Increase reductions with thread count

Respin of multi-thread idea that was simplified away recently: basically doing
more reductions with thread count since Lazy SMP naturally widens search. With
drawish book this idea got simplified away but with less drawish book it again
gains elo, maybe trying to reinstall other ideas that were simplified away
previously can be beneficial.

passed STC
LLR: 2.96 (-2.94,2.94) <-0.50,2.50>
Total: 39736 W: 10205 L: 9986 D: 19545
Ptnml(0-2): 45, 4254, 11064, 4447, 58
https://tests.stockfishchess.org/tests/view/615750702d02f48db3961b00

passed LTC
LLR: 2.97 (-2.94,2.94) <0.50,3.50>
Total: 60352 W: 15530 L: 15218 D: 29604
Ptnml(0-2): 24, 5900, 18016, 6212, 24
https://tests.stockfishchess.org/tests/view/6157d8935488e26ea5eace7f

closes https://github.com/official-stockfish/Stockfish/pull/3724

Bench 5714575

diff --git a/src/search.cpp b/src/search.cpp
index 48694cb2dedebcebaec8dd59491ade87c41d2daf..3eb8e9e1ee867e0595b6b2a247fe13db633575c5 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -173,7 +173,7 @@ namespace {
 void Search::init() {
 
   for (int i = 1; i < MAX_MOVES; ++i)
 void Search::init() {
 
   for (int i = 1; i < MAX_MOVES; ++i)

-      Reductions[i] = int(21.9 * std::log(i));
+      Reductions[i] = int((21.9 + std::log(Threads.size()) / 2) * std::log(i));

 }
 
 
 }