feat(serve): Opt-in parallelism for `deno serve` (#24920)

Adds a `parallel` flag to `deno serve`. When present, we spawn multiple
workers to parallelize serving requests.


```bash
deno serve --parallel main.ts
```

Currently on linux we use `SO_REUSEPORT` and rely on the fact that the
kernel will distribute connections in a round-robin manner.

On mac and windows, we sort of emulate this by cloning the underlying
file descriptor and passing a handle to each worker. The connections
will not be guaranteed to be fairly distributed (and in practice almost
certainly won't be), but the distribution is still spread enough to
provide a significant performance increase.

---
(Run on an Macbook Pro with an M3 Max, serving `deno.com`

baseline::
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   239.78ms   13.56ms 330.54ms   79.12%
    Req/Sec   258.58     35.56   360.00     70.64%
  Latency Distribution
     50%  236.72ms
     75%  248.46ms
     90%  256.84ms
     99%  268.23ms
  15458 requests in 30.02s, 2.47GB read
Requests/sec:    514.89
Transfer/sec:     84.33MB
```

this PR (`with --parallel` flag)
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   117.40ms  142.84ms 590.45ms   79.07%
    Req/Sec     1.33k   175.19     1.77k    69.00%
  Latency Distribution
     50%   22.34ms
     75%  223.67ms
     90%  357.32ms
     99%  460.50ms
  79636 requests in 30.07s, 12.74GB read
Requests/sec:   2647.96
Transfer/sec:    433.71MB
```

15 files changed, 616 insertions, 140 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 14437003e..3032b40b5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -229,6 +229,12 @@ opt-level = 'z' # Optimize for size
 inherits = "release"
 debug = true
 
+# Faster to compile than `release` but with similar performance.
+[profile.release-lite]
+inherits = "release"
+codegen-units = 128
+lto = "thin"
+
 # NB: the `bench` and `release` profiles must remain EXACTLY the same.
 [profile.bench]
 codegen-units = 1
diff --git a/cli/args/flags.rs b/cli/args/flags.rs
index f8577ed1b..800d6ff5a 100644
--- a/cli/args/flags.rs
+++ b/cli/args/flags.rs
@@ -339,6 +339,7 @@ pub struct ServeFlags {
   pub watch: Option<WatchFlagsWithPaths>,
   pub port: u16,
   pub host: String,
+  pub worker_count: Option<usize>,
 }
 
 impl ServeFlags {
@@ -349,6 +350,7 @@ impl ServeFlags {
       watch: None,
       port,
       host: host.to_owned(),
+      worker_count: None,
     }
   }
 }
@@ -2693,6 +2695,9 @@ fn serve_subcommand() -> Command {
         .help("The TCP address to serve on, defaulting to 0.0.0.0 (all interfaces).")
         .value_parser(serve_host_validator),
     )
+    .arg(
+      parallel_arg("multiple server workers", false)
+    )
     .arg(check_arg(false))
     .arg(watch_arg(true))
     .arg(watch_exclude_arg())
@@ -2854,11 +2859,7 @@ Directory arguments are expanded to all contained files matching the glob
           .action(ArgAction::SetTrue),
       )
       .arg(
-        Arg::new("parallel")
-          .long("parallel")
-          .help("Run test modules in parallel. Parallelism defaults to the number of available CPUs or the value in the DENO_JOBS environment variable.")
-          .conflicts_with("jobs")
-          .action(ArgAction::SetTrue)
+        parallel_arg("test modules", true)
       )
       .arg(
         Arg::new("jobs")
@@ -2901,6 +2902,18 @@ Directory arguments are expanded to all contained files matching the glob
     )
 }
 
+fn parallel_arg(descr: &str, jobs_fallback: bool) -> Arg {
+  let arg = Arg::new("parallel")
+    .long("parallel")
+    .help(format!("Run {descr} in parallel. Parallelism defaults to the number of available CPUs or the value in the DENO_JOBS environment variable."))
+    .action(ArgAction::SetTrue);
+  if jobs_fallback {
+    arg.conflicts_with("jobs")
+  } else {
+    arg
+  }
+}
+
 fn types_subcommand() -> Command {
   Command::new("types").about(
     "Print runtime TypeScript declarations.
@@ -4416,6 +4429,8 @@ fn serve_parse(
     .remove_one::<String>("host")
     .unwrap_or_else(|| "0.0.0.0".to_owned());
 
+  let worker_count = parallel_arg_parse(matches, false).map(|v| v.get());
+
   runtime_args_parse(flags, matches, true, true);
   // If the user didn't pass --allow-net, add this port to the network
   // allowlist. If the host is 0.0.0.0, we add :{port} and allow the same network perms
@@ -4455,6 +4470,7 @@ fn serve_parse(
     watch: watch_arg_parse_with_paths(matches),
     port,
     host,
+    worker_count,
   });
 
   Ok(())
@@ -4486,6 +4502,42 @@ fn task_parse(flags: &mut Flags, matches: &mut ArgMatches) {
   flags.subcommand = DenoSubcommand::Task(task_flags);
 }
 
