Sunday Jan 29, 2012

sweet numerology keyfob

(...Being a ramble through mnemonics, phonetics, information theory, pi, absolute zero, and cosmology, also touching on rutabagas and conches.)

Sometimes we need to commit a number to short-term memory, such as an street address or parking garage berth. More important numbers (for phones or credit cards, PINs or birthdays) call for long-term memorization although smart phones increasingly fill that gap. But even with a smart phone it is pleasantly useful to have the option to use one’s own brain for such things. To do this, I rely on something I call the Sweet Numerology Keyfob. Before I explain that outlandish phrase, here is some background... [Read More]

Friday Jun 24, 2011

Feynman's inbox

Here is Richard Feynman writing on the ease of criticizing theories, and the difficulty of forming them:

The problem is not just to say something might be wrong, but to replace it by something — and that is not so easy. As soon as any really definite idea is substituted it becomes almost immediately apparent that it does not work.

The second difficulty is that there is an infinite number of possibilities of these simple types. It is something like this. You are sitting working very hard, you have worked for a long time trying to open a safe. Then some Joe comes along who knows nothing about what you are doing, except that you are trying to open the safe. He says ‘Why don’t you try the combination 10:20:30?’ Because you are busy, you have tried a lot of things, maybe you have already tried 10:20:30. Maybe you know already that the middle number is 32 not 20. Maybe you know as a matter of fact that it is a five digit combination… So please do not send me any letters trying to tell me how the thing is going to work. I read them — I always read them to make sure that I have not already thought of what is suggested — but it takes too long to answer them, because they are usually in the class ‘try 10:20:30’.

(“Seeking New Laws”, page 161 in The Character of Physical Law.)

As a sometime designer (and longtime critic) of widely used computer systems, I have seen similar difficulties appear when anyone undertakes to publicly design a piece of software that may be used by many thousands of customers. (I have been on both sides of the fence, of course.) The design possibilities are endless, but the deep design problems are usually hidden beneath a mass of superfluous detail.

The sheer numbers can be daunting. Even if only one customer out of a thousand feels a need to express a passionately held idea, it can take a long time to read all the mail. And it is a fact of life that many of those strong suggestions are only weakly supported by reason or evidence. Opinions are plentiful, but substantive research is time-consuming, and hence rare. A related phenomenon commonly seen with software is bike-shedding, where interlocutors focus on surface details like naming and syntax… or (come to think of it) like lock combinations.

On the other hand, software is easier than quantum physics, and the population of people able to make substantial suggestions about software systems is several orders of magnitude bigger than Feynman’s circle of colleagues. My own work would be poorer without contributions — sometimes unsolicited, sometimes passionately urged on me — from the open source community. If a Nobel prize winner thought it was worthwhile to read his mail on the faint chance of learning a good idea, I am certainly not going to throw mine away.

(In case anyone is still reading this, and is wondering what provoked a meditation on the quality of one’s inbox contents, I’ll simply point out that the volume has been very high, for many months, on the Lambda-Dev mailing list, where the next version of the Java language is being discussed. Bravo to those of my colleagues who are surfing that wave.)

I started this note thinking there was an odd parallel between the life of the physicist and that of a software designer. On second thought, I’ll bet that is the story for anybody who works in public on something requiring special training. (And that would be pretty much anything worth doing.) In any case, Feynman saw it clearly and said it well.

Monday Feb 23, 2009

bloom filters in a nutshell

A Bloom filter is an array of M bits which is queried at K quasi-randomly selected positions pk (k < K). If all of the bits are set, then the query returns positive, indicating that someone has already visited the array, setting the bits at all the positions pk.

The filters are quite simple, but the math is a little slippery until you get the right grip on it. Here’s the way I like to grab it, presented in case it helps anyone else.

First of all, the bottom line: Size your Bloom filter to contain NK bits, plus an overhead of 44%. Put another way, for an error rate of ε, allocate lg(1/ε)·lg(e) bits for each key you intend your filter to hold.[Read More]

Saturday Mar 29, 2008

words of power from the ancient world

Hammering my way through a posting on the JVM and continuations, I realized again how oddly poetic are some of our terms of art. Some of them seem to have been with us from the dawn of the single-threaded stored-program computing machine. Terms like “continuation”, “closure”, “thunk”, even “call” and “loop” are metaphoric, evocative, polyvalent, elusive of final definition. What I mean is, they are poetic...[Read More]
About

John R. Rose

Java maven, HotSpot developer, Mac user, Scheme refugee.

Once Sun and present Oracle engineer.

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