Ask Slashdot: How Reproducible Is Arithmetic In the Cloud?

goodminton writes "I'm research the long-term consistency and reproducibility of math results in the cloud and have questions about floating point calculations. For example, say I create a virtual OS instance on a cloud provider (doesn't matter which one) and install Mathematica to run a precise calculation. Mathematica generates the result based on the combination of software version, operating system, hypervisor, firmware and hardware that are running at that time. In the cloud, hardware, firmware and hypervisors are invisible to the users but could still impact the implementation/operation of floating point math. Say I archive the virutal instance and in 5 or 10 years I fire it up on another cloud provider and run the same calculation. What's the likelihood that the results would be the same? What can be done to adjust for this? Currently, I know people who 'archive' hardware just for the purpose of ensuring reproducibility and I'm wondering how this tranlates to the world of cloud and virtualization across multiple hardware types."

Comments

Numerical instability

[Anonymous Coward]

If the value your computing is so dependent of the details of float point implementation that you'er worried about it, you probably have an issue of numerical stability and the results you are computing are likely useless, so this is really a mute point.

Re:Fixed-point arithmetic

[Anonymous Coward]

Submitter is entirely ignorant of floating point issues in general. Other than the buzzword "cloud" this is no different from any other clueless question about numerical issues in computing. "Help me, I don't know anything about the problem, but I just realized it exists!"

You need to know some numerical analysis

[daniel_mcl]

If your calculations are processor-dependent, that's a bad sign for your code. If your results really depend on things that can be altered by the specific floating-point implementation, you need to write code that's robust to changes in the way floating-point arithmetic is done, generally by tracking the uncertainty associated with each number in your calculation. (Obviously you don't need real-time performance since you're using cloud computing in the first place.) I'm not an expert on Mathematica, but it probably has such things built in if you go through the documentation, since Mathematica notebooks are supposed to exhibit reproduceable behavior on different machines. (Which is not to say that no matter what you write it's automatically going to be reproduceable.

Archiving hardware to get consistent results is mainly used when there are legal issues and some lawyer can jump in and say, "A-ha! This bit here is different, and therefore there's some kind of fraud going on!"

Re:You need to know some numerical analysis

[rockmuelle]

This.

Reproducibility (what we strive for in science) is not the same as repeatability (what the poster is actually trying to achieve). Results that are not robust on different platforms aren't really scientific results.

I wish more scientists understood this.

-Chris

Re:Fixed-point arithmetic

[Giant Electronic Bra]

I think the problem is that people PERCEIVE it to be a problem. Nothing is any more problematic than it was before, good numerical simulations will be stable over some range of inputs. It shouldn't MATTER if you get slightly different results for one given input. If that's all you tested, well, you did it wrong indeed. Mathematica is fine, people need to A) understand scientific computing and B) understand how to run and interpret models. I think most scientists that are doing a lot of modelling these days DO know these things. Its the occasional users that get it wrong I suspect.

Re:Fixed-point arithmetic

[immaterial]

For a guy who started off a reply with an emphatic "Wrong" you sure do seem to agree with the guy you quoted.