Index: clangd/index/Background.h
===================================================================
--- clangd/index/Background.h
+++ clangd/index/Background.h
@@ -16,6 +16,7 @@
 #include "index/Index.h"
 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/Support/SHA1.h"
+#include "TUScheduler.h"
 #include <condition_variable>
 #include <deque>
 #include <string>
@@ -34,7 +35,8 @@
   // FIXME: resource-dir injection should be hoisted somewhere common.
   BackgroundIndex(Context BackgroundContext, StringRef ResourceDir,
                   const FileSystemProvider &,
-                  ArrayRef<std::string> URISchemes = {});
+                  ArrayRef<std::string> URISchemes = {},
+                  size_t ThreadPoolSize = getDefaultAsyncThreadsCount());
   ~BackgroundIndex(); // Blocks while the current task finishes.
 
   // Enqueue a translation unit for indexing.
@@ -74,7 +76,8 @@
   std::condition_variable QueueCV;
   bool ShouldStop = false;
   std::deque<Task> Queue;
-  std::thread Thread; // Must be last, spawned thread reads instance vars.
+  // Must be last, spawned thread reads instance vars.
+  llvm::SmallVector<std::thread, 8> ThreadPool;
 };
 
 } // namespace clangd
Index: clangd/index/Background.cpp
===================================================================
--- clangd/index/Background.cpp
+++ clangd/index/Background.cpp
@@ -25,14 +25,20 @@
 BackgroundIndex::BackgroundIndex(Context BackgroundContext,
                                  StringRef ResourceDir,
                                  const FileSystemProvider &FSProvider,
-                                 ArrayRef<std::string> URISchemes)
+                                 ArrayRef<std::string> URISchemes,
+                                 size_t ThreadPoolSize)
     : SwapIndex(make_unique<MemIndex>()), ResourceDir(ResourceDir),
       FSProvider(FSProvider), BackgroundContext(std::move(BackgroundContext)),
-      URISchemes(URISchemes), Thread([this] { run(); }) {}
+      URISchemes(URISchemes) {
+  assert(ThreadPoolSize > 0 && "Thread pool size can't be zero.");
+  while(ThreadPoolSize--)
+    ThreadPool.emplace_back([this] { run(); });
+}
 
 BackgroundIndex::~BackgroundIndex() {
   stop();
-  Thread.join();
+  for (auto& Thread : ThreadPool)
+    Thread.join();
 }
 
 void BackgroundIndex::stop() {
@@ -44,7 +50,7 @@
 }
 
 void BackgroundIndex::run() {
-  WithContext Background(std::move(BackgroundContext));
+  WithContext Background(BackgroundContext.clone());
   while (true) {
     Optional<Task> Task;
     {
@@ -80,7 +86,7 @@
     std::lock_guard<std::mutex> Lock(QueueMu);
     enqueueLocked(std::move(Cmd));
   }
-  QueueCV.notify_all();
+  QueueCV.notify_one();
 }
 
 void BackgroundIndex::enqueueAll(StringRef Directory,