hume

    Nov 19 14:10:30 mail-tasks[1072039]: Acquired fork #8

    Nov 19 14:10:30 fork-check[1072079]: I have two clean forks

    Nov 19 14:10:30 consumption[1072134]: Eating...

    Nov 19 14:10:35 fork-release[1072229]: Cleaning forks

    Nov 19 14:10:35 fork-release[1072229]: Transferring forks

    Nov 19 14:10:35 fork-release[1072257]: Giving wittgenstein a clean shared fork

    Nov 19 14:10:35 fork-release[1072257]: Giving hume a clean shared fork

    ...



Look at today's contemplations for a philosopher



    agent1:~$ npcjournal --since=today --identifier=contemplation hegel | more

    Nov 19 00:00:39 contemplation[368515]: Currently contemplating the idea that contradiction and conflict are

    Nov 19 00:00:39 contemplation[368515]: inherent in the development of ideas and societies.

    Nov 19 00:01:32 contemplation[370247]: Currently contemplating the belief that reality is rational, and reason

    Nov 19 00:01:32 contemplation[370247]: is the ultimate essence of the world.

    Nov 19 00:03:08 contemplation[372866]: Currently contemplating the idea that contradiction and conflict are

    Nov 19 00:03:08 contemplation[372866]: inherent in the development of ideas and societies.



Get the gory details at systemd unit level



    agent1:~$ journalctl _UID=$(id -u kant) --since today

    Nov 19 14:16:46 srv-4mujm systemd[98014]: Starting Clean Fork Check...

    Nov 19 14:16:47 srv-4mujm systemd[98014]: fork-check.service: Main process exited, code=exited, status=75/TEMPFAIL

    Nov 19 14:16:47 srv-4mujm systemd[98014]: fork-check.service: Failed with result 'exit-code'.

    Nov 19 14:16:47 srv-4mujm systemd[98014]: Failed to start Clean Fork Check.

    Nov 19 14:16:47 srv-4mujm systemd[98014]: fork-check.service: Triggering OnFailure= dependencies.

    Nov 19 14:16:47 srv-4mujm systemd[98014]: Starting Neighbour Fork Request...

    Nov 19 14:16:47 srv-4mujm fork-request[1088801]: I don't have fork #11

    Nov 19 14:16:47 srv-4mujm fork-request[1088833]: Asking hobbes for the shared fork

    Nov 19 14:16:47 srv-4mujm sendmail[1088845]: 3AJEGlXl1088845: from=kant@srv-4mujm.gb1.brightbox.com, size=110, class=0, nrcpts=1, ...

    Nov 19 14:16:47 srv-4mujm sendmail[1088845]: 3AJEGlXl1088845: to=hobbes, ctladdr=kant@srv-4mujm.gb1.brightbox.com (1001/1002), ...

    Nov 19 14:16:47 srv-4mujm systemd[98014]: Finished Neighbour Fork Request.



See the philsopher's details and what they are thinking about



    agent1:~$ finger hume

    Login: hume           			        Name: David Hume

    Directory: /home/hume               	Shell: /bin/bash

    Never logged in.

    Mail forwarded to |/usr/local/bin/mail-processor

    No mail.

    Project:

    the empirical foundation of knowledge and the idea that all ideas derive from sensory impressions.

    the notion that causation is not a necessary connection but a habitual association based on experience.

    the self as a bundle of perceptions, with no enduring and unchanging identity.

    the principle that reason is the slave of the passions, and moral judgments are driven by sentiment.

    the idea that miracles, as violations of natural laws, are inherently improbable and should be met with skepticism.

    the understanding that probability is the guide of life, shaping our beliefs and actions based on likelihood.

    the distinction between 'is' and 'ought,' recognizing that moral values cannot be derived from factual statements.

    the empiricist view that all knowledge originates in experience, rejecting innate ideas as a source of understanding.

    the skeptical position that we can never know the true nature of things, only their appearances.

    the constant flux of impressions and ideas, emphasizing the impermanence and transience of all mental phenomena.

    No Plan.