From 4bd6ebb49584cff4d364d8586caa35f37e967147 Mon Sep 17 00:00:00 2001
From: Sam McCall <sam.mccall@gmail.com>
Date: Fri, 9 Aug 2019 23:03:32 +0000
Subject: [PATCH] clangd: use -j for background index pool

Summary:
clangd supports a -j option to limit the amount of threads to use for parsing
TUs. However, when using -background-index (the default in later versions of
clangd), the parallelism used by clangd defaults to the hardware_parallelisn,
i.e. number of physical cores.

On shared hardware environments, with large projects, this can significantly
affect performance with no way to tune it down.

This change makes the -j parameter apply equally to parsing and background
index. It's not perfect, because the total number of threads is 2x the -j value,
which may still be unexpected. But at least this change allows users to prevent
clangd using all CPU cores.

Reviewers: kadircet, sammccall

Reviewed By: sammccall

Subscribers: javed.absar, jfb, sammccall, ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66031

llvm-svn: 368498
---
 clang-tools-extra/clangd/ClangdServer.cpp    | 8 +++++---
 clang-tools-extra/clangd/TUScheduler.cpp     | 9 +++++----
 clang-tools-extra/clangd/tool/ClangdMain.cpp | 6 ++++--
 3 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
index 816c423..ae2b2d9 100644
--- 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 @@ ClangdServer::ClangdServer(const GlobalCompilationDatabase &CDB,
                      : 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 @@ ClangdServer::ClangdServer(const GlobalCompilationDatabase &CDB,
     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)
diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
index bcfddb9..040c1bd 100644
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/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 @@ std::string renderTUAction(const TUAction &Action) {
 } // 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;
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
index 337f525..bfc7ad9 100644
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -267,7 +267,8 @@ list<std::string> TweakList{
 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<PCHStorageFlag> PCHStorage{
 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,
 };
-- 
2.7.4