Dead-for-Now (DFN) Code Splitting

Demand-loading of program code is an old idea that is very important for web applications. Ajax applications typically have 2-4 megabytes of JavaScript code, and it takes enough time to download and parse that code that there are large user-visible pauses. Web sites need to go lightning fast whenever possible, and one tool in the toolbox is to load only part of the code to begin with and load the rest later.

This idea isn't new, of course. Unix loads programs on demand, one page at a time. When you start an application on Unix, the OS loads only its first page. Whenever execution goes off of that page onto a new page, it traps into the OS to load a new page. On machines too memory-constrained for that to work, a manual-control variation called memory overlays are often used. Also, on the web, Java applets attempted to solve this problem by loading one class at a time. Unlike the prior two approaches, the class-by-class demand loading of Java applets failed to ever become practical, and it was abandoned in favor of loading at the jar granularity.

For about a year now, I've been maintaining GWT's implementation of demand loading, which was designed by Bruce Johnson. One of Bruce's key ideas was to break with the page-by-page loading done for applications loading from disks. Java applets initially stuck to that model, but there are severe problems with it, essentially equivalent to the problems of synchronous XHR that I previously posted about. Bruce is in the "yes, it's evil" camp, and I agree. The problems are just too hard to overcome. Therefore, a key part of GWT's demand loading system is that whenever you request code that might not be available yet, you supply a callback that is invoked asynchronously.

Another key part is that deployed GWT apps always go through a whole-program compile. As a result, GWT's code splitter can use static analysis as part of the system. A common static analysis is the identification of dead code, code that is part of a program but will never run. We like to say that GWT identifies code that is "dead for now."

The Fable of the Fable

Gene Expression is normally an interesting blog, but they have
now joined the ranks of Qwerty basher bashing.
They start by being merely incorrect:

In 1990, Stan Liebowitz and Stephen Margolis wrote an article
detailing the history of the now standard QWERTY keyboard layout
vs. its main competitor, the Dvorak Simplified Keyboard. In brief,
the greatest results in favor of the DSK came from a study that was
never officially published and that was headed by none other than
Dvorak himself. Later, when researchers tried to devise more
controlled experiments, the supposed superiority of the DSK mostly
evaporated.

They then go from this bad footing to launch stink bombs like this one:

Shielded from the dynamics of survival-of-the-fittest, all manner of
silly ideas can catch on and become endemic. In this case, the
enduring popularity of the idea is accounted for by the
Microsoft-hating religion of most academics and of geeks outside the
universities. For them, Microsoft is not a company that introduced the
best word processors and spreadsheets to date, and that is largely
responsible for driving down software prices, but instead a folk devil
upon which the cult projects whatever evil forces it can dream
up. Psychologically, though, it's pretty tough to just make shit up
like that. It's easier to give it the veneer of science -- and that's
just what the ideas behind the QWERTY and Betamax examples were able
to give them.

To take this backwards, I read the "Fable" article they link back in the 90s,
and I found it uncompelling. I went on to spend a month retraining with
Dvorak, because I expect to be typing for at least another decade. I believe
the benefits of Dvorak are modest, but given the enormous amount of
typing I expect to do, it will add up.

Microsoft never entered the picture except in a positive way. It occurred to me
at the time that, unlike with the computers of my childhood, computers
with operating systems like Microsoft's make it exceptionally easy to switch
the layout to Dvorak. Far from believing that Microsoft locked in Qwerty,
I have considered them part of the revolution. For me, Microsoft
does not explain the mystery. It deepens it.

The case for Dvorak looks good. The famous "Fables" article
mostly only argues that that Dvorak's study, from nearly a century
ago, is by itself not very compelling. It ignores substantial other
evidence that has accumulated since then. For one, the fastest
typist in the world prefers Dvorak. Further, the evidence is
overwhelming that Dvorak reduces finger travel distance
when typing English text. By Occam's Razor, we should
assume until proven otherwise that this longer finger distance
implies slower speed and greater wear and tear.

At this point, it looks like Dvorak really is better. The economic
study of how we got here, far from being based on a fable,
is a legitimate scientific probe into a surprising aspect of
our world.