chore: replace num_cpus with available_parallelism (#6709)

benches/Cargo.toml

@@ -12,7 +12,6 @@ tokio = { version = "1.5.0", path = "../tokio", features = ["full"] }
 criterion = "0.5.1"
 rand = "0.8"
 rand_chacha = "0.3"
-num_cpus = "1.16.0"
 
 [dev-dependencies]
 tokio-util = { version = "0.7.0", path = "../tokio-util", features = ["full"] }

tokio/Cargo.toml

@@ -71,10 +71,7 @@ process = [
 ]
 # Includes basic task execution capabilities
 rt = []
-rt-multi-thread = [
+rt-multi-thread = ["rt"]
-  "num_cpus",
-  "rt",
-]
 signal = [
   "libc",
   "mio/os-poll",
@@ -96,7 +93,6 @@ pin-project-lite = "0.2.11"
 # Everything else is optional...
 bytes = { version = "1.0.0", optional = true }
 mio = { version = "0.8.9", optional = true, default-features = false }
-num_cpus = { version = "1.8.0", optional = true }
 parking_lot = { version = "0.12.0", optional = true }
 
 [target.'cfg(not(target_family = "wasm"))'.dependencies]

tokio/src/loom/std/mod.rs

@@ -84,6 +84,8 @@ pub(crate) mod sync {
 pub(crate) mod sys {
     #[cfg(feature = "rt-multi-thread")]
     pub(crate) fn num_cpus() -> usize {
+        use std::num::NonZeroUsize;
+
         const ENV_WORKER_THREADS: &str = "TOKIO_WORKER_THREADS";
 
         match std::env::var(ENV_WORKER_THREADS) {
@@ -97,7 +99,9 @@ pub(crate) mod sys {
                 assert!(n > 0, "\"{}\" cannot be set to 0", ENV_WORKER_THREADS);
                 n
             }
-            Err(std::env::VarError::NotPresent) => usize::max(1, num_cpus::get()),
+            Err(std::env::VarError::NotPresent) => {
+                std::thread::available_parallelism().map_or(1, NonZeroUsize::get)
+            }
             Err(std::env::VarError::NotUnicode(e)) => {
                 panic!(
                     "\"{}\" must be valid unicode, error: {:?}",