moderated Re: Call for information about PC systems being used for WSJT-X #general
On 09/06/2021 00:37, Bill Somerville wrote:
Hi all WSJT-X users,Hi all,
thanks for all those who took the time to check their systems and report those without AVX support. That has helped to get a broad picture of the numbers with older CPUs that pre-date this feature. Not unsurprisingly a significant proportion lack AVX, this is almost certainly due to systems being acquired second-hand, repurposed from other uses, or kept for extended periods as they are more than adequate for the average shack PC. Some have suggested that we should not abandon owners of these older PCs, don't worry as that has never been the intention of this exercise, here is some background that should help to clarify what may happen.
The MAP65 application, as of WSJT-X v2.5.0 RC1 has been updated to decode Q65 signals, this is because we feel certain that Q65 is superior for EME use on all bands and the prior JT65 decoding ability will be superseded by Q65. The MAP65 decoder is able to decode many signals across a wide pass-band, and also implements polarization diversity with suitably equipped stations. Automatic linear drift compensation has also been added to compensate for less well specified stations. This all requires a lot of signal processing effort, but users expect signals to be decoded in the short interval between the end of transmission and the start of the next period (note with EME the path delay means that up to 2 1/2 seconds of that interval is lost compared with terrestrial paths). The first use of hand coded micro-optimizations using AVX instructions on suitable CPUs will be aimed at getting Q65 decodes done faster in MAP65. Because the Q65 decoder is shared by WSJT-X and MAP65, the same optimizations will be there for WSJT-X Q65 users. None of this is particularly relevant to the survey of CPUs done here as I am sure that PC costs are such a small part of the typical EME station investment that users will find a way to upgrade their PCs if necessary.
So why did I ask the question about AVX? Once we start using AVX for some parts of WSJT-X it makes sense to find other opportunities for similar hand coded micro-optimizations elsewhere in out code base, not only that but once implemented we may well choose to increase the decoding depth of other decoders by taking advantage of such performance gains. The net effect would be that those with AVX equipped PCs will see faster and deeper decoding, those with older PCs will see the same extra depth but overall decoding will take longer than before. My aim was to judge what proportion of users might suffer this speed degradation versus those that will see both faster and deeper decoding.
To reassure those that may have misunderstood, there is no intention to exclude users from the latest WSJT-X enhancements just because they have older CPUs. We would implement AVX implementations of critical algorithms alongside their current linear implementations and the choice of which to use would be made at runtime according to the available CPU features. Note exactly this already happens in the FFT library we use called FFTW3, so WSJT-X and MAP65 users have always had AVX specific algorithm implementations for FFT calculations if the CPU they run on supports them. We are investigating coding other critical algorithms in a similar fashion. Notwithstanding that, we also have no intention of dropping support for ARM CPU architectures like the Raspberry Pi, yet we have no intention of similar hand coded micro-optimizations for that platform since the required tools do not exist, so for that platform our linear implementations would still be used, just like on non-AVX Intel or AMD CPUs.