TUHS April 2019

tuhs@tuhs.org

63 participants
43 discussions

by jnc＠mercury.lcs.mit.edu

> From: Richard Salz > Any view on this? > https://www.microsoft.com/en-us/research/publication/a-fork-in-the-road/ Having read this, and seen the subsequent discussion, I think both sides have good points. What I perceive to be happening is something I've described previously, but never named, which is that as a system scales up, it can be necessary to take one subsystem which did two things, and split it up so there's a custom subsystem for each. I've seen this a lot in networking; I've been trying to remember some of the examples I've seen, and here's the best one I can come up with at the moment: having the routing track 'unique-ID network interface names' (i.e. interface 'addresses') - think 48-bit IEEE interface IDs' - directly. In a small network, this works fine for routing traffic, and as a side-benefit, gives you mobility. Doesn't scale, though - you have to build an 'interface ID to location name mapping system', and use 'location names' (i.e. 'addresses') in the routing. So classic Unix 'fork' does two things: i) creates a new process, and ii) replicates the environment/etc of an existing process. (In early Unix, the latter was pretty simple, but as the paper points out, it has now become a) complex and b) expensive.) I think the answer has to include decomposing the functionality of old fork() into several separate sub-primitives (albeit not all necessarily directly accessible to the user): a new-process primitive, which can be bundled with a number of different alternatives (e.g. i) exec(), ii) environment replication, iii) address-space replication, etc) - perhaps more than one at once. So that shell would want a form of fork() which bundled in i) and ii), but large applications might want something else. And there might be several variants of ii), e.g. one might replicate only environment variables, another might add I/O channels, etc. In a larger system, there's just no 'one size fits all' answer, I think. Noel

5 years, 8 months

"Fork considered harmful"

by Richard Salz

Any view on this? https://www.microsoft.com/en-us/research/publication/a-fork-in-the-road/

5 years, 8 months

Re: [TUHS] Unix filesystem semantics history - files with holes

by Richard Tobin

> When did the Unix filesystem add the semantics for "files with holes" (large, > sparse files)? It was there in the first edition: https://www.bell-labs.com/usr/dmr/www/pdfs/man51.pdf The FILE SYSTEM (V) man page includes a last paragraph identical to that of FILSYS (V) in seventh edition: If block b in a file exists, it is not necessary that all blocks less than b exist. A zero block number either in the address words of the the i-node or in an indirect block indicates that the corresponding block has never been allocated. Such a missing block reads as if it contained all zero words. The first edition indirect blocks were a bit different though: if the file was bigger than 8 blocks (4kB), all the blocks in the inode were (singly) indirect. -- Richard -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

5 years, 8 months

Unix filesystem semantics history - files with holes

by Erik E. Fair

When did the Unix filesystem add the semantics for "files with holes" (large, sparse files)? Just an idle question that was sparked by a conversation I had today. Erik <fair(a)clock.org>

5 years, 8 months

Re: [TUHS] Paper discussing Unix boot process?

by Pat Barron

The more I think about this, the more I'm sure I'm barking up the wrong tree... >From bits and pieces I've been able to recall, the thing I am looking for was not about Unix - it was about TOPS-20. It was a timeline of the system bootstrap activities from power-on to the point where users could log in. I still don't remember where I found it originally, but at least now I'm pretty sure I've been looking in all the wrong places... I believe it originated at CMU, but I don't know for sure that that's where I originally located it. The actual problem I'm trying to solve is, at this point in my professional career, I'm starting to interact with a lot of people (even experienced software developers) who just have no clue of what has to happen to get a computer from the point of "power-on" to the point where they can actually use it to do things. This makes me sad... So, I'm looking for something that I can point these people to that could clue them in... I think the whole bootstrap process is useful to understand for a lot of reasons, partly because it makes you think about all the little fiddly details that have to be attended to to make the computer do what you want - when I was first learning about this, I remember being particularly fascinated by what had to happen to prepare for that moment at which you turn on the MMU, to make sure that the system continues executing in a place you expect it to, in the right processor mode. I know most people that I interact with are using Linux or Windows on Intel-architecture machines, but the boot process for Unix on the PDP-10 or VAX (or even TOPS-20 on the PDP-10) I thought would be a much simpler thing to understand. Though maybe that's the wrong thought process, maybe I should just find something related to Linux that is comparable (even though I think it's more complicated). While searching, I also came across a decent presentation by a friend of mine who teaches at CMU, and discusses hardware that people probably actually work with right now, but I think it would be best consumed along with the actual lecture that it goes with. http://www.cs.cmu.edu/~410-f08/lectures/L20_Bootstrap.pdf Maybe I'll find what I was originally looking for at some point, but after spinning on this for most of the day, I don't think it's related to Unix... --Pat.

5 years, 8 months

Re: [TUHS] TUHS Digest, Vol 41, Issue 2

by Doug McIlroy

here

5 years, 8 months

Re: [TUHS] List ping

by jnc＠mercury.lcs.mit.edu

This thread brings to mind a wonderful saying which I just saw in another forum: "A wise man speaks when he feels he has something to say - a fool speaks when he feels he has to say something." And to reply in advance to the 'but I did't realize so many other people had sent in replies' - try scanning your emailbox before replying to messages to a large list. S/N is to be hallowed. Noel

5 years, 8 months

Re: [TUHS] Does anybody have this book and can look for something for me?

by Pat Barron

This book can apparently be "borrowed" from the Internet Archive. I'm not sure how they do that, haven't tried to, I just see it says you need to log in to borrow the book for 14 days. https://archive.org/details/unixprimerplus00wait --Pat.

5 years, 8 months

Does anybody have this book and can look for something for me? - solved

by Jon Steinhart

Thanks to Pat, I looked at the book at the Internet Archive and it is there.

5 years, 8 months

Does anybody have this book and can look for something for me?

by Jon Steinhart

OK, something that's not a ping :-) I'm trying to track down the author of a cartoon that I'd like to use in my book so that I can try to get permission. Last one that I need! It's a cartoon that I only have on paper and don't know where it came from. It has two frames, then and now, the first with a bunch of cavemen grunting awk, grep, mkdir, yank, the second with a bunch of people sitting at computers uttering the same. I recently stumbled upon something that said that these were in a book called "UNIX Primer PLUS". Anybody have a copy of that? If so, can you please check to see if that's the original or whether they got it from somewhere else? Thanks, Jon

5 years, 8 months

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TUHS April 2019