Fix NNCache sizing with playout/visits limits.

* Limit the NNcache size when visits are limited.
* Fixed handling of -p 0 and -v 0 which mean "no limit".

Also avoid potential overflow in the size calculation.

Pull request leela-zero#1281.

src/Leela.cpp

@@ -210,10 +210,22 @@ static void parse_commandline(int argc, char *argv[]) {
                    "Add --noponder if you want a weakened engine.\n");
             exit(EXIT_FAILURE);
         }
+
+        // 0 may be specified to mean "no limit"
+        if (cfg_max_playouts == 0) {
+            cfg_max_playouts =
+                std::numeric_limits<decltype(cfg_max_playouts)>::max();
+        }
     }
 
     if (vm.count("visits")) {
         cfg_max_visits = vm["visits"].as<int>();
+
+        // 0 may be specified to mean "no limit"
+        if (cfg_max_visits == 0) {
+            cfg_max_visits =
+                std::numeric_limits<decltype(cfg_max_visits)>::max();
+        }
     }
 
     if (vm.count("resignpct")) {
@@ -305,7 +317,9 @@ void init_global_objects() {
     // improves reproducibility across platforms.
     Random::get_Rng().seedrandom(cfg_rng_seed);
 
-    NNCache::get_NNCache().set_size_from_playouts(cfg_max_playouts);
+    // When visits are limited ensure cache size is still limited.
+    auto playouts = std::min(cfg_max_playouts, cfg_max_visits);
+    NNCache::get_NNCache().set_size_from_playouts(playouts);
 
     // Initialize network
     Network::initialize();

src/NNCache.cpp

@@ -77,7 +77,13 @@ void NNCache::set_size_from_playouts(int max_playouts) {
     // cache hits are generally from last several moves so setting cache
     // size based on playouts increases the hit rate while balancing memory
     // usage for low playout instances. 150'000 cache entries is ~225 MB
-    auto max_size = std::min(150'000, std::max(6'000, 3 * max_playouts));
+    constexpr auto num_cache_moves = 3;
+    auto max_playouts_per_move =
+        std::min(max_playouts,
+            std::numeric_limits<decltype(max_playouts)>::max() /
+            num_cache_moves);
+    auto max_size = num_cache_moves * max_playouts_per_move;
+    max_size = std::min(150'000, std::max(6'000, max_size));
     NNCache::get_NNCache().resize(max_size);
 }
 

src/UCTSearch.cpp

@@ -696,24 +696,18 @@ void UCTSearch::set_playout_limit(int playouts) {
     static_assert(std::is_convertible<decltype(playouts),
                                       decltype(m_maxplayouts)>::value,
                   "Inconsistent types for playout amount.");
-    if (playouts == 0) {
-        // Divide max by 2 to prevent overflow when multithreading.
-        m_maxplayouts = std::numeric_limits<decltype(m_maxplayouts)>::max()
-                        / 2;
-    } else {
-        m_maxplayouts = playouts;
-    }
+    // Limit to type max / 2 to prevent overflow when multithreading.
+    m_maxplayouts =
+        std::min(playouts,
+            std::numeric_limits<decltype(m_maxplayouts)>::max() / 2);
 }
 
 void UCTSearch::set_visit_limit(int visits) {
     static_assert(std::is_convertible<decltype(visits),
                                       decltype(m_maxvisits)>::value,
                   "Inconsistent types for visits amount.");
-    if (visits == 0) {
-        // Divide max by 2 to prevent overflow when multithreading.
-        m_maxvisits = std::numeric_limits<decltype(m_maxvisits)>::max()
-                      / 2;
-    } else {
-        m_maxvisits = visits;
-    }
+    // Limit to type max / 2 to prevent overflow when multithreading.
+    m_maxvisits =
+        std::min(visits,
+            std::numeric_limits<decltype(m_maxvisits)>::max() / 2);
 }