Add an env var to configure the runtime threadpool

Author: jbr

src/rt/mod.rs

@@ -12,7 +12,11 @@ pub struct Runtime {}
 /// The global runtime.
 pub static RUNTIME: Lazy<Runtime> = Lazy::new(|| {
     // Create an executor thread pool.
-    let num_threads = num_cpus::get().max(1);
+    let num_threads = std::env::var("ASYNC_STD_RT_THREADS")
+        .map(|env| env.parse().expect("ASYNC_STD_RT_THREADS must be a number"))
+        .unwrap_or_else(|_| num_cpus::get())
+        .max(1);
+
     for _ in 0..num_threads {
         thread::Builder::new()
             .name("async-std/runtime".to_string())