TUHS

tuhs@tuhs.org

5 participants
6532 discussions

by jnc＠mercury.lcs.mit.edu

> From: Warren Toomey > xv6 is a Unix-like OS written for teaching purposes. I'm fairly well-aware of Xv6; I too am planning to use it in a project. But back to the original topic, it sounds like there's a huge amount of variance in the semantics of doing fstat() on a pipe. V6 doesn't special-case it in any way, but it sounds as if other systems do. What V6 does (to complete the list) is grow the temporary file being used to buffer the pipe contents up to a certain maximum size, whereupon it halts the writer, and waits for the reader to catch up - at which point it truncates the file, and adjusts the read and write pointers back to 0. So fstat() on V6, which doesn't special-case pipes in any way for fstat(), apparently returns 'waiting_to_be_read' plus 'already_read'. >>> xv6 has fstat() but returns an error if the file descriptor isn't >>> associated with an i-node. >> ?? All pipe file descriptors should have an inode? To answer my own question, after a quick look at the Xv6 sources (on my desktop ;-); it turns out that Xv6 handles pipes completely differently; instead of borrowing an inode, they have special 'pipe' structures. Hence the error return in fstat() on Xv6. (That difference also limits the amount of buffered data in a pipe to 512 bytes. So don't expect high throughput from a pipe on Xv6! :-) So I guess you get to pick which semantics you want fstat() on a pipe to have there: V6's, V7's (see below), or something else! :-) > 7th Ed seems to return the amount of free space in the pipe, if I read > the code correctly: I'm not sure of that (see below), but I think it would make more sense to return the amount of un-read data (which is what I think it does do), as the closest semantics to fstat() on a file. It might also make sense to return the amount of free space (to a writer), and the amount of data available to read (to a reader), since those are the numbers users will care about. (Although then fstat() on the write side of a pipe will have semantics which are inconsistent with fstat() on files. And if the user code knows the maximum amount of buffering in a pipe, it could work out the available write space from that, and the amount currently un-read.) > fstat() > { > ... > /* Call stat1() with the current offset in the pipe */ > stat1(fp->f_inode, uap->sb, fp->f_flag&FPIPE? fp->f_un.f_offset: 0); > } > stat1() > { > ... > ds.st_size = ip->i_size - pipeadj; I'm too lazy to go read the code (even though I already have it :-), but V7 seems to usually be very similar to V6. So, what I suspect this code does is pass the expression: ((fp->f_flag & FPIPE) ? fp->f_un.f_offset : 0) as 'pipeadj' (to account for the amount that's already been read), and then returns (ip->i_size - pipeadj), i.e. the amount remaining un-read, as the size. Noel

8 years, 10 months

Re: [TUHS] fstat(2) on pipes?

by jnc＠mercury.lcs.mit.edu

> From: Warren Toomey > I'm trying to find out what fstat(2) returns when the file descriptor > is a pipe. In V6, it returns information about the file (inode) used as a temporary storage area for data which has been written into the pipe, but not yet read; i.e. it's an un-named file with a length which varies between 0 and 4KB. > xv6 has fstat() but returns an error if the file descriptor isn't > associated with an i-node. ?? All pipe file descriptors should have an inode? Noel

8 years, 10 months

Help with a Unix-ish project?

by Warren Toomey

Hi all, I'm working on a Unix-related project, and I thought I'd ask if anybody here might help. There's a pared-down Unix-like system, xv6, which is inspired by 6th Edition Unix and the Lions Commentary. Its purpose is to teach OS principles. The website and book are here: https://pdos.csail.mit.edu/6.828/2014/xv6.html https://pdos.csail.mit.edu/6.828/2014/xv6/book-rev8.pdf Unfortunately, while the kernel is nice, they don't provide much of a run-time environment, so it feels too much of a toy to use. I had the idea of porting a small set of libraries and commands over to get it to the point where it feels a bit like 7th Edition. I've made a start by using the Minix 2.0 libraries and commands, see https://github.com/DoctorWkt/xv6-minix2 and the NOTES file. I now realise that bringing up a libc plus associated commands will involve a fair bit of work. So, if anybody is interested in helping, let me know. Thanks in advance, Warren

8 years, 10 months

Re: [TUHS] History repeating itself (was: Unix v6 problem with /tmp)

by Norman Wilson

Dave Horsfall: Not Henry Spencer, perchance? ===== Since the Canadian in question had been working in the US since 1964 or so, he must by now be pushing 70 years old. I haven't seen Henry for some years, but I don't think he has aged that much. Norman Wilson Toronto ON