+fn parallel_arg_parse(
+  matches: &mut ArgMatches,
+  fallback_to_jobs: bool,
+) -> Option<NonZeroUsize> {
+  if matches.get_flag("parallel") {
+    if let Ok(value) = env::var("DENO_JOBS") {
+      value.parse::<NonZeroUsize>().ok()
+    } else {
+      std::thread::available_parallelism().ok()
+    }
+  } else if fallback_to_jobs && matches.contains_id("jobs") {
+    // We can't change this to use the log crate because its not configured
+    // yet at this point since the flags haven't been parsed. This flag is
+    // deprecated though so it's not worth changing the code to use the log
+    // crate here and this is only done for testing anyway.
+    #[allow(clippy::print_stderr)]
+    {
+      eprintln!(
+        "⚠️ {}",
+        crate::colors::yellow(concat!(
+          "The `--jobs` flag is deprecated and will be removed in Deno 2.0.\n",
+          "Use the `--parallel` flag with possibly the `DENO_JOBS` environment variable instead.\n",
+          "Learn more at: https://docs.deno.com/runtime/manual/basics/env_variables"
+        )),
+      );
+    }
+    if let Some(value) = matches.remove_one::<NonZeroUsize>("jobs") {
+      Some(value)
+    } else {
+      std::thread::available_parallelism().ok()
+    }
+  } else {
+    None
+  }
+}
+
 fn test_parse(flags: &mut Flags, matches: &mut ArgMatches) {
   flags.type_check_mode = TypeCheckMode::Local;
   runtime_args_parse(flags, matches, true, true);
@@ -4552,36 +4604,7 @@ fn test_parse(flags: &mut Flags, matches: &mut ArgMatches) {
     flags.argv.extend(script_arg);
   }
 
-  let concurrent_jobs = if matches.get_flag("parallel") {
-    if let Ok(value) = env::var("DENO_JOBS") {
-      value.parse::<NonZeroUsize>().ok()
-    } else {
-      std::thread::available_parallelism().ok()
-    }
-  } else if matches.contains_id("jobs") {
-    // We can't change this to use the log crate because its not configured
-    // yet at this point since the flags haven't been parsed. This flag is
-    // deprecated though so it's not worth changing the code to use the log
-    // crate here and this is only done for testing anyway.
-    #[allow(clippy::print_stderr)]
-    {
-      eprintln!(
-        "⚠️ {}",
-        crate::colors::yellow(concat!(
-        "The `--jobs` flag is deprecated and will be removed in Deno 2.0.\n",
-        "Use the `--parallel` flag with possibly the `DENO_JOBS` environment variable instead.\n",
-        "Learn more at: https://docs.deno.com/runtime/manual/basics/env_variables"
-        )),
-      );
-    }
-    if let Some(value) = matches.remove_one::<NonZeroUsize>("jobs") {
-      Some(value)
-    } else {
-      std::thread::available_parallelism().ok()
-    }
-  } else {
-    None
-  };
+  let concurrent_jobs = parallel_arg_parse(matches, true);
 
   let include = if let Some(files) = matches.remove_many::<String>("files") {
     files.collect()
diff --git a/cli/main.rs b/cli/main.rs
index e8ecaa393..1752c3373 100644
--- a/cli/main.rs
+++ b/cli/main.rs
@@ -236,7 +236,7 @@ async fn run_subcommand(flags: Arc<Flags>) -> Result<i32, AnyError> {
       }
     }),
     DenoSubcommand::Serve(serve_flags) => spawn_subcommand(async move {
-      tools::run::run_script(WorkerExecutionMode::Serve, flags, serve_flags.watch).await
+      tools::serve::serve(flags, serve_flags).await
     }),
     DenoSubcommand::Task(task_flags) => spawn_subcommand(async {
       tools::task::execute_script(flags, task_flags, false).await
diff --git a/cli/tools/mod.rs b/cli/tools/mod.rs
index 7bb9b7cf6..1e1c65565 100644
--- a/cli/tools/mod.rs
+++ b/cli/tools/mod.rs
@@ -16,6 +16,7 @@ pub mod lint;
 pub mod registry;
 pub mod repl;
 pub mod run;
+pub mod serve;
 pub mod task;
 pub mod test;
 pub mod upgrade;
diff --git a/cli/tools/run/mod.rs b/cli/tools/run/mod.rs
index 65044fbad..1964cfdd9 100644
--- a/cli/tools/run/mod.rs
+++ b/cli/tools/run/mod.rs
@@ -18,11 +18,7 @@ use crate::util::file_watcher::WatcherRestartMode;
 
 pub mod hmr;
 
-pub async fn run_script(
-  mode: WorkerExecutionMode,
-  flags: Arc<Flags>,
-  watch: Option<WatchFlagsWithPaths>,
-) -> Result<i32, AnyError> {
+pub fn check_permission_before_script(flags: &Flags) {
   if !flags.has_permission() && flags.has_permission_in_argv() {
     log::warn!(
       "{}",
@@ -33,6 +29,14 @@ To grant permissions, set them before the script argument. For example:
       )
     );
   }
+}
+
+pub async fn run_script(
+  mode: WorkerExecutionMode,
+  flags: Arc<Flags>,
+  watch: Option<WatchFlagsWithPaths>,
+) -> Result<i32, AnyError> {
+  check_permission_before_script(&flags);
 
   if let Some(watch_flags) = watch {
     return run_with_watch(mode, flags, watch_flags).await;
@@ -187,7 +191,7 @@ pub async fn eval_command(
   Ok(exit_code)
 }
 
