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Author: kris

Swap and Memory Pressure: How Developers think to how Operations people think

There is a very useful and interesting article by Chris Down: “In defence of swap: common misconceptions“. Chris explains what Swap is, and how it provides a backing store of anonymous pages as opposed to the actual code files, which provide backing store for file based pages.

I have no problem with the information and background knowledge he provides. This is correct and useful stuff, and I even learned a thing about what cgroups can do for me.

I do have a problem with some attitudes here. They are coming from a developers or desktop perspective, and they are not useful in a data center. At least not in mine. :-)

Chris writes:

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Knock, Knock

What does your Mac do on Startup? Knockknock knows.

It’s not properly updated for current versions of MacOS, but it is still useful. “git clone https://github.com/synack/knockknock” and “/usr/bin/python knockknock.py” is sufficient to test.

TOTAL ITEMS FOUND: 44

That’s quite a bit. Apparently, I am starting

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Conway’s Law

Melvin Conway is a compiler developer and systems designer, who is well known for the eponymous Conway’s Law. Various phrasings exist of that, and one popular is

Organizations which design systems are constrained to produce designs which are copies of the communication structures of these organizations.

The original paper and an introductory paragraph can be found on his website. It’s worth reading, because there are more useful insights to be found in the original writeup.

So what does this even mean? Can you give examples from your current or previous work environments?

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Plex and TLS vs. beA and failure

Filippo Valsorda wrote an article “How Plex is doing https for all its users” two years ago. In the article, Filippo explains how the self-hosted media server Plex can offer TLS to secure all connections, including those to the user’s servers.

Plex is a server software running on your machine, and a discovery service somewhere out on the internet. Using your login, you connect to the discovery service, and then connects directly to your server, using XHR.

The XHR part means you are on a page https://plex.tv/web.app, and because that is a https page, the XHRs also need to be encrypted and trusted. That means your server needs to be able to do https, and that means your server needs to have a valid certificate to do this.

How does your server get this cert?

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In those days Caesar Augustus issued a decree…

In those days Caesar Augustus issued a decree that a great repair should be made of the entire computing world. And every child went to their parents to have a look at their installation.

And so it will come to pass that I will show up in the city of Kiel this weekend, actually between Friday and Sunday.

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The new Connexxion

So today is not only a day with a snowflake coming down and touching the ground, which in itself is always an event that puts the Netherlands into chaos (The Dutch are generally awesome with liquid water, but are completely unable to deal with the same substance in a frozen state).

Our annual day of winter.

Today is also the day where Connexxion switches to the new Dienstregling, stops accepting cash in the buses and also switches to new equipment. This is not without it’s share of problems.

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Fertig gelesen: The Apollo Guidance Computer: Architecture and Operation

Frank O’Brien explains an outstanding piece of engineering: The Apollo Guidance Computer (AGC) was the computer that controlled the thrusters and navigation on the Apollo Command Module, and a second instance, on the Lunar Module.

It’s a 32kg box with core memory and core rope ROM, a 16 bit discrete CPU (15 data bits and parity) running at 1 MHz effective, and some very special hardware behind its I/O ports. Memory was 2k words of writeable core memory, and some 36k of ROM.

The processor architecture is stackless, which complicates many things, and despite being designed in 1966, already has multiplication and division machine instructions. Addresses are effectively 12 bit, so sophisticated bank switching to extend the address space is needed, writeable and read-only memory are banked separately. Some interesting interaction with interrupt processing and bank switching exists.

There exists a very basic multitasking system, which at the same time is made simpler and more complicated by the stackless architecture. On top of the multitasking executive sits the interpreter, a virtual machine that was much more comfortable than the actual hardware and had a stack and index-registers, as well as trigonometric math functions. Navigational code ran on the interpreter, basic hardware control (thrusters, sextant, inertial control system and anything timer related) ran on the executive.

The book gives a guided tour into a hardware design over fifty years old, which itself was already foreshadowing and in some way surpassing the more modern integrated chip CPUs of seventies home computers. It also demonstrates a number of creative and interesting ideas of how to achieve remarkable things with very little hardware.

The Apollo Guidance Computer: Architecture and Operation
Frank O’Brien
EUR 30.71 (purchased as Kindle edition, no longer available as eBook)
Paperback

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