make it a config

crates/ide/src/prime_caches.rs

@@ -108,7 +108,8 @@ pub(crate) fn parallel_prime_caches(
         }
 
         // recv_timeout is somewhat a hack, we need a way to from this thread check to see if the current salsa revision
-        // is cancelled.
+        // is cancelled on a regular basis. workers will only exit if they are processing a task that is cancelled, or
+        // if this thread exits, and closes the work channel.
         let worker_progress = match progress_receiver.recv_timeout(Duration::from_millis(10)) {
             Ok(p) => p,
             Err(crossbeam_channel::RecvTimeoutError::Timeout) => {

crates/ide/src/prime_caches/topologic_sort.rs

@@ -1,3 +1,4 @@
+//! helper data structure to schedule work for parallel prime caches.
 use std::{collections::VecDeque, hash::Hash};
 
 use rustc_hash::FxHashMap;

crates/rust-analyzer/src/config.rs

@@ -298,6 +298,9 @@ config_data! {
         /// Whether to show `can't find Cargo.toml` error message.
         notifications_cargoTomlNotFound: bool      = "true",
 
+        /// How many worker threads to to handle priming caches. The default `0` means to pick automatically.
+        primeCaches_numThreads: ParallelPrimeCachesNumThreads = "0",
+
         /// Enable support for procedural macros, implies `#rust-analyzer.cargo.runBuildScripts#`.
         procMacro_enable: bool                     = "true",
         /// Internal config, path to proc-macro server executable (typically,
@@ -1016,6 +1019,13 @@ impl Config {
             yield_points: self.data.highlightRelated_yieldPoints,
         }
     }
+
+    pub fn prime_caches_num_threads(&self) -> u8 {
+        match self.data.primeCaches_numThreads {
+            0 => num_cpus::get_physical().try_into().unwrap_or(u8::MAX),
+            n => n,
+        }
+    }
 }
 
 #[derive(Deserialize, Debug, Clone, Copy)]
@@ -1130,6 +1140,8 @@ enum WorkspaceSymbolSearchKindDef {
     AllSymbols,
 }
 
+type ParallelPrimeCachesNumThreads = u8;
+
 macro_rules! _config_data {
     (struct $name:ident {
         $(
@@ -1351,6 +1363,11 @@ fn field_props(field: &str, ty: &str, doc: &[&str], default: &str) -> serde_json
                 "Search for all symbols kinds"
             ],
         },
+        "ParallelPrimeCachesNumThreads" => set! {
+            "type": "number",
+            "minimum": 0,
+            "maximum": 255
+        },
         _ => panic!("{}: {}", ty, default),
     }
 

crates/rust-analyzer/src/main_loop.rs

@@ -505,17 +505,16 @@ impl GlobalState {
             self.fetch_build_data();
         }
         if self.prime_caches_queue.should_start_op() {
+            let num_worker_threads = self.config.prime_caches_num_threads();
+
             self.task_pool.handle.spawn_with_sender({
                 let analysis = self.snapshot().analysis;
                 move |sender| {
                     sender.send(Task::PrimeCaches(PrimeCachesProgress::Begin)).unwrap();
-                    let res = analysis.parallel_prime_caches(
-                        num_cpus::get_physical().try_into().unwrap_or(u8::MAX),
-                        |progress| {
-                            let report = PrimeCachesProgress::Report(progress);
-                            sender.send(Task::PrimeCaches(report)).unwrap();
-                        },
-                    );
+                    let res = analysis.parallel_prime_caches(num_worker_threads, |progress| {
+                        let report = PrimeCachesProgress::Report(progress);
+                        sender.send(Task::PrimeCaches(report)).unwrap();
+                    });
                     sender
                         .send(Task::PrimeCaches(PrimeCachesProgress::End {
                             cancelled: res.is_err(),

docs/user/generated_config.adoc

@@ -454,6 +454,11 @@ Number of syntax trees rust-analyzer keeps in memory. Defaults to 128.
 --
 Whether to show `can't find Cargo.toml` error message.
 --
+[[rust-analyzer.primeCaches.numThreads]]rust-analyzer.primeCaches.numThreads (default: `0`)::
++
+--
+How many worker threads to to handle priming caches. The default `0` means to pick automatically.
+--
 [[rust-analyzer.procMacro.enable]]rust-analyzer.procMacro.enable (default: `true`)::
 +
 --

editors/code/package.json

@@ -880,6 +880,13 @@
                     "default": true,
                     "type": "boolean"
                 },
+                "rust-analyzer.primeCaches.numThreads": {
+                    "markdownDescription": "How many worker threads to to handle priming caches. The default `0` means to pick automatically.",
+                    "default": 0,
+                    "type": "number",
+                    "minimum": 0,
+                    "maximum": 255
+                },
                 "rust-analyzer.procMacro.enable": {
                     "markdownDescription": "Enable support for procedural macros, implies `#rust-analyzer.cargo.runBuildScripts#`.",
                     "default": true,