-async fn maybe_npm_install(factory: &CliFactory) -> Result<(), AnyError> {
+pub async fn maybe_npm_install(factory: &CliFactory) -> Result<(), AnyError> {
   // ensure an "npm install" is done if the user has explicitly
   // opted into using a managed node_modules directory
   if factory.cli_options()?.node_modules_dir_enablement() == Some(true) {
diff --git a/cli/tools/serve.rs b/cli/tools/serve.rs
new file mode 100644
index 000000000..24666b8f6
--- /dev/null
+++ b/cli/tools/serve.rs
@@ -0,0 +1,192 @@
+// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
+
+use std::sync::Arc;
+
+use deno_core::error::AnyError;
+use deno_core::futures::TryFutureExt;
+use deno_core::ModuleSpecifier;
+use deno_runtime::deno_permissions::Permissions;
+use deno_runtime::deno_permissions::PermissionsContainer;
+
+use super::run::check_permission_before_script;
+use super::run::maybe_npm_install;
+use crate::args::Flags;
+use crate::args::ServeFlags;
+use crate::args::WatchFlagsWithPaths;
+use crate::factory::CliFactory;
+use crate::util::file_watcher::WatcherRestartMode;
+use crate::worker::CliMainWorkerFactory;
+
+pub async fn serve(
+  flags: Arc<Flags>,
+  serve_flags: ServeFlags,
+) -> Result<i32, AnyError> {
+  check_permission_before_script(&flags);
+
+  if let Some(watch_flags) = serve_flags.watch {
+    return serve_with_watch(flags, watch_flags, serve_flags.worker_count)
+      .await;
+  }
+
+  let factory = CliFactory::from_flags(flags);
+  let cli_options = factory.cli_options()?;
+  let deno_dir = factory.deno_dir()?;
+  let http_client = factory.http_client_provider();
+
+  // Run a background task that checks for available upgrades or output
+  // if an earlier run of this background task found a new version of Deno.
+  #[cfg(feature = "upgrade")]
+  super::upgrade::check_for_upgrades(
+    http_client.clone(),
+    deno_dir.upgrade_check_file_path(),
+  );
+
+  let main_module = cli_options.resolve_main_module()?;
+
+  maybe_npm_install(&factory).await?;
+
+  let permissions = PermissionsContainer::new(Permissions::from_options(
+    &cli_options.permissions_options()?,
+  )?);
+  let worker_factory = factory.create_cli_main_worker_factory().await?;
+
+  do_serve(
+    worker_factory,
+    main_module,
+    permissions,
+    serve_flags.worker_count,
+    false,
+  )
+  .await
+}
+
+async fn do_serve(
+  worker_factory: CliMainWorkerFactory,
+  main_module: ModuleSpecifier,
+  permissions: PermissionsContainer,
+  worker_count: Option<usize>,
+  hmr: bool,
+) -> Result<i32, AnyError> {
+  let mut worker = worker_factory
+    .create_main_worker(
+      deno_runtime::WorkerExecutionMode::Serve {
+        is_main: true,
+        worker_count,
+      },
+      main_module.clone(),
+      permissions.clone(),
+    )
+    .await?;
+  let worker_count = match worker_count {
+    None | Some(1) => return worker.run().await,
+    Some(c) => c,
+  };
+
+  let main = deno_core::unsync::spawn(async move { worker.run().await });
+
+  let extra_workers = worker_count.saturating_sub(1);
+
+  let mut channels = Vec::with_capacity(extra_workers);
+  for i in 0..extra_workers {
+    let worker_factory = worker_factory.clone();
+    let main_module = main_module.clone();
+    let permissions = permissions.clone();
+    let (tx, rx) = tokio::sync::oneshot::channel();
+    channels.push(rx);
+    std::thread::Builder::new()
+      .name(format!("serve-worker-{i}"))
+      .spawn(move || {
+        deno_runtime::tokio_util::create_and_run_current_thread(async move {
+          let result =
+            run_worker(i, worker_factory, main_module, permissions, hmr).await;
+          let _ = tx.send(result);
+        });
+      })?;
+  }
+
+  let (main_result, worker_results) = tokio::try_join!(
+    main.map_err(AnyError::from),
+    deno_core::futures::future::try_join_all(
+      channels.into_iter().map(|r| r.map_err(AnyError::from))
+    )
+  )?;
+
+  let mut exit_code = main_result?;
+  for res in worker_results {
+    let ret = res?;
+    if ret != 0 && exit_code == 0 {
+      exit_code = ret;
+    }
+  }
+  Ok(exit_code)
+
+  // main.await?
+}
+
+async fn run_worker(
+  worker_count: usize,
+  worker_factory: CliMainWorkerFactory,
+  main_module: ModuleSpecifier,
+  permissions: PermissionsContainer,
+  hmr: bool,
+) -> Result<i32, AnyError> {
+  let mut worker = worker_factory
+    .create_main_worker(
+      deno_runtime::WorkerExecutionMode::Serve {
+        is_main: false,
+        worker_count: Some(worker_count),
+      },
+      main_module,
+      permissions,
+    )
+    .await?;
+  if hmr {
+    worker.run_for_watcher().await?;
+    Ok(0)
+  } else {
+    worker.run().await
+  }
+}
+
+async fn serve_with_watch(
+  flags: Arc<Flags>,
+  watch_flags: WatchFlagsWithPaths,
+  worker_count: Option<usize>,
+) -> Result<i32, AnyError> {
+  let hmr = watch_flags.hmr;
+  crate::util::file_watcher::watch_recv(
+    flags,
+    crate::util::file_watcher::PrintConfig::new_with_banner(
+      if watch_flags.hmr { "HMR" } else { "Watcher" },
+      "Process",
+      !watch_flags.no_clear_screen,
+    ),
+    WatcherRestartMode::Automatic,
+    move |flags, watcher_communicator, _changed_paths| {
+      Ok(async move {
+        let factory = CliFactory::from_flags_for_watcher(
+          flags,
+          watcher_communicator.clone(),
+        );
+        let cli_options = factory.cli_options()?;
+        let main_module = cli_options.resolve_main_module()?;
+
+        maybe_npm_install(&factory).await?;
+
+        let _ = watcher_communicator.watch_paths(cli_options.watch_paths());
+
+        let permissions = PermissionsContainer::new(Permissions::from_options(
+          &cli_options.permissions_options()?,
+        )?);
+        let worker_factory = factory.create_cli_main_worker_factory().await?;
+
+        do_serve(worker_factory, main_module, permissions, worker_count, hmr)
+          .await?;
+
+        Ok(())
+      })
+    },
+  )
+  .await?;
+  Ok(0)
+}
diff --git a/cli/worker.rs b/cli/worker.rs
index 82051da6c..7f04e8846 100644
--- a/cli/worker.rs
+++ b/cli/worker.rs
@@ -414,6 +414,7 @@ impl CliMainWorker {
   }
 }
 
