Why I think that gxt-1080-sc is a real GPU miner
Gxt-1080-sc miner is back and dominates the queue again. Yet this time the increase in queue length and mining complexity is not that dramatic, at least for now.
In this post @alphabeta noticed that nonce values look different in gxt-1080-sc pow2 records. Probably this miner starts searching from zero or some small nonce value instead of using a value based on current time like it is done in the official implementation. AFAIU, nothing is wrong about this, but it hints that gxt-1080-sc may be using a completely different miner implementation.
My speculation is that some time ago the person behind supercomputing and gx-1080-sc accounts started to work on a GPU miner which required reimplementing the POW algorithm from scratch and led him to discovery of the pre-HF13 short cut. After the short cut was fixed he completed the GPU miner implementation and now started to use it.
Btw, the CPU miner also has some good potential for performance improvements (no exploits or short cuts, just removing unnecessary overhead, I'll write about it later).
Update: I missed the comment from @arhag on the @alphabeta's post:
I've explained the likely reason for that in the #mining channel on steemit.chat.
From what I've seen, my only conclusion is that he is most likely not using an exploit, but rather using his own more efficient custom implementation of the miner. If we take his account name at face value, it likely means that he has built a GPU miner.
It seems that a GPU miner for CUDA has been in the works from @djm34 for a while now, though he has not publicly released updates lately: https://steemit.com/mining/@djm34/djm34-sneak-peek
If this is true, then we can expect this user to continue to dominate the mining queue. I would expect smaller miners to start to drop out if this continues, which would only increase this person's market share. What are the implications if we end up with basically one single user doing most to all of the mining?
Then that single miner will have the same profit and influence as a top 19 witness.