Lower the default local parallelism, and align the number of swimlane…

…s to it. (#10499)

### Problem

It's pretty easy to overwhelm a machine with `2*multiprocessing.cpu_count()` processes running (especially since it frequently reports the number of virtual cores rather than physical cores), so this isn't a great default for local parallelism. Additionally, the number of swimlanes rendered in the dynamic UI is out of alignment with our configured parallelism.

### Solution

Lower the default parallelism to `multiprocessing.cpu_count()`, and align the swimlane count.

[ci skip-build-wheels]

src/python/pants/option/global_options.py

@@ -113,7 +113,7 @@ def from_bootstrap_options(cls, bootstrap_options):
     remote_store_chunk_upload_timeout_seconds=60,
     remote_store_rpc_retries=2,
     remote_store_connection_limit=5,
-    process_execution_local_parallelism=multiprocessing.cpu_count() * 2,
+    process_execution_local_parallelism=multiprocessing.cpu_count(),
     process_execution_remote_parallelism=128,
     process_execution_cleanup_local_dirs=True,
     process_execution_speculation_delay=1,

src/rust/engine/src/context.rs

@@ -50,6 +50,7 @@ pub struct Core {
   pub vfs: PosixFS,
   pub watcher: Arc<InvalidationWatcher>,
   pub build_root: PathBuf,
+  pub local_parallelism: usize,
 }
 
 #[derive(Clone, Debug)]
@@ -284,6 +285,7 @@ impl Core {
         .map_err(|e| format!("Could not initialize VFS: {:?}", e))?,
       build_root,
       watcher,
+      local_parallelism: exec_strategy_opts.local_parallelism,
     })
   }
 

src/rust/engine/src/scheduler.rs

@@ -87,7 +87,10 @@ impl Session {
   ) -> Session {
     let workunit_store = WorkunitStore::new(should_render_ui);
     let display = if should_render_ui {
-      Some(Mutex::new(ConsoleUI::new(workunit_store.clone())))
+      Some(Mutex::new(ConsoleUI::new(
+        workunit_store.clone(),
+        scheduler.core.local_parallelism,
+      )))
     } else {
       None
     };

src/rust/engine/ui/Cargo.toml

@@ -16,4 +16,3 @@ uuid = { version = "0.7", features = ["v4"] }
 logging = { path = "../logging" }
 task_executor = { path = "../task_executor" }
 workunit_store = { path = "../workunit_store" }
-num_cpus = "1"

src/rust/engine/ui/src/console_ui.rs

@@ -42,14 +42,16 @@ use workunit_store::WorkunitStore;
 
 pub struct ConsoleUI {
   workunit_store: WorkunitStore,
+  local_parallelism: usize,
   // While the UI is running, there will be an Instance present.
   instance: Option<Instance>,
 }
 
 impl ConsoleUI {
-  pub fn new(workunit_store: WorkunitStore) -> ConsoleUI {
+  pub fn new(workunit_store: WorkunitStore, local_parallelism: usize) -> ConsoleUI {
     ConsoleUI {
       workunit_store,
+      local_parallelism,
       instance: None,
     }
   }
@@ -180,7 +182,7 @@ impl ConsoleUI {
     }
 
     // Setup bars, and then spawning rendering of the bars into a background task.
-    let (multi_progress, bars) = Self::setup_bars(num_cpus::get());
+    let (multi_progress, bars) = Self::setup_bars(self.local_parallelism);
     let multi_progress_task = {
       executor
         .spawn_blocking(move || multi_progress.join())