Index: clang-tools-extra/trunk/clangd/Threading.h
===================================================================
--- clang-tools-extra/trunk/clangd/Threading.h
+++ clang-tools-extra/trunk/clangd/Threading.h
@@ -17,6 +17,7 @@
 #include <condition_variable>
 #include <memory>
 #include <mutex>
+#include <thread>
 #include <vector>
 
 namespace clang {
@@ -115,6 +116,13 @@
   mutable std::condition_variable TasksReachedZero;
   std::size_t InFlightTasks = 0;
 };
+
+enum class ThreadPriority {
+  Low = 0,
+  Normal = 1,
+};
+void setThreadPriority(std::thread &T, ThreadPriority Priority);
+
 } // namespace clangd
 } // namespace clang
 #endif
Index: clang-tools-extra/trunk/clangd/Threading.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/Threading.cpp
+++ clang-tools-extra/trunk/clangd/Threading.cpp
@@ -1,9 +1,13 @@
 #include "Threading.h"
 #include "Trace.h"
 #include "llvm/ADT/ScopeExit.h"
+#include "llvm/Config/config.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/Threading.h"
 #include <thread>
+#ifdef HAVE_PTHREAD_H
+#include <pthread.h>
+#endif
 
 using namespace llvm;
 namespace clang {
@@ -97,5 +101,15 @@
   CV.wait_until(Lock, D.time());
 }
 
+void setThreadPriority(std::thread &T, ThreadPriority Priority) {
+#ifdef HAVE_PTHREAD_H
+  sched_param priority;
+  priority.sched_priority = 0;
+  pthread_setschedparam(
+      T.native_handle(),
+      Priority == ThreadPriority::Low ? SCHED_IDLE : SCHED_OTHER, &priority);
+#endif
+}
+
 } // namespace clangd
 } // namespace clang
Index: clang-tools-extra/trunk/clangd/index/Background.h
===================================================================
--- clang-tools-extra/trunk/clangd/index/Background.h
+++ clang-tools-extra/trunk/clangd/index/Background.h
@@ -16,6 +16,7 @@
 #include "index/Index.h"
 #include "clang/Tooling/CompilationDatabase.h"
 #include "llvm/Support/SHA1.h"
+#include "llvm/Support/Threading.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 = llvm::hardware_concurrency());
   ~BackgroundIndex(); // Blocks while the current task finishes.
 
   // Enqueue a translation unit for indexing.
@@ -66,7 +68,7 @@
   llvm::StringMap<Hash> FileHash; // Digest of indexed file.
 
   // queue management
-  using Task = std::function<void()>; // FIXME: use multiple worker threads.
+  using Task = std::function<void()>;
   void run(); // Main loop executed by Thread. Runs tasks from Queue.
   void enqueueLocked(tooling::CompileCommand Cmd);
   std::mutex QueueMu;
@@ -74,7 +76,7 @@
   std::condition_variable QueueCV;
   bool ShouldStop = false;
   std::deque<Task> Queue;
-  std::thread Thread; // Must be last, spawned thread reads instance vars.
+  std::vector<std::thread> ThreadPool; // FIXME: Abstract this away.
 };
 
 } // namespace clangd
Index: clang-tools-extra/trunk/clangd/index/Background.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/index/Background.cpp
+++ clang-tools-extra/trunk/clangd/index/Background.cpp
@@ -11,6 +11,7 @@
 #include "ClangdUnit.h"
 #include "Compiler.h"
 #include "Logger.h"
+#include "Threading.h"
 #include "Trace.h"
 #include "index/IndexAction.h"
 #include "index/MemIndex.h"
@@ -25,14 +26,26 @@
 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(); });
+    // Set priority to low, since background indexing is a long running task we
+    // do not want to eat up cpu when there are any other high priority threads.
+    // FIXME: In the future we might want a more general way of handling this to
+    // support a tasks with various priorities.
+    setThreadPriority(ThreadPool.back(), ThreadPriority::Low);
+  }
+}
 
 BackgroundIndex::~BackgroundIndex() {
   stop();
-  Thread.join();
+  for (auto &Thread : ThreadPool)
+    Thread.join();
 }
 
 void BackgroundIndex::stop() {
@@ -44,7 +57,7 @@
 }
 
 void BackgroundIndex::run() {
-  WithContext Background(std::move(BackgroundContext));
+  WithContext Background(BackgroundContext.clone());
   while (true) {
     Optional<Task> Task;
     {