8 years, 11 months

Re: [TUHS] History repeating itself (was: Unix v6 problem with /tmp)

by Rudi Blom

> Date: Sat, 30 Jul 2016 15:30:36 +0000 > From: Michael Kjörling <michael(a)kjorling.se> > To: tuhs(a)tuhs.org > Subject: Re: [TUHS] History repeating itself > Message-ID: <20160730153036.GI3375(a)yeono.kjorling.se> > Content-Type: text/plain; charset=utf-8 > > On 30 Jul 2016 10:15 -0400, from cowan(a)mercury.ccil.org (John Cowan): >>> Who needs FedEx? >> >> Well, latency counts for something too, as does radius: if I want to >> send bulk data from New York to London (a very normal thing to do), >> your station wagon isn't going to count for much. > > You could, however, get an economy class flight ticket and load up > your suitcase with either HDDs or SDXCs (I suspect SDXCs would be > better per amount of data from the perspective of both volume and > weight, and would take better to handling). Given FedEx's prices, > _once you have the infrastructure set up_ (which you'll need whether > you have someone travel with the media, by air or by stationwagon, or > FedEx it), that _might_ even compare favorably in terms of bytes > transferred per second per dollar. (Now that's a measurement of > throughput I don't think I've seen before; B/s/$.) Of course, you'd > need someone who can babysit the suitcase, which potentially adds to > the cost, but the stationwagon traditionally hasn't been self-driving > either, and most of a transatlantic flight isn't active time on part > of the person travelling with the suitcase so you could go with an > overnight flight and allow the person to sleep. > > If you want to reduce the risk of the bag getting handled roughly or > lost in handling, reduce the above to carry-on luggage; it will still > provide a quite respectable throughput. > > ... ... > > It might not be the absolute cheapest approach, but it seems rather > hard to beat in terms of throughput per dollar for bulk data transfer, > especially if you already have someone who would travel anyway and can > be convinced to take a company-approved suitcase in return for having > their ticket paid for. > > -- > Michael Kjörling • https://michael.kjorling.se • michael(a)kjorling.se > “People who think they know everything really annoy > those of us who know we don’t.” (Bjarne Stroustrup) > To setup the 'infrastructure might be the tricky part. Many years ago I flew from Montreal to Amsterdam and had two stacks of 5-1/4" diskettes with me. No papers, confiscated in Amsterdam. Cheers, Rudi

8 years, 11 months

Unix v6 problem with /tmp

by Mark Longridge

Hi folks, My root partition for Unix v6 is almost full and /dev/rk0 only has 83 blocks. The trouble is I wanted to compile bc.y and I think it needs around 300 blocks of temporary space. I was wondering if there was a way to set up Unix v6 so that it could use one of the other drives for tmp space. I tried to set up a link using ln but it seems I can't link across filesystems. The exact error is "26: Intermediate file error". I managed to rearrange things so that /dev/rk0 had over 300 blocks of free space and it fixed the problem, but I'm curious if there was another solution. Mark

8 years, 11 months

Re: [TUHS] Unix v6 problem with /tmp

by Norman Wilson

Clem Cole: Also to be fair, Dennis did symlinks before 4.2. They were part of the V8 I believe. ======= I'm pretty sure they came from Berkeley nevertheless. I don't know the exact order of events, but the 8th Edition kernel was essentially that from one of the later 4.1x BSDs, hacked in 1127 to remove sockets and FFS (were they even there yet), then to add Dennis's stream I/O system, Tom Killian's original /proc, and Peter Weinberger's neta network-file-system client. Perhaps a few other hooks as well. Symlinks were already there, and although we made some limited careful use of them, made nobody very happy because they made such a big irregular lump in so many things: file system no longer a tree, difference between stat and lstat, and so on. One thing 8/e did differently from Berkeley was that ls by default hid symlinks rather than trotting them out proudly. If f was a symlink, ls -l f showed the state of the target file, not that of the link; one had to do ls -lL f to see the symlink itself. That reflected a general feeling that symlinks should be neither seen nor heard unless necessary. Norman Wilson Toronto ON

8 years, 11 months

Re: [TUHS] Unix v6 problem with /tmp

by Norman Wilson

William Pechter: Only thing I can think of is add another drive or partition and mount it as /tmp. ===== You say that as if it's a bad thing. Norman Wilson Toronto ON mount >> ln -s

8 years, 11 months

Re: [TUHS] Of login names

by Norman Wilson

Just to be clear: I don't pine at all for UUCP. I do still think it's a mistake that e-mail addresses and domain names run backwards from the way directories and filenames run. That's what I miss about !norman vs norman@. But it's all a Beta-vs-VHS matter these days, like a lot of unfortunate design decisions that have become standard over the years. Like git winning out over hg, which is sort of like the VAX/VMS command language winning out over the Bourne shell. (To toss another pebble into the pond to see what the ripples look like, rather in the manner of Rob and Dave.) Norman Wilson Toronto ON

8 years, 11 months

lpr with symlink

by Nemo

I recently noticed that lpr has a symlink option ("-s") on Solaris but not on Apple. Is there anything here historically except prudence and small drives? N.

8 years, 11 months

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TUHS