diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -43,6 +43,7 @@
 #include <future>
 #include <memory>
 #include <mutex>
+#include <type_traits>
 
 namespace clang {
 namespace clangd {
@@ -141,10 +142,19 @@
   if (Opts.StaticIndex)
     AddIndex(Opts.StaticIndex);
   if (Opts.BackgroundIndex) {
-    BackgroundIdx = llvm::make_unique<BackgroundIndex>(
-        Context::current().clone(), FSProvider, CDB,
-        BackgroundIndexStorage::createDiskBackedStorageFactory(
-            [&CDB](llvm::StringRef File) { return CDB.getProjectInfo(File); }));
+    auto&& DBSF = BackgroundIndexStorage::createDiskBackedStorageFactory(
+            [&CDB](llvm::StringRef File) { return CDB.getProjectInfo(File); });
+
+    if (Opts.AsyncThreadsCount) {
+      BackgroundIdx = llvm::make_unique<BackgroundIndex>(
+          Context::current().clone(), FSProvider, CDB,
+          std::forward<decltype(DBSF)>(DBSF),
+          Opts.AsyncThreadsCount );
+    } else {
+      BackgroundIdx = llvm::make_unique<BackgroundIndex>(
+          Context::current().clone(), FSProvider, CDB,
+          std::forward<decltype(DBSF)>( DBSF ) );
+    }
     AddIndex(BackgroundIdx.get());
   }
   if (DynamicIdx)
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -308,7 +308,7 @@
 opt<bool> Sync{
     "sync",
     cat(Misc),
-    desc("Parse on main thread. If set, -j is ignored"),
+    desc("Parse on main thread. If set, -j is only applied to background index"),
     init(false),
     Hidden,
 };