htop explained*

I was looking forward for web protocol, but alas...

> I would hope that the system knows precisely what is using every single byte of physical and virtual memory.

Of course the system knows what is using every page. The difficulty is really in how to account for pages that are backed by disk.

If you count all of those as free, that's not accurate. If you count all of those as used, that's not accurate either. Additionally, FreeBSD (at least) doesn't have separate queues for disk backed pages, so there's not really a good way to know how much of your active (or inactive) memory is disk backed.

As an additional caveat that measuring active/inactive has costs. In the past, FreeBSD wouldn't really do the work for that until it needed to... I know some stuff changed, but I don't remember where it ended up; it wasn't great when it bulk marked a ton of pages as inactive and then the active ones would fault back in.

The thing is it's easy to define free, unused memory. But a lot of the used memory is your system caching stuff that would be free if you needed more than what's actually free. So you can see you have 1g of free memory out of your 4g, but then you allocate 3g and it will do without a sweat and you'd be confused. So you have to go and dig for what those caches are and report that they're effectively free too.

You will be surprised by how inaccurate memory measurements are.

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I'm a professor at a community college in Silicon Valley, and my students use online textbooks. I try to use Creative Commons or other libre textbooks, but sometimes I use paid textbooks when they are heads-and-shoulders better than their libre alternatives. Some e-textbooks can be accessed on a subscription basis. I admit I prefer non-subscription materials, but a colleague advised me that often the book that students learn from is different from a good reference book that students can use once they've already learned the material. For example, my colleagues and I have had great success with an online, interactive textbook for discrete math. While the subscription is unfortunately only valid for the duration of the course, once students have learned discrete math, they could buy a used copy of Rosen's discrete math textbook as a reference.

The nice thing about e-textbooks is not needing to carry around a bunch of heavy books. I remember the tomes I had in my college days, such as Stewart's Calculus.

I bought as few textbooks as I could, but the few that I did buy are sitting in my parents' basement bookshelves somewhere.

FreeBSD didn’t have memory overcommit and instead used strict swap reservation - each allocated anonymous memory page was supposed to have a corresponding swap page. This required 2x RAM swap space, otherwise you would get “out of swap” when forking a large process. FreeBSD implemented memory overcommit around 2000.

Yes, It's just not every tool is aware of ZFS ARC. Which is what this post is about. Author just describes in an odd way.

I remember how NetBSD promoted itself as running on many more toasters than Linux once.

Then some NetBSD dev wrote on their mailing list that this is no longer true. Linux runs on more toasters now. (And also top 500 supercomputers, but toasters are the real metal to the petal test.)

These fights always remind me of:

https://www.jwz.org/doc/worse-is-better.html

It's an interesting piece of history too. I kind of evaluate it a bit differently, e. g. my summary is "momentum beats academic perfection". Which is not completely what it is about, but it is my own imperfect TL;DR summary.

I've noticed that the various tools report different amounts of free memory. I appreciate finally having an explanation for why.

Sure, but also some tools needed fixing.

The OS Crusades are over man

No ZFS no problem :)