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
@@ -40,9 +40,11 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
+#include <algorithm>
 #include <future>
 #include <memory>
 #include <mutex>
+#include <type_traits>
 
 namespace clang {
 namespace clangd {
@@ -117,8 +119,7 @@
                      : nullptr),
       GetClangTidyOptions(Opts.GetClangTidyOptions),
       SuggestMissingIncludes(Opts.SuggestMissingIncludes),
-      TweakFilter(Opts.TweakFilter),
-      WorkspaceRoot(Opts.WorkspaceRoot),
+      TweakFilter(Opts.TweakFilter), WorkspaceRoot(Opts.WorkspaceRoot),
       // Pass a callback into `WorkScheduler` to extract symbols from a newly
       // parsed file and rebuild the file index synchronously each time an AST
       // is parsed.
@@ -144,7 +145,8 @@
     BackgroundIdx = llvm::make_unique<BackgroundIndex>(
         Context::current().clone(), FSProvider, CDB,
         BackgroundIndexStorage::createDiskBackedStorageFactory(
-            [&CDB](llvm::StringRef File) { return CDB.getProjectInfo(File); }));
+            [&CDB](llvm::StringRef File) { return CDB.getProjectInfo(File); }),
+        std::max(Opts.AsyncThreadsCount, static_cast<unsigned int>(1)));
     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
@@ -267,7 +267,8 @@
 opt<unsigned> WorkerThreadsCount{
     "j",
     cat(Misc),
-    desc("Number of async workers used by clangd"),
+    desc("Number of async workers used by clangd. Background index also "
+         "uses this many workers."),
     init(getDefaultAsyncThreadsCount()),
 };
 
@@ -308,7 +309,8 @@
 opt<bool> Sync{
     "sync",
     cat(Misc),
-    desc("Parse on main thread. If set, -j is ignored"),
+    desc("Handle client requests on main thread. Background index still uses "
+         "its own thread."),
     init(false),
     Hidden,
 };