diff --git a/clang-tools-extra/clangd/index/Background.h b/clang-tools-extra/clangd/index/Background.h
--- a/clang-tools-extra/clangd/index/Background.h
+++ b/clang-tools-extra/clangd/index/Background.h
@@ -135,7 +135,9 @@
       Context BackgroundContext, const ThreadsafeFS &,
       const GlobalCompilationDatabase &CDB,
       BackgroundIndexStorage::Factory IndexStorageFactory,
-      size_t ThreadPoolSize = 0, // 0 = use all hardware threads
+      // Arbitrary value to ensure some concurrency in tests.
+      // In production an explicit value is passed.
+      size_t ThreadPoolSize = 4,
       std::function<void(BackgroundQueue::Stats)> OnProgress = nullptr);
   ~BackgroundIndex(); // Blocks while the current task finishes.
 
diff --git a/clang-tools-extra/clangd/index/Background.cpp b/clang-tools-extra/clangd/index/Background.cpp
--- a/clang-tools-extra/clangd/index/Background.cpp
+++ b/clang-tools-extra/clangd/index/Background.cpp
@@ -103,10 +103,9 @@
           CDB.watch([&](const std::vector<std::string> &ChangedFiles) {
             enqueue(ChangedFiles);
           })) {
-  assert(Rebuilder.TUsBeforeFirstBuild > 0 &&
-         "Thread pool size can't be zero.");
+  assert(ThreadPoolSize > 0 && "Thread pool size can't be zero.");
   assert(this->IndexStorageFactory && "Storage factory can not be null!");
-  for (unsigned I = 0; I < Rebuilder.TUsBeforeFirstBuild; ++I) {
+  for (unsigned I = 0; I < ThreadPoolSize; ++I) {
     ThreadPool.runAsync("background-worker-" + llvm::Twine(I + 1), [this] {
       WithContext Ctx(this->BackgroundContext.clone());
       Queue.work([&] { Rebuilder.idle(); });
diff --git a/clang-tools-extra/clangd/index/BackgroundRebuild.h b/clang-tools-extra/clangd/index/BackgroundRebuild.h
--- a/clang-tools-extra/clangd/index/BackgroundRebuild.h
+++ b/clang-tools-extra/clangd/index/BackgroundRebuild.h
@@ -49,9 +49,7 @@
 public:
   BackgroundIndexRebuilder(SwapIndex *Target, FileSymbols *Source,
                            unsigned Threads)
-      : TUsBeforeFirstBuild(llvm::heavyweight_hardware_concurrency(Threads)
-                                .compute_thread_count()),
-        Target(Target), Source(Source) {}
+      : TUsBeforeFirstBuild(Threads), Target(Target), Source(Source) {}
 
   // Called to indicate a TU has been indexed.
   // May rebuild, if enough TUs have been indexed.
@@ -72,7 +70,7 @@
   // Ensures we won't start any more rebuilds.
   void shutdown();
 
-  // Thresholds for rebuilding as TUs get indexed.
+  // Thresholds for rebuilding as TUs get indexed. Exposed for testing.
   const unsigned TUsBeforeFirstBuild; // Typically one per worker thread.
   const unsigned TUsBeforeRebuild = 100;