diff --git a/clang-tools-extra/clangd/index/FileIndex.cpp b/clang-tools-extra/clangd/index/FileIndex.cpp
--- a/clang-tools-extra/clangd/index/FileIndex.cpp
+++ b/clang-tools-extra/clangd/index/FileIndex.cpp
@@ -32,6 +32,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
@@ -40,11 +41,17 @@
 #include <tuple>
 #include <utility>
 #include <vector>
+#include <malloc.h>
 
 namespace clang {
 namespace clangd {
 namespace {
 
+void trimMemory() {
+  if (malloc_trim(0))
+    vlog("Trimmed memory");
+}
+
 SlabTuple indexSymbols(ASTContext &AST, std::shared_ptr<Preprocessor> PP,
                        llvm::ArrayRef<Decl *> DeclsToIndex,
                        const MainFileMacros *MacroRefsToIndex,
@@ -263,6 +270,10 @@
 std::unique_ptr<SymbolIndex>
 FileSymbols::buildIndex(IndexType Type, DuplicateHandling DuplicateHandle,
                         size_t *Version) {
+  // Building the index often leaves a lof of freed but unreleased memory
+  // (at the OS level): manually trim the memory after the index is built
+  auto _ = llvm::make_scope_exit(trimMemory);
+
   std::vector<std::shared_ptr<SymbolSlab>> SymbolSlabs;
   std::vector<std::shared_ptr<RefSlab>> RefSlabs;
   std::vector<std::shared_ptr<RelationSlab>> RelationSlabs;