+#[derive(Clone)]
 pub struct CliMainWorkerFactory {
   shared: Arc<SharedWorkerState>,
 }
@@ -546,7 +547,7 @@ impl CliMainWorkerFactory {
     let maybe_inspector_server = shared.maybe_inspector_server.clone();
 
     let create_web_worker_cb =
-      create_web_worker_callback(mode, shared.clone(), stdio.clone());
+      create_web_worker_callback(shared.clone(), stdio.clone());
 
     let maybe_storage_key = shared
       .storage_key_resolver
@@ -739,7 +740,6 @@ impl CliMainWorkerFactory {
 }
 
 fn create_web_worker_callback(
-  mode: WorkerExecutionMode,
   shared: Arc<SharedWorkerState>,
   stdio: deno_runtime::deno_io::Stdio,
 ) -> Arc<CreateWebWorkerCb> {
@@ -752,7 +752,7 @@ fn create_web_worker_callback(
         args.permissions.clone(),
       );
     let create_web_worker_cb =
-      create_web_worker_callback(mode, shared.clone(), stdio.clone());
+      create_web_worker_callback(shared.clone(), stdio.clone());
 
     let maybe_storage_key = shared
       .storage_key_resolver
@@ -802,7 +802,7 @@ fn create_web_worker_callback(
         disable_deprecated_api_warning: shared.disable_deprecated_api_warning,
         verbose_deprecated_api_warning: shared.verbose_deprecated_api_warning,
         future: shared.enable_future_features,
-        mode,
+        mode: WorkerExecutionMode::Worker,
         serve_port: shared.serve_port,
         serve_host: shared.serve_host.clone(),
       },
diff --git a/ext/http/00_serve.ts b/ext/http/00_serve.ts
index 8ed1a1d04..9c6f80552 100644
--- a/ext/http/00_serve.ts
+++ b/ext/http/00_serve.ts
@@ -579,6 +579,8 @@ type RawServeOptions = {
   handler?: RawHandler;
 };
 
+const kLoadBalanced = Symbol("kLoadBalanced");
+
 function serve(arg1, arg2) {
   let options: RawServeOptions | undefined;
   let handler: RawHandler | undefined;
@@ -634,6 +636,7 @@ function serve(arg1, arg2) {
     hostname: options.hostname ?? "0.0.0.0",
     port: options.port ?? 8000,
     reusePort: options.reusePort ?? false,
+    loadBalanced: options[kLoadBalanced] ?? false,
   };
 
