Comment by DanielBMarkham | Hacker News Reader

Comment by DanielBMarkham | Hacker News Reader

So quantum computing says that instead of one calculation, I perform multiple calculations simultaneously in different universes. Now this guy says that I have to give up the idea of computing as being deterministic and always getting the right answer?

It has to be true, because it makes no sense whatsoever. Web designers of the future will have a great reply to customers. "The site gives wrong answers? Well, you need to realize that asking for correct answers isn't what we do with computers. And besides that, in another universe the answers are correct. Boy, are you really happy over there!"

That was perhaps a too-long-winded snark with a point: it's not that I don't feel that we're headed in the right direction, it's that visionaries always seem to concentrate on the brain-exploding part of what they're saying instead of talking about how it all integrates together. While I think this might sell some website views, it doesn't do much for the practitioner looking for insight.

Taking into account the snark, it doesn't change the fact that you're presenting a false dichotomy.

Ungar isn't saying that we have to give up determinism. What he's saying is that to take advantage of massively parallel systems, determinism comes at a high cost.

Clearly, we can write programs on current architectures that are deterministic (well, perceived as such, at least) and we don't have to forgo that. But there is no reason to believe it has to be the only architecture. Ungar is looking at what happens when you try to do computation in a highly networked environment with low latency (like say, a brain).

Also, if you don't see it helping for the practitioner, perhaps practitioners aren't asking good (or enough?) questions.

Ungar could have said what you said, but didn't. Clearly some gee-whiz marketing going on.

And no, practitioners shouldn't have to ask the question "what good it is?" Every research result begs that question.

Determinism comes at a high cost? So the more processors we add, the more our computers become like us, very smart but not entirely accurate?

More like: the more processors we add, the more performance can be gained by switching to nondeterministic algorithms for problems where getting a solution that is within 99% of the perfectly correct one and arrives in 1 second is better than getting a perfect solution in 2 hours.

I like the way you've phrased this, so let me give a contrived but quasi-realistic example: I have a network of nodes which calculates and returns some floating point value. I then accumulate those in the arbitrary order in which they come back from in the network. (According to Google) 1111.0000000000001 * 2 * 1111.00000000000001 = 2468642 BUT 1111.0000000000001 * 1111.00000000000001 * .2 = 246864.2. By using floating point we've already decided it's OK to give up some precision, however by applying the commutative property of multiplication now we've also given up determinism.

As I understand it, the alternative to giving the correct answer isn't giving a wrong answer - it's giving an answer that's right to a high - but not 100% - degree of confidence.

There are plenty of domains where a very fast almost-correct answer is preferable to a perfectly correct, slower one.

Take search, for an example. First off - it's very hard to determine an exact definition of the right answer. If the provided answer was good enough, it was right.

Another domain is route planning, especially with multiple stops in arbitrary order (travelling salesman). There are plenty of applications where the 99% answer in one second is vastly preferable to the 100% answer in an hour.

An algorithm can be indeterministic and still have a determined outcome. Actually many parallel algorithms already are very indeterministic since the computations when they perform depend on the thread scheduling.

The family of problems where computing is non-deterministic and there is no single right answer is very large. Many gossiping algorithms belong to that family, as do several finance problems with an underlying stochastic model. Here's a very practical problem that I solve everyday that can benefit hugely from Ungar's model of nondeterministic concurrent computation : a bank makes n loans, with each loan having a different probability of default. The loans are all connected by a covariance matrix ie. given any two loans (p,q), we have a best guess of the chances of p defaulting if q defaults. At the end of 1 year, m of those n loans will default. But today, right now, the bank has to submit to the Fed its best guess of how much money it might lose at the end of next year! In finance, we call this "estimation of the conditional value at risk" aka cvar estimation. There is no right answer, m and n are in the millions, the covariance matrix is huge, the number of possible losses is scala.math.pow(2,m) which is essentially un-computable for even a very small m like 100 loans, so it is out of the question if m is millions of loans. Yet I perform this very calculation using a modest 100,000 simulations ( via Actors in Scala, 1 Monte Carlo simulation becomes 1 Actor iterating over a list of loans & computing their losses concurrently ). While I sort of converge to an answer, the computation is non-deterministic, there is no real right answer, and Ungar's model would be a clear win for this problem.

I think it is fair to say that this approach only works in some domains. We won't be using it where giving the 'wrong' answer upsets the people with the money. But if you listen to the channel 9 interview put up by 6ren he clearly outlines a number of scenarios that are appropriate.

<not-snarky>It isn't clear what quantum computing has to do with this discussion.</not-snarky>

Edit: Mitigate snarkiness of final comment.

"...It isn't clear what quantum computing has to do with this discussion..."

My entire point was that the way we describe future tech emphasizes the differences and exotic nature of the concepts instead of how it's all going to eventually work together anyway. Quantum computing was my second example.

Apologies if I didn't make that clear enough.

I agree with your sentiment that it isn't clear how this will fit into software development in general. Fair enough about the quantum comment. Also, my comments sounds snarkier that it was intented, so apologies for that :) (what a polite bunch we all are)

Don't think of it as a quantum CPU. Think of it as a quantum coprocessor that the CPU uses to accelerate some operations.

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