[Beowulf] Power calculations , double precision, ECC and power of APU's

Geoffrey Jacobs gdjacobs at gmail.com
Mon Mar 18 22:02:08 PDT 2013

On 03/18/2013 07:09 PM, Lux, Jim (337C) wrote:
> Another case where SP might be ok is in converting from fairly low precision gridded observations and doing some sort of model function retrieval, and perhaps regridding.
> If your measurements of ocean backscatter are on 10km grids, and the uncertainty of the measurement after model function retrieval is 0.5 m/s out of a maximum value of 50 m/sec, SP is probably more than accurate enough, since the random noise in the input data set is enormous compared to any roundoff precision things.  
> Likewise, if you're taking data that's unevenly spaced from an orbiting sensor (e.g. Seawinds on QuikScat) and regridding to a 10km mesh, I doubt that going to DP will make any difference in the interpolation.
> SAR (synthetic aperture radar) data processing is another case where SP is probably good enough. If the radar is collecting data with a 12bit ADC, it's unlikely that you'd need more than 24 bits of mantissa even after all the range processing.  SAR is a notorious bandwidth hog, as well.  1 Gbit/s kinds of rates aren't unusual.  For on board processing, often they do some form of block floating point, so the math is really all integer.
> But even in FP, the dynamic range of the signal isn't all that great.  Compare the 9.4 GHz radar cross section of an aphid head on is about 5E-6 cm^2 (from Riley, IEEE Proc, 1985), or 5E-10 m^2.  Let's consider something really big as a target, like the moon with a RCS of around 1E13 square meters (I just took the cross sectional area..the moon's round and rough, so its RCS is lower).   That's a range of 24 orders of magnitude, which would justify DP, but, on the other hand, it's unlikely we'll be looking for individual aphids on the moon.
> If you were processing all the RF signals available in the HF band you might need DP.  The instantaneous dynamic range can be around 130dB within a small subband, and that's on the order of 21-22 bits, and if you look at the entire 2-30 MHz spectrum, you probably have more range than can be accommodated in the 24 bit mantissa of a SP floating point.
> Jim Lux

I would be happy to be corrected, but isn't an SNR of 1000:1 considered
to be excellent for a spectrograph (I'm thinking of particular datasets
from Victor Blanco and CFHT). I don't believe any reduction pipeline or
analysis for such data benefits from DP.

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