Hacker News Reader

Comment by valm- | Hacker News Reader

I don't entirely disagree, but I would like to make one point. Taking 1 millisecond from 1000 people, does not translate to 1 second of lost human productivity. There's no way to collect all those little slices of time and they are not appreciable individually. (Assuming those milliseconds are sufficiently spread out, i.e., not 1 ms out of every 2, or something like that)

View on HN

> Taking 1 millisecond from 1000 people, does not translate to 1 second of lost human productivity

I believe that it does. If you average over a large population even tiny imperceptible increases in e.g. page load time have a measurable effect. Perhaps 1000 people isn't a large enough sample to measure the effect from 1ms delay considering all the confounding factors at play, but I absolutely believe you could see it with precise enough measurement and a large enough sample.

Reminds me of this semi-relevant story about improving the boot times of the original Macintosh — http://www.folklore.org/StoryView.py?story=Saving_Lives.txt

The irony being that Jobs himself limited the memory in the original Mac, so it would have to go to disk more often!

Source?

Its a fairly well known story. The engineers ignored him and built in capability for 512k anyway.

http://www.informationweek.com/mobile/mobile-devices/steve-j...

He also hated fans and expansion slots.

Hmm I think the heart of the question is whether productivity is linear with time. I guess he's right that, for small values of t, productivity is highly superlinear. So giving 1 person an extra minute has an impact on that person, but because of superlinearity, if you distribute that among 60 persons the extra second won't allow anything new, like an original thought or whatever.

You're thinking about it wrong. Of course if you give 60 people 1 second each they won't be able to accomplish a task that takes a minute; that's obvious. Instead what if you give a group of people 59 seconds each to complete a task that takes 1 minute +-10 seconds, and then you give a different group of people 60 seconds each to complete the same task. More people in the second group will complete the task.

Well in your case there's a "phase transition" for the task around 1 minute, so in that case the return function is sublinear in t (if you assume given time G starts at 0. G=t), or linear in it (if you assume it starts at 50, G=50+t). I just wanted to highlight this principle: his statement is equivalent to "For small t, the return for freeing t seconds from computer tasks in superlinear on t". And in general: "The analysis of time redistribution hinges on the linearity of return function for the freed time t".

I do believe that for most persons and most situations, they do tasks in a segmented fashion, working to completion. So if you give them a small amount of extra time it's just going to shift their segments until it hits a synchronized event, like sleeping at a predetermined time or having a meeting -- so all that happens is they get e.g. a millisecond each before they sleep. Under this model it's quite clear to me for very small t the return function is superlinear, and it's better to give 1 person 100 seconds than 100k persons a millisecond (assuming this is one-off of course): for the 100k persons the millisecond is insufficient to elicit any change whatsoever. It's much less than a neuron firing time.

However the carbon footprint of the code does reduce like that. It would be very wasteful in terms of energy not to optimize a widely used binary.

Beware: optimising for speed and optimising for energy don't yield the same results.

If you want speed, you typically want to use your CPU as much as possible. If you want to save energy, you'll want to underclock your CPU. If we go all the way to hardware design, saving energy means going massively multicore, with simple, slow, in-order cores.

You can optimise both, to some extent. But there is a point where you have to chose one or the other.

> If you want to save energy, you'll want to underclock your CPU.

This is incorrect. To save energy, the strategy is to do your thing as quickly as possible, with maximum CPU clocks and enter power saving mode as soon as possible. Otherwise the leakage current in the CPU will consume more energy than can be saved by underclocking.

It may be counterintuitive but fast code consumes the least energy.

I insist: speed and energy savings are correlated to a point.

To. A. Point. Some instructions in your CPU may be faster, but slower alternative often consume less total energy (even taking everything into account —there have been experiments and power measurements). Even if it means your CPU is up a bit longer. Then there are considerations that save energy, but don't matter for speed at all, such as the width of integer operations: no need to go 32 bits wide or 64 bits wide, when no value exceeds 127.

Also, I'm not sure about current leakage exceeding the benefits of underclocking. Those benefits are massive. Lowering the frequency also means you can lower the voltage. In the talks I have seen, halving the speed of a core (and lowering the voltage accordingly) makes it dissipate about a fourth the power. Half the energy, considering everything takes twice the time. This of course ignores leakage, but I doubt it would counter such a strong effect.

Besides, with a phone running a number of background processes, I wonder how much power saving time you can get. Not to mention, the potential costs of switching in and out of CPU saving mode.

"Race to sleep" is a well known power saving strategy.

> This of course ignores leakage

You can't ignore leakage. The ability of a chip to turn off currently unused silicon is crucial to power management on modern devices.

Of course you can't. But if leakage consume 25% of CPU power, then halving the frequency is still worth it. Leakage may be important, but that doesn't mean it dominates.

Now if as an app developer you have zero control over the CPU frequency, sure, the best strategy is probably to write your code to be as fast as possible. But if you're really big on battery life, someone will have control over CPU clocking or voltage. In which case race to sleep is no longer the only viable strategy.

Reply on news.ycombinator.com