Re: [Bug-gnubg] Simple multi-threading...

[Øystein Johansen]

Jonathan Kinsey wrote:
> Can someone explain how the NN works in terms of the NNEVAL_SAVE and
> NNEVAL_FROMBASE operations?

It's a smart trick invented by Joseph. Notice that there in the original 
implementation (before the sse vectorization) was a "if (input_node)" 
and if not it just moved a pointer. This condition test saved a lot of 
time when the input node was 0 (and it often is). An evaluation with 
lots of zeros in the input uses considerably shorter processing time 
than a input with non-zeros.

So Joseph, ingenious as he is, invented the trick of storing the input 
array and the resulting hidden layer array, when the first candidate 
move is considered in a movelist. When evaluating the next move 
candidate, we simply subtract the new input array from the stored input 
array, and in that way we're hoping to "generate" a lot of zeros to the 
evaluator. (As described above, zeros are faster!) Now, to compensate at 
the hidden layer, the result from the modified input array evaluation is 
subtracted from the stored hidden layer array, to get the right result. 
The evaluation step from the hidden layer to the output layer is unchanged.

When I first saw and understood this clever trick, I was mighty 
impressed. The speedup is not superb, but significant. About 10% I 
guess. Joseph may have more accurate results. It's not as big speed 
improvement as the pruning nets, but it is (or was) worth having.

The SSE vectorization doesn't do this trick. Of course it then has to 
check if all four inputs are zeros to just move the result pointer to 
the next position, without doing any arithmetics. Maybe it's worth doing 
 something similar for the vectorized code? If not, we may remove the 
cleaver trick from the SSE code, so it's not wasting cycles subtracting 
the input array for no use.

NNEVAL_SAVE is then a flag to the evaluator to do a normal evaluation 
and save the input array and the hidden layer array.

NNEVAL_FROMBASE is using a flag to evaluate from a saved set of inputs 
and saved set of hidden layer values.

-Øystein