Perf: Throttle parallel typechecks in Find All References

vsintegration/src/FSharp.Editor/Common/CancellableTasks.fs

@@ -1098,6 +1098,28 @@ module CancellableTasks =
                 return! Task.WhenAll (tasks)
             }
 
+        /// Runs the given tasks concurrently, but caps the number of tasks that are running at
+        /// the same time to at most maxDegreeOfParallelism, to avoid launching an unbounded number
+        /// of parallel typechecks (e.g. one per document/project) at once.
+        let inline whenAllThrottled maxDegreeOfParallelism (tasks: CancellableTask<'a> seq) =
+            cancellableTask {
+                let! ct = getCancellationToken ()
+                use semaphore = new SemaphoreSlim(maxDegreeOfParallelism: int)
+
+                let runThrottled (task: CancellableTask<'a>) =
+                    backgroundTask {
+                        do! semaphore.WaitAsync(ct)
+
+                        try
+                            return! start ct task
+                        finally
+                            semaphore.Release() |> ignore
+                    }
+
+                let tasks = seq { for task in tasks do yield runThrottled task }
+                return! Task.WhenAll (tasks)
+            }
+
         let inline whenAllTasks (tasks: CancellableTask seq) =
             cancellableTask {
                 let! ct = getCancellationToken ()

vsintegration/src/FSharp.Editor/LanguageService/WorkspaceExtensions.fs

@@ -1,4 +1,4 @@
-[<AutoOpen>]
+[<AutoOpen>]
 module internal Microsoft.VisualStudio.FSharp.Editor.WorkspaceExtensions
 
 open System
@@ -707,7 +707,8 @@ type Project with
                     documents
                     |> Seq.map (fun doc ->
                         doc.FindFSharpReferencesAsync(symbol, projectSnapshot, (fun range -> onFound doc range), userOpName))
-                    |> CancellableTask.whenAll
+                    // Throttle to avoid launching a typecheck per document in the project all at once.
+                    |> CancellableTask.whenAllThrottled (max 1 Environment.ProcessorCount)
             else
                 for doc in documents do
                     do! doc.FindFSharpReferencesAsync(symbol, projectSnapshot, (onFound doc), userOpName)