   if (options.certFile || options.keyFile) {
@@ -842,18 +845,25 @@ function registerDeclarativeServer(exports) {
         "Invalid type for fetch: must be a function with a single or no parameter",
       );
     }
-    return ({ servePort, serveHost }) => {
+    return ({ servePort, serveHost, serveIsMain, serveWorkerCount }) => {
       Deno.serve({
         port: servePort,
         hostname: serveHost,
+        [kLoadBalanced]: (serveIsMain && serveWorkerCount > 1) ||
+          (serveWorkerCount !== null),
         onListen: ({ port, hostname }) => {
-          console.debug(
-            `%cdeno serve%c: Listening on %chttp://${hostname}:${port}/%c`,
-            "color: green",
-            "color: inherit",
-            "color: yellow",
-            "color: inherit",
-          );
+          if (serveIsMain) {
+            const nThreads = serveWorkerCount > 1
+              ? ` with ${serveWorkerCount} threads`
+              : "";
+            console.debug(
+              `%cdeno serve%c: Listening on %chttp://${hostname}:${port}/%c${nThreads}`,
+              "color: green",
+              "color: inherit",
+              "color: yellow",
+              "color: inherit",
+            );
+          }
         },
         handler: (req) => {
           return exports.fetch(req);
diff --git a/ext/net/01_net.js b/ext/net/01_net.js
index 517ab127e..536f79bbf 100644
--- a/ext/net/01_net.js
+++ b/ext/net/01_net.js
@@ -531,10 +531,14 @@ const listenOptionApiName = Symbol("listenOptionApiName");
 function listen(args) {
   switch (args.transport ?? "tcp") {
     case "tcp": {
-      const { 0: rid, 1: addr } = op_net_listen_tcp({
-        hostname: args.hostname ?? "0.0.0.0",
-        port: Number(args.port),
-      }, args.reusePort);
+      const { 0: rid, 1: addr } = op_net_listen_tcp(
+        {
+          hostname: args.hostname ?? "0.0.0.0",
+          port: Number(args.port),
+        },
+        args.reusePort,
+        args.loadBalanced ?? false,
+      );
       addr.transport = "tcp";
       return new Listener(rid, addr);
     }
diff --git a/ext/net/ops.rs b/ext/net/ops.rs
index f28778d29..b74dc8d75 100644
--- a/ext/net/ops.rs
+++ b/ext/net/ops.rs
@@ -353,6 +353,7 @@ pub fn op_net_listen_tcp<NP>(
   state: &mut OpState,
   #[serde] addr: IpAddr,
   reuse_port: bool,
+  load_balanced: bool,
 ) -> Result<(ResourceId, IpAddr), AnyError>
 where
   NP: NetPermissions + 'static,
@@ -367,7 +368,11 @@ where
     .next()
     .ok_or_else(|| generic_error("No resolved address found"))?;
 
-  let listener = TcpListener::bind_direct(addr, reuse_port)?;
+  let listener = if load_balanced {
+    TcpListener::bind_load_balanced(addr)
+  } else {
+    TcpListener::bind_direct(addr, reuse_port)
+  }?;
   let local_addr = listener.local_addr()?;
   let listener_resource = NetworkListenerResource::new(listener);
   let rid = state.resource_table.add(listener_resource);
diff --git a/ext/net/ops_tls.rs b/ext/net/ops_tls.rs
index a2a27c4ad..8483e7e66 100644
--- a/ext/net/ops_tls.rs
+++ b/ext/net/ops_tls.rs
@@ -475,6 +475,8 @@ fn load_private_keys_from_file(
 pub struct ListenTlsArgs {
   alpn_protocols: Option<Vec<String>>,
   reuse_port: bool,
+  #[serde(default)]
+  load_balanced: bool,
 }
 
 #[op2]
@@ -502,7 +504,11 @@ where
     .next()
     .ok_or_else(|| generic_error("No resolved address found"))?;
 
-  let tcp_listener = TcpListener::bind_direct(bind_addr, args.reuse_port)?;
+  let tcp_listener = if args.load_balanced {
+    TcpListener::bind_load_balanced(bind_addr)
+  } else {
+    TcpListener::bind_direct(bind_addr, args.reuse_port)
+  }?;
   let local_addr = tcp_listener.local_addr()?;
   let alpn = args
     .alpn_protocols
diff --git a/runtime/js/99_main.js b/runtime/js/99_main.js
index ca96e34b7..5e25a3818 100644
--- a/runtime/js/99_main.js
+++ b/runtime/js/99_main.js
@@ -45,6 +45,7 @@ const {
   PromiseResolve,
   SafeSet,
   StringPrototypeIncludes,
+  StringPrototypePadEnd,
   StringPrototypeSplit,
   StringPrototypeTrim,
   Symbol,
@@ -709,8 +710,37 @@ function bootstrapMainRuntime(runtimeOptions, warmup = false) {
       11: mode,
       12: servePort,
       13: serveHost,
+      14: serveIsMain,
+      15: serveWorkerCount,
     } = runtimeOptions;
 
