ENH: Optimized radius calculation.

[yipihey]

PR Summary

Changes one of the routines that calculated spherical radius.
get_radius in fields/field_functions.py
It runs approximately twice as fast as the original routine it replaces.
Performance benefits come from minimizing allocations and triggering unit conversions.

The script I used for testing is here:

import yt

for i in range(10):
    ds = yt.frontends.enzo.EnzoDataset(fn)
    ds._periodicity = (False, False, False) # Checking periodicity is slow in yt
    readit = ds.all_data()  # already defines xyz

    r = readit[("index", "radius")]

yt.SlicePlot(ds, "z", ("index", "radius")).save()

where fn can be obtained in a separate script as

import yt
fn = yt.load_sample("HiresIsolatedGalaxy").filename

[neutrinoceros]

pre-commit.ci autofix

[neutrinoceros]

I made minor adjustments to the original post (showed what fn is supposed to be, added import yt, made the loop run 10 times instead of 1).
I find that this reduces the runtime for the example script by about 10%
I'm impressed how much you're able to gain with such small edits, this is great !

[neutrinoceros]

I'd like at least one other maintainer to sign off given how sensitive this kind of computation is, but this looks sensible to me !

[matthewturk]

Thank you, @yipihey -- I really like where this is going. If we could get @jzuhone to take a quick look I'm happy with it going in.

[matthewturk]

I like this.

[brittonsmith]

The removal of the unit conversion (i.e., in_base(unit_system.name)) has changed radius values for frontends where the default unit is not "code_length". I'm not sure how much performance is lost by adding it back, but I think it is necessary unless we mandate position units always come back in "code_length", which I don't think we do.

[yipihey]

Yes perhaps we are mixing two concepts.
One is "code_units" and one is the "original_units", i.e. which units are used in the file we are using. I think it is advantageous to require frontends to not modify the numbers it reads from disk.
I think of two different use cases of yt.

1. Debugging a code that generates the data we are reading in. It is desirable that yt attempts to not modify the data it reads so we can have some certainty we are debugging the code that generates the data.
2. If I use yt to convert from one data format to another. E.g. I read all particle positions in enzo and then store them as a simpler hdf5 file where the positions are a simple array. In this case I want to make sure that I'm just changing the shape of the data but do not change any of the numbers.

The positive side effect that it also avoid unnecessary computation which saves a little time.

Is there already the concept of "original_units" ? I.e. give us a simple way to check in what units the data was provided?