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Why physicists still use Fortran (2015)

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Re: Why physicists still use Fortran (2015)

#81

Post by pjmlp »

nejenendn wrote: post #25
dahart wrote: post #5

> Interestingly, C/C++ beats Fortran on all but two of the benchmarks, although they are fairly close on most. I think this is fairly recent that C/C++ wins. I don’t know how recent exactly, but I remember a colloquium not too long ago by a compiler researcher who said that cross-compiling to Fortran and then optimizing almost always produced faster code than the C/C++ compiler could. Fortran is apparently easier to…

Fortran is easier to optimize compared to c/++ if you don’t use restrict for the c end. If you do use restrict (iirc) the compilers are competitive.

Kind of, compilers don't enforce correct use of restrict which opens the doors to strange errors, as it is UB if they actually do overlap.

Which is the main reason why ANSI C++ members are not keen in having restrict in C++.

Re: Why physicists still use Fortran (2015)

#83

Post by RhysU »

> Even if old code is hard to read, poorly documented, and not the most efficient, it is often faster to use old validated code than to write new code.

Amen. A one-character mistake might take a week to find as it exhibits only subtly wrong behavior (e.g. wrong grid convergence rate, overly noisy boundary condition, odd symmetry breaking beyond IEEE floating point). During that week no science happens.

Re: Why physicists still use Fortran (2015)

#84

Post by rdiddly »

The paragraph on "legacy code" is a bit weak and half-hearted because it underemphasizes one of the most important arguments for using old code: it's been thoroughly debugged already. The most the author can summon on the topic is the fact that legacy code "takes uncertainty out of the debugging process." What? There is no debugging process, because that code has been debugged for 40 years and is damn near bulletproof at this point!

Everybody is used to cringing when they hear "legacy code," and that's justifiable for several good reasons. Note that "not wanting to learn an unfamiliar language" isn't one of them. And "not having, or not being willing to use/cultivate, the skill set of reading someone else's code" isn't one of them either.

But there is obviously a lot of bad code out there. And that's the thing, there are only two kinds of code: good code and bad code. And by extension there is bad legacy code and there is good legacy code. Don't assume legacy code is always bad code. If something has been used successfully for 40 years, do yourself a favor and try to have the humility to assume people implemented it well, found all the bugs, know what they're doing, and/or generally are rational-thinking adults who make good choices... instead of the usual naïve assumption that everybody's an idiot but I'm going to change all that! No, you're going to duplicate a lot of effort, and possibly (depending on the faithfulness of your reading of the code) reintroduce some of the same bugs that were dealt with years ago.

Re: Why physicists still use Fortran (2015)

#85

Post by jlg23 »

rxhernandez wrote: post #68
thicknavyrain wrote: post #47

Earlier quoted context omitted.

Pretty much just size! There was about 20,000 lines of it, it's legacy code that has been gradually added to since the 1980s, so it would have taken a while to rewrite all the parts to work with each other. Perhaps some day though.

20,000 lines of code really doesn't seem like that much. I probably output that much in about 2-3 months of biomedical research so there has to be more to it than that.

>> legacy code that has been gradually added to since the 1980s

> 20,000 lines of code really doesn't seem like that much.

It's not "20k lines of code" but "n lines of code that grew organically over decades to 20k lines" - with the help of probably way more than 100 people who all are not trained as developers. I think it's a safe bet to say the current state only has a faint memory of being a consistent code base.

Re: Why physicists still use Fortran (2015)

#86

Post by roel_v »

dagw wrote: post #80
PascLeRasc wrote: post #36

Earlier quoted context omitted.

That was without a doubt pretty bad for those grad students' careers. That kind of work shouldn't have been done by anyone without tenure.

I don't know. Having "rewrote X kloc of scientific Fortran to modern idiomatic C++" on your CV should get you to the head of the line in many places when looking for a job.

Only if you go after run of the mill coding jobs after your phd, in which case why bother at all. When applying for a postdoc, you'll very much want to bury that part of your work.

Re: Why physicists still use Fortran (2015)

#87

Post by adwn »

drngdds wrote: post #29

Seeing the author talk about "C/C++" like it's one language (and basically just C) is frustrating.

That was my initial reaction as well. However, I would guess that C++ is mostly used as a "C with classes" in the domain the author is talking about, so "C/C++" wouldn't be so incorrect.

Re: Why physicists still use Fortran (2015)

#88

Post by sampo »

ChuckMcM wrote: post #62
thicknavyrain wrote: post #24

"Professors usually have this legacy code on hand (often code they wrote themselves decades ago) and pass this code on to their students. This saves their students time, and also takes uncertainty out of the debugging process." This is so true. I'm a PhD student in physics using Fortran for pretty much that reason. At the start of my PhD, in response to my supervisor telling me I should learn Fortran to modify our cu…

Now you need to do a follow on study to see how much science the 'rival' group does with a more modern codebase than your 'legacy' group does. Would you know if there are enough examples of two groups who have diverged like this to get meaningful (as in statistically significant) results on the cost benefit of porting / not porting?

> Now you need to do a follow on study to see how much science the 'rival' group does with a more modern codebase than your 'legacy' group does.

I would guess that a C++ codebase written by PhD students, not by seasoned C++ experts, is more complicated and much slower to debug, than a corresponding Fortran codebase.

Re: Why physicists still use Fortran (2015)

#89

Post by jxcole »

So the article basically says:

1) Some stuff is already written in Fortran so they don't want to rewrite that. I dig it.

2) It's fast (except C sometimes) but easier to write than c. Like 100x faster than python.

I'm not sure about number two. With the gpu processing revolution wouldn't a python/TensorFlow stack be faster than Fortran? Am I missing something?

I remember talking to someone who had worked heavily on atmospheric weather predictors recently and her description of the program was: we divide the space into tiny little cubes and then run some differential physics equations to predict what will happen next. My basic questions to her:

1) From a computational perspective this seems very GPU friendly.

2) Why not use a convolutional neural network? If you use the same data for training you will probably wind up with a more accurate prediction than a theoretically based physics system.

Her reaction was basically that she hadn't heard of these things before so my impression is in fact that the physics community is just behind and they will catch up when they are ready.

Hope I'm not missing something here.

Re: Why physicists still use Fortran (2015)

#90

Post by openasocket »

jxcole wrote: post #89

So the article basically says: 1) Some stuff is already written in Fortran so they don't want to rewrite that. I dig it. 2) It's fast (except C sometimes) but easier to write than c. Like 100x faster than python. I'm not sure about number two. With the gpu processing revolution wouldn't a python/TensorFlow stack be faster than Fortran? Am I missing something? I remember talking to someone who had worked heavily on at…

TensorFlow is for machine learning, not general purpose computations. And no, you will not get better results from a neural network than state of the art computational fluid dynamics.

As for general purpose GPU programming, some parts physics are GPU friendly put not all of them.

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