reduce regression parallelism

xtask/src/main.rs

@@ -261,7 +261,7 @@ fn try_regression() -> Result<(usize, usize, usize), DynError> {
     const GAMES: usize = 100;
     // limit the parallelism
     rayon::ThreadPoolBuilder::new()
-        .num_threads(std::thread::available_parallelism()?.get() / 3)
+        .num_threads(std::thread::available_parallelism()?.get() / 4)
         .build_global()
         .unwrap();