diff --git a/Cargo.lock b/Cargo.lock
index c52c3b20e1..14c9e776db 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1475,6 +1475,7 @@ dependencies = [
  "home",
  "lazy_static",
  "libc",
+ "num_cpus",
  "opener",
  "openssl",
  "pgp",
diff --git a/Cargo.toml b/Cargo.toml
index d1c0cae22e..86b11f531e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -31,6 +31,7 @@ git-testament = "0.1.4"
 home = "0.5"
 lazy_static = "1"
 libc = "0.2"
+num_cpus = "1.13"
 opener = "0.4.0"
 # Used by `curl` or `reqwest` backend although it isn't imported
 # by our rustup.
diff --git a/README.md b/README.md
index c4864e462b..92ae3dbebb 100644
--- a/README.md
+++ b/README.md
@@ -697,7 +697,7 @@ currently can define only `default_toolchain`.
   Sets the root URL for downloading self-updates.
 
 - `RUSTUP_IO_THREADS` *unstable* (defaults to reported cpu count). Sets the
-  number of threads to perform close IO in. Set to `disabled` to force
+  number of threads to perform close IO in. Set to `1` to force
   single-threaded IO for troubleshooting, or an arbitrary number to
   override automatic detection.
 
diff --git a/src/diskio/mod.rs b/src/diskio/mod.rs
index a655479397..51932d1660 100644
--- a/src/diskio/mod.rs
+++ b/src/diskio/mod.rs
@@ -59,6 +59,7 @@ use std::io::{self, Write};
 use std::path::{Path, PathBuf};
 use std::time::{Duration, Instant};
 
+use crate::errors::{Result, ResultExt};
 use crate::process;
 use crate::utils::notifications::Notification;
 
@@ -194,20 +195,16 @@ pub fn create_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
 /// Get the executor for disk IO.
 pub fn get_executor<'a>(
     notify_handler: Option<&'a dyn Fn(Notification<'_>)>,
-) -> Box<dyn Executor + 'a> {
+) -> Result<Box<dyn Executor + 'a>> {
     // If this gets lots of use, consider exposing via the config file.
-    if let Ok(thread_str) = process().var("RUSTUP_IO_THREADS") {
-        if thread_str == "disabled" {
-            Box::new(immediate::ImmediateUnpacker::new())
-        } else if let Ok(thread_count) = thread_str.parse::<usize>() {
-            Box::new(threaded::Threaded::new_with_threads(
-                notify_handler,
-                thread_count,
-            ))
-        } else {
-            Box::new(threaded::Threaded::new(notify_handler))
-        }
-    } else {
-        Box::new(threaded::Threaded::new(notify_handler))
-    }
+    let thread_count = match process().var("RUSTUP_IO_THREADS") {
+        Err(_) => num_cpus::get(),
+        Ok(n) => n
+            .parse::<usize>()
+            .chain_err(|| "invalid value in RUSTUP_IO_THREADS. Must be a natural number")?,
+    };
+    Ok(match thread_count {
+        0 | 1 => Box::new(immediate::ImmediateUnpacker::new()),
+        n => Box::new(threaded::Threaded::new(notify_handler, n)),
+    })
 }
diff --git a/src/diskio/threaded.rs b/src/diskio/threaded.rs
index b71b2ee78b..07a01d03f4 100644
--- a/src/diskio/threaded.rs
+++ b/src/diskio/threaded.rs
@@ -34,29 +34,7 @@ pub struct Threaded<'a> {
 }
 
 impl<'a> Threaded<'a> {
-    pub fn new(notify_handler: Option<&'a dyn Fn(Notification<'_>)>) -> Self {
-        // Defaults to hardware thread count threads; this is suitable for
-        // our needs as IO bound operations tend to show up as write latencies
-        // rather than close latencies, so we don't need to look at
-        // more threads to get more IO dispatched at this stage in the process.
-        let pool = threadpool::Builder::new()
-            .thread_name("CloseHandle".into())
-            .thread_stack_size(1_048_576)
-            .build();
-        let (tx, rx) = channel();
-        Self {
-            n_files: Arc::new(AtomicUsize::new(0)),
-            pool,
-            notify_handler,
-            rx,
-            tx,
-        }
-    }
-
-    pub fn new_with_threads(
-        notify_handler: Option<&'a dyn Fn(Notification<'_>)>,
-        thread_count: usize,
-    ) -> Self {
+    pub fn new(notify_handler: Option<&'a dyn Fn(Notification<'_>)>, thread_count: usize) -> Self {
         // Defaults to hardware thread count threads; this is suitable for
         // our needs as IO bound operations tend to show up as write latencies
         // rather than close latencies, so we don't need to look at
diff --git a/src/dist/component/package.rs b/src/dist/component/package.rs
index 02469a9989..3beddf54fe 100644
--- a/src/dist/component/package.rs
+++ b/src/dist/component/package.rs
@@ -293,7 +293,7 @@ fn unpack_without_first_dir<'a, R: Read>(
     path: &Path,
     notify_handler: Option<&'a dyn Fn(Notification<'_>)>,
 ) -> Result<()> {
-    let mut io_executor: Box<dyn Executor> = get_executor(notify_handler);
+    let mut io_executor: Box<dyn Executor> = get_executor(notify_handler)?;
     let entries = archive
         .entries()
         .chain_err(|| ErrorKind::ExtractingPackage)?;