Insomnia (by alaric)

yawn

5:30am and I haven't slept a wink yet! I really need to sort out my lifestyle so I get (a) exercise and (b) time to think every day. Time to think is important for me; if I don't get enough, then when I go to bed, I lie there and think. Lots.

Tonights thoughts have included:

1. Some ideas about how whole-program transformations (eg, the macroexpander/partial evaluator and the OO system) in CHROME might be handled. The OO system needs to be a whole-program transformation rather than just some macros using the normal macroexpander since things like inheritance graphs and method lists for generic functions need to be accumulated together; most Lisps handle that with macros that destructively update data structures, but I'm trying to design a system without rampant mutation, so need a whole-program code walk to do this. Clearly, since we want to be able to write macros that produce generic functions, methods, and the like, we need to do this AFTER normal macro expansion, but before the compiler gets its hands on it.
2. Some ideas about separating the generic function/method system - the dispatch part of Lispy OO - from the classes-inheriting thing. Subtype relationships that are used to dispatch GFs should be doable with plain predicates - pair? my-record-type? etc. Or more complex predicate expressions on groups of arguments, so we can support multivariate typeclasses in the Haskell sense, as a rich interface/implementation system as well as a traditional records-with-single-inheritance class system. To do this properly we also need declarations that one predicate implies another - (number? x) -> (integer? x) - so that a method on numbers will be used for integers, yet a more specific integer method can override it. I'm not sure how decidable the "most specific method wins" thing can be with complex multivariate type predicates, though. Must experiment and ask knowledgeable formal logic folks.
3. Thoughts about future computer architectures. The drive is for more speed, but these days, most CPUs are idle waiting for memory to feed them code and data, or (much more often) for the disk, network, or user to feed them. The only places where the CPU maxes out tend to be highly parallelisable tasks - server stuff handling lots of requests at once, games and other audiovisual things, and scientific number crunching. This suggests to me that a future direction of growth would be one or more high-bang-per-buck MISC processors embedded directly into SRAM chips (sort of like a CPU with an onboard cache... but the other way around, since the CPU would be much smaller than the SRAM array) which are bonded to the same chip carrier module as a set of DRAMs. One or more of CPU-and-SRAM and some DRAM chips are then all designed together as a tightly-coupled integrated unit for maximum speed due to short traces and the lack of standardised modular interfaces between them (like DIMMs and CPU socket standards) meaning that the interface can evolve rapidly. The whole CPU+SRAM+DRAM unit is then pluggable into a standardised socket, which motherboards will have several of. The result? Lots of cores of low power consumption reasonably fast CPU with high speed access to high speed memory. And for those demanding games/media/scientific tasks? The higher-end modules will have FPGAs on as well...
4. Forget nasty complex unpredictable memory caches: have a nonuniform physical address space (with regions of memory of varying speed) and let the OS use the virtual memory system to allocate pages to regions based upon application hints and/or access statistics. Not having cache tag and management facilities makes for more chip area to put actual memory in.
5. We've been wondering about getting goats lately. Goats are useful creatures; they produce milk (which can be turned into cheese) and they produce decent wool (just not in the quantities sheep produce it). Their milk and cheese don't make Sarah ill the way cow-derived versions do. Plus, we need something to come and graze our paddock. We've been doing a little bit of research and apparently two goats would be about right for the space we have. We'd need to put an inner layer of fence around the paddock to keep them in while still allowing the public footpath, and we'd need a little shed for them to shelter in. But thinking about setting things up in the paddock, I'm now wondering if it would be a good idea to build a duck run in there too, down at the bottom by the stream, all securely fenced against foxes and mink and with a duck-house up on stilts in case of flooding, but with a little pond dug out for them (connected to the stream by a channel with a grille over it to prevent escapage). It would be a convenient place to have the ducks, and it would make a good home for them, I think.

It's now 6am. Do I try and go to sleep, or try and last the day out? Hmmm...

Web server upgrade (by alaric)

Whew. On Monday I upgraded some of the software on my primary web server, since it was running some old stuff with security holes in.

Annoyingly, the www/apache2 package in NetBSD seemed to now conflict with devel/subversion-base since apache 2 required devel/apr0 while devel/subversion-base required devel/apr and they were conflicting packages. So, I had to upgrade to www/apache22. Fair enough.

One recompile later, and I start apache, and start checking out different web applications I host to see if they all still work...

...and my browser times out. Hmm, OK. I go to an open ssh window to look at the log files, and it's frozen.

I quickly check the network hasn't failed, then resign myself to the fact that my server has just dropped off of the net. It won't even ping, and I can't reach any of the services it forwards in to the backend server either, so the network stack is totally down.

So that evening I head down to the datacentre and take a look... to find that it's died handling the exit() syscall from Apache. Apparently an assertion failure inside knote_destroy or something.

Reboot. Start Apache. Start taking a look at sites.

Kerboom! It dies again in the same way.

Hmmm... Clearly, my three year old NetBSD 2.0 kernel is none too happy with Apache 2.2. It looks like Apache's doing something that triggers a bug in the kernel; knotes are event notification things, so I bet Apache's doing some kind of asynch I/O, and triggering a bug in the kernel code that implements it, causing it to leave the knote state of the process in an invalid state, so that the kernel panics when trying to close down the process state after process termination.

So I reboot it again, stop Apache starting, and leave it at that for the time being. No web service, but everything else works.

Then this evening (the day after), I returned, now with a shiny NetBSD 4.0 install CD in hand. Nervously I backed up some critical directories, then bit the bullet and did an upgrade.

