Index: clang-tools-extra/trunk/clangd/ClangdServer.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/ClangdServer.cpp
+++ clang-tools-extra/trunk/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, 1u));
     AddIndex(BackgroundIdx.get());
   }
   if (DynamicIdx)
Index: clang-tools-extra/trunk/clangd/TUScheduler.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/TUScheduler.cpp
+++ clang-tools-extra/trunk/clangd/TUScheduler.cpp
@@ -54,6 +54,7 @@
 #include "llvm/ADT/ScopeExit.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Threading.h"
 #include <algorithm>
 #include <memory>
 #include <queue>
@@ -801,10 +802,10 @@
 } // namespace
 
 unsigned getDefaultAsyncThreadsCount() {
-  unsigned HardwareConcurrency = std::thread::hardware_concurrency();
-  // C++ standard says that hardware_concurrency()
-  // may return 0, fallback to 1 worker thread in
-  // that case.
+  unsigned HardwareConcurrency = llvm::heavyweight_hardware_concurrency();
+  // heavyweight_hardware_concurrency may fall back to hardware_concurrency.
+  // C++ standard says that hardware_concurrency() may return 0; fallback to 1
+  // worker thread in that case.
   if (HardwareConcurrency == 0)
     return 1;
   return HardwareConcurrency;
Index: clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp
+++ clang-tools-extra/trunk/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,
 };