2K (2048) seems to be the recommended difficulty for a 2TH/s miner.
Just to demystify "diff" for those that may not know: you can think of it as simply how often the hardware goes back to the pool asking for work. For a beast like the S4, going back say 1000 times a second will mean its starved for work (a low diff) and why it doesn't report doing much processing. Apparently 2048 (and diff is always a power of 2) suffices to allow the S4 to actually work most of the time. A higher diff, like 16284 (just 3 doublings from 2048) simply mean the pool is handing work over 1/8th as often (and 8 times as much work) as it did at 2048.
Most dynamic diff pools play with the assigned worker diff so that it "checks in" about once every 15 seconds.
The only downside to a high diff is the potential for wasted work. A GOOD pool will notify all workers when a block is found and immediately assign new work, a bad pool won't. Even with the best of intentions, your hardware may submit shares after the pool knows a block was found (say 0.1 second after that happened), in which case you get the "stale shares" message.
Cassey
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