And, to my delight, it was nearly seamless. The NetBSD installer upgraded and rebooted into a nearly perfectly working system. All my existing software, compiled under 2.0, ran fine under 4.0's 2.0 emulation, with the mysterious exception of net/bind9, which wouldn't start. A quick cd /usr/pkgsrc/net/bind9; make install later, and it was starting fine. Even Apache worked without hosing the system!

I had to compile a custom kernel with routing enabled, to allow the NAT that the server provides between the single public IP of the love.warhead.org.uk cluster and the backend server infatuation; then a quick reboot and that was working too.

All in all a successful mission, and it only took an hour or two. I still need to recompile all of my packages, but only to avoid the risk of there being a problem in the 2.0 emulation. While I was there I recompiled bash and sudo, just because it's nice to be able to rely on them.

Error correcting codes (by alaric)

I've just finished reading A Commonsense Approach to the Theory of Error-Correcting Codes. The book does exactly what it says on the tin; it explains error correcting codes in terms of linear modulo-2 algebra, only getting into Galois fields and all that in an appendix.

And, as such, it does little more than scratch the surface. It only goes into Reed-Solomon codes that can correct single word errors, for example. But hey, I'm not complaining - it's done a great job of giving me an intuitive understanding of Hamming codes, cyclic codes (such as CRCs), the single-word-correcting RS codes, and so on. And I've learnt a lot about Linear feedback shift registers.

But it strikes me that the whole field of error correcting code is a bit insular. The maths required to really grasp it are really complex. Finite fields are bizarre things. While lots of people can experiment with things like data compression or basic encryption, error correction codes, the third cornerstone of low-level coding technology, is quite inscrutable.

This sucks.

Read more »

Derren Brown at Southend (by sarah)

Tuesaday saw numero-uno of my friend Clares Birthday celebrations and so we headed off to South end (of all places) to see Derren Brown. We saw him in London about two years ago when I was still on the crutches - I got hit by a monky that time so Clare got to go up on stage and win £50 off of him which was fantasmigorical as you can imagine.

This time none of us made it onto the stage but the show was fantastic! What with gorrilaz and a truelly spectacular Oricle show that had me wowing - he does he do it? Clare almost took out half the threater with her egerness to put a question in his pot! We really enjoyed the show 🙂

However the pre-show meal was a bit of a disastor, they just took too long to bring it out and got the order wronge and in one case brought out something that was inedible ie Lianes cake could smash plates! They are the resturant in the theatre so you'd have thought they would have been better but no we had to leave half our desserts and had no coffee (which was all part of the set meal) and we only just got in for the beginning of the show.

Also poor Al had to do a bit of a marathon to get to us for the meal which he should have been late for but becuase of how they where being we had bearly ordered! Still us girls had a nice wonder around the place and as me and Lianne are obessed with taking photos and neither of us had a camera Clares new camera was getting a good old try out. I'm hoping to see them soon so I can post them up on here 🙂

Tonight sees round two of the celebrations - Karyoky mucho mucho fun 🙂

The Castle – The Return of Sarah (by sarah)

Yesturday I went to the Castle climbing centre I was cery nervous as I didn't know if I would even be able to get up onto the bouldering walls from the floor, nor how the pelvis would react to such an activity.

I haven't attempted climbing since the day after our wedding, sure we've taken the scouts climbing a few times but with Jean being there and time constraints etc... we have never managed to have a little climb at such events. I was excited and worried and then too my suprise my body sort of remebered things though I was still pretty dreadful!

First off I refused to try things until we found a stretch of relatively flattish wall with like no one else around using it. I pulled myself onto it and intially had the problem that I always want to go up but once I realised I was supposed to go along thats what I did. Being me though I got a bit confused as there where too many hand holds and foots holds so the first time I went along the wall I just used what ever was handy then Al showed me that the different colours where different hardnesses of route so I did the 4a holds which are basically the easiest and then had a little go at the black holds later on but had to give up with trying them as pelvis had gone click.

My pelvis actually with stood me falling off onto crash matts whilst trying to navagate a tricky corner - Al tended to fall off at this point too. This was in the bit behind the protrusions they've called envy and lust. I did alot of trying to climb round the pink one of those too, again just using any old hand holds! Al said lots of girls tend to climb on that one becuase it is pink - I thumped him. He on the other hand was obessed with getting around using the wooden hand and foot holds on Envy which are apparently 4a's aswell though he had to stretch to do them so I don't know how I could manage them.

I managed another horizontal stretch of boldering wall but alas I tried to do a specific route and basically stretched too wide with my legs resulting in pelvis click. It didn't hurt but I know from bitter experience that once it has clicked it is very easy to make to properlly mobile and therefore painfull once more so I did the thing I find the hardest to do and stopped.

We had a sandwitch in the cafe place and then went back - I was sort of optomistically hoping the pelvis would have settled down but it hadn't so I just helped Al for our second half showing him where the holds where and sorting out the slight issue of why he was getting so breathless whilst climbing - to wit - he was concentrating so hard he was holding his breath and trying to wizz around the bolder before oxygen deprivation got him!

The after math today is that I have slightly acted up the pelvis but my disc (which I acted up on Wednesday probably as I didn't do my normal amount of daily wallking as I had acted up my knee on Monday by us walking from Angel to Highgate) feels alot better and my shoulder has so far really behaved itself much to my joy :). This makes me really happy as I really miss doing all the climbing and stuff and it makes me feel more like me 🙂