+    if (mode === executionModes.serve) {
+      if (serveIsMain && serveWorkerCount) {
+        const origLog = console.log;
+        const origError = console.error;
+        const prefix = `[serve-worker-0 ]`;
+        console.log = (...args) => {
+          return origLog(prefix, ...new primordials.SafeArrayIterator(args));
+        };
+        console.error = (...args) => {
+          return origError(prefix, ...new primordials.SafeArrayIterator(args));
+        };
+      } else if (serveWorkerCount !== null) {
+        const origLog = console.log;
+        const origError = console.error;
+        const base = `serve-worker-${serveWorkerCount + 1}`;
+        // 15 = "serve-worker-nn".length, assuming
+        // serveWorkerCount < 100
+        const prefix = `[${StringPrototypePadEnd(base, 15, " ")}]`;
+        console.log = (...args) => {
+          return origLog(prefix, ...new primordials.SafeArrayIterator(args));
+        };
+        console.error = (...args) => {
+          return origError(prefix, ...new primordials.SafeArrayIterator(args));
+        };
+      }
+    }
+
     if (mode === executionModes.run || mode === executionModes.serve) {
       let serve = undefined;
       core.addMainModuleHandler((main) => {
@@ -725,13 +755,16 @@ function bootstrapMainRuntime(runtimeOptions, warmup = false) {
         }
 
         if (mode === executionModes.serve && !serve) {
-          console.error(
-            `%cerror: %cdeno serve requires %cexport default { fetch }%c in the main module, did you mean to run \"deno run\"?`,
-            "color: yellow;",
-            "color: inherit;",
-            "font-weight: bold;",
-            "font-weight: normal;",
-          );
+          if (serveIsMain) {
+            // Only error if main worker
+            console.error(
+              `%cerror: %cdeno serve requires %cexport default { fetch }%c in the main module, did you mean to run \"deno run\"?`,
+              "color: yellow;",
+              "color: inherit;",
+              "font-weight: bold;",
+              "font-weight: normal;",
+            );
+          }
           return;
         }
 
@@ -746,7 +779,7 @@ function bootstrapMainRuntime(runtimeOptions, warmup = false) {
             );
           }
           if (mode === executionModes.serve) {
-            serve({ servePort, serveHost });
+            serve({ servePort, serveHost, serveIsMain, serveWorkerCount });
           }
         }
       });
diff --git a/runtime/worker_bootstrap.rs b/runtime/worker_bootstrap.rs
index b13c3c428..afd3242e8 100644
--- a/runtime/worker_bootstrap.rs
+++ b/runtime/worker_bootstrap.rs
@@ -10,7 +10,6 @@ use deno_terminal::colors;
 
 /// The execution mode for this worker. Some modes may have implicit behaviour.
 #[derive(Copy, Clone)]
-#[repr(u8)]
 pub enum WorkerExecutionMode {
   /// No special behaviour.
   None,
@@ -28,11 +27,39 @@ pub enum WorkerExecutionMode {
   /// `deno bench`
   Bench,
   /// `deno serve`
-  Serve,
+  Serve {
+    is_main: bool,
+    worker_count: Option<usize>,
+  },
   /// `deno jupyter`
   Jupyter,
 }
 
+impl WorkerExecutionMode {
+  pub fn discriminant(&self) -> u8 {
+    match self {
+      WorkerExecutionMode::None => 0,
+      WorkerExecutionMode::Worker => 1,
+      WorkerExecutionMode::Run => 2,
+      WorkerExecutionMode::Repl => 3,
+      WorkerExecutionMode::Eval => 4,
+      WorkerExecutionMode::Test => 5,
+      WorkerExecutionMode::Bench => 6,
+      WorkerExecutionMode::Serve { .. } => 7,
+      WorkerExecutionMode::Jupyter => 8,
+    }
+  }
+  pub fn serve_info(&self) -> (Option<bool>, Option<usize>) {
+    match *self {
+      WorkerExecutionMode::Serve {
+        is_main,
+        worker_count,
+      } => (Some(is_main), worker_count),
+      _ => (None, None),
+    }
+  }
+}
+
 /// The log level to use when printing diagnostic log messages, warnings,
 /// or errors in the worker.
 ///
@@ -175,6 +202,10 @@ struct BootstrapV8<'a>(
   u16,
   // serve host
   Option<&'a str>,
+  // serve is main
+  Option<bool>,
+  // serve worker count
+  Option<usize>,
 );
 
 impl BootstrapOptions {
@@ -186,6 +217,7 @@ impl BootstrapOptions {
     let scope = RefCell::new(scope);
     let ser = deno_core::serde_v8::Serializer::new(&scope);
 
+    let (serve_is_main, serve_worker_count) = self.mode.serve_info();
     let bootstrap = BootstrapV8(
       self.location.as_ref().map(|l| l.as_str()),
       self.unstable,
@@ -198,9 +230,11 @@ impl BootstrapOptions {
       self.disable_deprecated_api_warning,
       self.verbose_deprecated_api_warning,
       self.future,
-      self.mode as u8 as _,
+      self.mode.discriminant() as _,
       self.serve_port.unwrap_or_default(),
       self.serve_host.as_deref(),
+      serve_is_main,
+      serve_worker_count,
     );
 
     bootstrap.serialize(ser).unwrap()
diff --git a/tests/integration/serve_tests.rs b/tests/integration/serve_tests.rs
index cfe7e4d6a..f3d887ec2 100644
--- a/tests/integration/serve_tests.rs
+++ b/tests/integration/serve_tests.rs
@@ -1,93 +1,244 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 
+use std::cell::RefCell;
+use std::collections::HashMap;
 use std::io::Read;
+use std::time::Duration;
 
