Use multiple threads for Relm async tasks depending on system

src/main.rs

@@ -4,6 +4,9 @@ mod persist;
 mod ui;
 mod views;
 
+use std::cmp::max;
+use std::thread;
+
 use gtk4::gdk::Display;
 use gtk4::gio::File;
 use gtk4::{CssProvider, Settings, gio};
@@ -18,8 +21,13 @@ fn main() {
 
   include_app_css();
 
-  // TODO: Set this to nr of cores if using Relm commands heavily.
+  // Set number of threads used by Relm for async tasks to the number of available
-  // RELM_THREADS.set(4).unwrap();
+  // CPU cores minus two, thereby reserving one for the main thread and one for
+  // the database connection thread spawned by async_sqlite.
+  let cores = thread::available_parallelism()
+    .map(|cores| cores.get() - 2)
+    .unwrap_or(1);
+  relm4::RELM_THREADS.set(max(cores, 1)).unwrap();
 
   app.run::<App>(());
 }