Introducing NNUE Evaluation

As of August 6, the efficiently updatable neural network (NNUE) evaluation has landed in the Stockfish repo!

What is NNUE?

Both the NNUE and the classical evaluations are available, and can be used to assign a value to a position that is later used in alpha-beta (PVS) search to find the best move. The classical evaluation computes this value as a function of various chess concepts, handcrafted by experts, tested and tuned using fishtest. The NNUE evaluation computes this value with a neural network based on basic inputs. The network is optimized and trained on the evaluations of millions of positions at moderate search depth.

The NNUE evaluation was first introduced in shogi, and ported to Stockfish afterward. It can be evaluated efficiently on CPUs, and exploits the fact that only parts of the neural network need to be updated after a typical chess move. The nodchip repository provides additional tools to train and develop the NNUE networks.

Results

The performance of the NNUE evaluation relative to the classical evaluation depends somewhat on the hardware, and is expected to improve quickly, but is currently on > 80 Elo on fishtest:

60000 @ 10+0.1 th 1
https://tests.stockfishchess.org/tests/view/5f28fe6ea5abc164f05e4c4c
ELO: 92.77 +-2.1 (95%) LOS: 100.0%
Total: 60000 W: 24193 L: 8543 D: 27264
Ptnml(0-2): 609, 3850, 9708, 10948, 4885

40000 @ 20+0.2 th 8
https://tests.stockfishchess.org/tests/view/5f290229a5abc164f05e4c58
ELO: 89.47 +-2.0 (95%) LOS: 100.0%
Total: 40000 W: 12756 L: 2677 D: 24567
Ptnml(0-2): 74, 1583, 8550, 7776, 2017

Trying it out

Stockfish 12 is not expected to be released imminently–we want some time to let this major change bake for a bit. But you might still want to try out NNUE! Three simple steps:

Download the latest development build of Stockfish (or build yourself, from GitHub)
Download the default net. Currently that would be nn-9931db908a9b.nnue but you can check which is the default one as indicated by the EvalFile UCI option. Place the .nnue file in the same directory as the engine binary. (Tip: you could also run the make net command from the src directory to automatically download the default net)
By default NNUE is off, but it can be enabled with the Use NNUE (true/false) UCI option, provided the EvalFile option points to the network file (if you put it in the same directory as the engine it should work, but in some cases you may need to specify the full path).

Acknowledgements

This patch is the result of contributions of various authors, from various communities, including: nodchip, ynasu87, yaneurao (initial port and NNUE authors), domschl, FireFather, rqs, xXH4CKST3RXx, tttak, zz4032, joergoster, mstembera, nguyenpham, erbsenzaehler, dorzechowski, and vondele.

This new evaluation needed various changes to fishtest and the corresponding infrastructure, for which tomtor, ppigazzini, noobpwnftw, daylen, and vondele are gratefully acknowledged.

The first networks have been provided by gekkehenker and sergiovieri, with the latter net being the current default.

For Developers

Guidelines for testing new nets can be found here.

Integration has been discussed in various Github issues:

The pull requests:

WIP (not merged) https://github.com/official-stockfish/Stockfish/pull/2825
Actual PR (merged) https://github.com/official-stockfish/Stockfish/pull/2912

This will be an exciting time for computer chess, looking forward to seeing the evolution of this approach.