 use pretty_assertions::assert_eq;
 use regex::Regex;
+use reqwest::RequestBuilder;
 use test_util as util;
+use test_util::DenoChild;
+use tokio::time::timeout;
+
+struct ServeClient {
+  child: RefCell<DenoChild>,
+  client: reqwest::Client,
+  output_buf: RefCell<Vec<u8>>,
+  endpoint: RefCell<Option<String>>,
+}
+
+impl Drop for ServeClient {
+  fn drop(&mut self) {
+    let mut child = self.child.borrow_mut();
+    child.kill().unwrap();
+    child.wait().unwrap();
+  }
+}
+
+struct ServeClientBuilder(util::TestCommandBuilder, Option<String>);
+
+impl ServeClientBuilder {
+  fn build(self) -> ServeClient {
+    let Some(entry_point) = self.1 else {
+      panic!("entry point required");
+    };
+    let cmd = self.0.arg(entry_point);
+    let child = cmd.spawn().unwrap();
+
+    ServeClient::with_child(child)
+  }
+  fn map(
+    self,
+    f: impl FnOnce(util::TestCommandBuilder) -> util::TestCommandBuilder,
+  ) -> Self {
+    Self(f(self.0), self.1)
+  }
+  fn entry_point(self, file: impl AsRef<str>) -> Self {
+    Self(self.0, Some(file.as_ref().into()))
+  }
+  fn worker_count(self, n: Option<u64>) -> Self {
+    self.map(|t| {
+      let t = t.arg("--parallel");
+      if let Some(n) = n {
+        t.env("DENO_JOBS", n.to_string())
+      } else {
+        t
+      }
+    })
+  }
+  fn new() -> Self {
+    Self(
+      util::deno_cmd()
+        .current_dir(util::testdata_path())
+        .arg("serve")
+        .arg("--port")
+        .arg("0")
+        .stdout_piped(),
+      None,
+    )
+  }
+}
+
+impl ServeClient {
+  fn builder() -> ServeClientBuilder {
+    ServeClientBuilder::new()
+  }
+  fn with_child(child: DenoChild) -> Self {
+    Self {
+      child: RefCell::new(child),
+      output_buf: Default::default(),
+      endpoint: Default::default(),
+      client: reqwest::Client::builder()
+        .add_root_certificate(
+          reqwest::Certificate::from_pem(include_bytes!(
+            "../testdata/tls/RootCA.crt"
+          ))
+          .unwrap(),
+        )
+        // disable connection pooling so we create a new connection per request
+        // which allows us to distribute requests across workers
+        .pool_max_idle_per_host(0)
+        .pool_idle_timeout(Duration::from_nanos(1))
+        .http2_prior_knowledge()
+        .build()
+        .unwrap(),
+    }
+  }
+
+  fn kill(self) {
+    let mut child = self.child.borrow_mut();
+    child.kill().unwrap();
+    child.wait().unwrap();
+  }
+
+  fn output(self) -> String {
+    let mut child = self.child.borrow_mut();
+    child.kill().unwrap();
+    let mut stdout = child.stdout.take().unwrap();
+    child.wait().unwrap();
+
+    let mut output_buf = self.output_buf.borrow_mut();
+
+    stdout.read_to_end(&mut output_buf).unwrap();
+
+    String::from_utf8(std::mem::take(&mut *output_buf)).unwrap()
+  }
+
+  fn get(&self) -> RequestBuilder {
+    let endpoint = self.endpoint();
+    self.client.get(&*endpoint)
+  }
+
+  fn endpoint(&self) -> String {
+    if let Some(e) = self.endpoint.borrow().as_ref() {
+      return e.to_string();
+    };
+    let mut buffer = self.output_buf.borrow_mut();
+    let mut temp_buf = [0u8; 64];
+    let mut child = self.child.borrow_mut();
+    let stdout = child.stdout.as_mut().unwrap();
+    let port_regex = regex::bytes::Regex::new(r":(\d+)").unwrap();
+
+    let start = std::time::Instant::now();
+    // try to find the port number in the output
+    // it may not be the first line, so we need to read the output in a loop
+    let port = loop {
+      if start.elapsed() > Duration::from_secs(5) {
+        panic!(
+          "timed out waiting for serve to start. serve output:\n{}",
+          String::from_utf8_lossy(&buffer)
+        );
+      }
+      let read = stdout.read(&mut temp_buf).unwrap();
+      buffer.extend_from_slice(&temp_buf[..read]);
+      if let Some(p) = port_regex
+        .captures(&buffer)
+        .and_then(|c| c.get(1))
+        .map(|v| std::str::from_utf8(v.as_bytes()).unwrap().to_owned())
+      {
+        break p;
+      }
+      // this is technically blocking, but it's just a test and
+      // I don't want to switch RefCell to Mutex just for this
+      std::thread::sleep(Duration::from_millis(10));
+    };
+    self
+      .endpoint
+      .replace(Some(format!("http://127.0.0.1:{port}")));
+
+    return self.endpoint.borrow().clone().unwrap();
+  }
+}
 
