On 11/17/22 8:53 PM, Charles R Harris wrote:
On Thu, Nov 17, 2022 at 6:30 PM Scott Ransom <sransom@nrao.edu <mailto:sransom@nrao.edu>> wrote:
On 11/17/22 7:13 PM, Charles R Harris wrote: > > > On Thu, Nov 17, 2022 at 3:15 PM Ralf Gommers <ralf.gommers@gmail.com <mailto:ralf.gommers@gmail.com> > <mailto:ralf.gommers@gmail.com <mailto:ralf.gommers@gmail.com>>> wrote: > > Hi all, > > We have to do something about long double support. This is something I wanted to propose a long > time ago already, and moving build systems has resurfaced the pain yet again. > > This is not a full proposal yet, but the start of a discussion and gradual plan of attack. <snip> > I would agree that extended precision is pretty useless, IIRC, it was mostly intended as an accurate > way to produce double precision results. That idea was eventually dropped as not very useful. I'd > happily do away with subnormal doubles as well, they were another not very useful idea. And strictly > speaking, we should not support IBM double-double either, it is not in the IEEE standard. > > That said, I would like to have a quad precision type. That precision is useful for some things, and > I have a dream that someday it can be used for a time type. Unfortunately, last time I looked > around, none of the available implementations had a NumPy compatible license. > > The tricky thing here is to not break downstream projects, but that may be unavoidable. I suspect > the fallout will not be that bad. > > Chuck
A quick response from one of the leaders of a team that requires 80bit extended precision for astronomical work...
"extended precision is pretty useless" unless you need it. And the high-precision pulsar timing community needs it. Standard double precision (64-bit) values do not contain enough precision for us to pass relative astronomical times via a single float without extended precision (the precision ends up being at the ~1 microsec level over decades of time differences, and we need it at the ~1-10ns level) nor can we store the measured spin frequencies (or do calculations on them) of our millisecond pulsars with enough precision. Those spin frequencies can have 16-17 digits of base-10 precision (i.e. we measure them to that precision). This is why we use 80-bit floats (usually via Linux, but also on non X1 Mac hardware if you use the correct compilers) extensively.
Numpy is a key component of the PINT software to do high-precision pulsar timing, and we use it partly *because* it has long double support (with 80-bit extended precision): https://github.com/nanograv/PINT <https://github.com/nanograv/PINT> And see the published paper here, particularly Sec 3.3.1 and footnote #42: https://ui.adsabs.harvard.edu/abs/2021ApJ...911...45L/abstract <https://ui.adsabs.harvard.edu/abs/2021ApJ...911...45L/abstract>
Going to software quad precision would certainly work, but it would definitely make things much slower for our matrix and vector math.
We would definitely love to see a solution for this that allows us to get the extra precision we need on other platforms besides Intel/AMD64+Linux (primarily), but giving up extended precision on those platforms would *definitely* hurt. I can tell you that the pulsar community would definitely be against option "B". And I suspect that there are other users out there as well.
Scott NANOGrav Chair www.nanograv.org <http://www.nanograv.org>
Pulsar timing is one reason I wanted a quad precision time type. I thought Astropy was using a self implemented double-double type to work around that?
That is correct. For non-compute-intensive time calculations, Astropy as a Time object that internally uses two 64-bit floats. We use it, and it works great for high precision timekeeping over astronomical times. *However*, it ain't fast. So you can't do fast matrix/vector math on time differences where your precision exceeds a single 64-bit float. That's exactly where we are with extended precision for our pulsar timing work. Scott -- Scott M. Ransom Address: NRAO Phone: (434) 296-0320 520 Edgemont Rd. email: sransom@nrao.edu Charlottesville, VA 22903 USA GPG Fingerprint: A40A 94F2 3F48 4136 3AC4 9598 92D5 25CB 22A6 7B65