 #[tokio::test]
 async fn deno_serve_port_0() {
-  let mut child = util::deno_cmd()
-    .current_dir(util::testdata_path())
-    .arg("serve")
-    .arg("--port")
-    .arg("0")
-    .arg("./serve/port_0.ts")
-    .stdout_piped()
-    .spawn()
-    .unwrap();
-  let stdout = child.stdout.as_mut().unwrap();
-  let mut buffer = [0; 52];
-  let _read = stdout.read(&mut buffer).unwrap();
-  let msg = std::str::from_utf8(&buffer).unwrap();
-  let port_regex = Regex::new(r":(\d+)").unwrap();
-  let port = port_regex.captures(msg).unwrap().get(1).unwrap().as_str();
-
-  let cert = reqwest::Certificate::from_pem(include_bytes!(
-    "../testdata/tls/RootCA.crt"
-  ))
-  .unwrap();
-
-  let client = reqwest::Client::builder()
-    .add_root_certificate(cert)
-    .http2_prior_knowledge()
-    .build()
-    .unwrap();
-
-  let res = client
-    .get(&format!("http://127.0.0.1:{port}"))
-    .send()
-    .await
-    .unwrap();
+  let client = ServeClient::builder()
+    .entry_point("./serve/port_0.ts")
+    .build();
+  let res = client.get().send().await.unwrap();
   assert_eq!(200, res.status());
 
   let body = res.text().await.unwrap();
   assert_eq!(body, "deno serve --port 0 works!");
-
-  child.kill().unwrap();
-  child.wait().unwrap();
+  client.kill();
 }
 
 #[tokio::test]
 async fn deno_serve_no_args() {
-  let mut child = util::deno_cmd()
-    .current_dir(util::testdata_path())
-    .arg("serve")
-    .arg("--port")
-    .arg("0")
-    .arg("./serve/no_args.ts")
-    .stdout_piped()
-    .spawn()
-    .unwrap();
-  let stdout = child.stdout.as_mut().unwrap();
-  let mut buffer = [0; 52];
-  let _read = stdout.read(&mut buffer).unwrap();
-  let msg = std::str::from_utf8(&buffer).unwrap();
-  let port_regex = Regex::new(r":(\d+)").unwrap();
-  let port = port_regex.captures(msg).unwrap().get(1).unwrap().as_str();
-
-  let cert = reqwest::Certificate::from_pem(include_bytes!(
-    "../testdata/tls/RootCA.crt"
-  ))
-  .unwrap();
-
-  let client = reqwest::Client::builder()
-    .add_root_certificate(cert)
-    .http2_prior_knowledge()
-    .build()
-    .unwrap();
-
-  let res = client
-    .get(&format!("http://127.0.0.1:{port}"))
-    .send()
-    .await
-    .unwrap();
+  let client = ServeClient::builder()
+    .entry_point("./serve/no_args.ts")
+    .build();
+  let res = client.get().send().await.unwrap();
   assert_eq!(200, res.status());
 
   let body = res.text().await.unwrap();
   assert_eq!(body, "deno serve with no args in fetch() works!");
+}
+
+#[tokio::test]
+async fn deno_serve_parallel() {
+  let client = ServeClient::builder()
+    .entry_point("./serve/parallel.ts")
+    .worker_count(Some(4))
+    .build();
+
+  let mut serve_counts = HashMap::<u32, u32>::new();
+
+  tokio::time::sleep(Duration::from_millis(1000)).await;
+
+  let serve_regex =
+    Regex::new(r"\[serve\-worker\-(\d+)\s*\] serving request").unwrap();
+
+  for _ in 0..100 {
+    let response = timeout(Duration::from_secs(2), client.get().send())
+      .await
+      .unwrap()
+      .unwrap();
+    assert_eq!(200, response.status());
+    let body = response.text().await.unwrap();
+    assert_eq!(body, "deno serve parallel");
+    tokio::time::sleep(Duration::from_millis(1)).await;
+  }
+
+  let output = client.output();
+
+  let listening_regex =
+    Regex::new(r"Listening on http[\w:/\.]+ with (\d+) threads").unwrap();
+
+  eprintln!("serve output:\n{output}");
+  assert_eq!(
+    listening_regex
+      .captures(&output)
+      .unwrap()
+      .get(1)
+      .unwrap()
+      .as_str()
+      .trim(),
+    "4"
+  );
+
+  for capture in serve_regex.captures_iter(&output) {
+    if let Some(worker_number) =
+      capture.get(1).and_then(|m| m.as_str().parse::<u32>().ok())
+    {
+      *serve_counts.entry(worker_number).or_default() += 1;
+    }
+  }
 
-  child.kill().unwrap();
-  child.wait().unwrap();
+  assert!(
+    serve_counts.values().filter(|&&n| n > 2).count() >= 2,
+    "bad {serve_counts:?}"
+  );
 }
diff --git a/tests/testdata/serve/parallel.ts b/tests/testdata/serve/parallel.ts
new file mode 100644
index 000000000..f1f118c71
--- /dev/null
+++ b/tests/testdata/serve/parallel.ts
@@ -0,0 +1,7 @@
+console.log("starting serve");
+export default {
+  fetch(_req: Request) {
+    console.log("serving request");
+    return new Response("deno serve parallel");
+  },
+};