[base] add build flag to use MADV_DONTFORK

Keyhan Vakil · V8 LUCI CQ · commit 1a782f6543ae · 2023-06-22T16:35:11.000Z
Embedders like Node.js and Electron expose fork(2)/execve(2) to their users. Unfortunately when the V8 heap is very large, these APIs become rather slow on Linux, due to the kernel needing to do all the bookkeeping for the forked process (in clone's dup_mmap and execve's exec_mmap). Of course, this is useless because the forked child thread will never actually need to access the V8 heap. Add a new build flag v8_enable_private_mapping_fork_optimization which marks all pages allocated by OS::Allocate as MADV_DONTFORK. This improves the performance of Node.js's fork/execve combination by 10x on a 600 MB heap. Fixed: v8:7381 Change-Id: Ib649f774d4a932b41886313ce89acc369923699d Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4602858 Commit-Queue: Michael Lippautz <mlippautz@chromium.org> Reviewed-by: Michael Lippautz <mlippautz@chromium.org> Cr-Commit-Position: refs/heads/main@{#88447}
diff --git a/BUILD.gn b/BUILD.gn
@@ -83,6 +83,17 @@ declare_args() {
   # Sets -dENABLE_HUGEPAGE
   v8_enable_hugepage = false
 
+  # Sets -dV8_ENABLE_PRIVATE_MAPPING_FORK_OPTIMIZATION.
+  #
+  # This flag speeds up the performance of fork/execve on Linux systems for
+  # embedders which use it (like Node.js). It works by marking the pages that
+  # V8 allocates as MADV_DONTFORK. Without MADV_DONTFORK, the Linux kernel
+  # spends a long time manipulating page mappings on fork and exec which the
+  # child process doesn't generally need to access.
+  #
+  # See v8:7381 for more details.
+  v8_enable_private_mapping_fork_optimization = false
+
   # Sets -dENABLE_HANDLE_ZAPPING.
   v8_enable_handle_zapping = is_asan || is_debug
 
@@ -1005,6 +1016,9 @@ config("features") {
   if (v8_enable_hugepage) {
     defines += [ "ENABLE_HUGEPAGE" ]
   }
+  if (v8_enable_private_mapping_fork_optimization) {
+    defines += [ "V8_ENABLE_PRIVATE_MAPPING_FORK_OPTIMIZATION" ]
+  }
   if (v8_enable_object_print) {
     defines += [ "OBJECT_PRINT" ]
   }
diff --git a/src/base/platform/platform-posix.cc b/src/base/platform/platform-posix.cc
@@ -160,6 +160,12 @@ void* Allocate(void* hint, size_t size, OS::MemoryPermission access,
   int flags = GetFlagsForMemoryPermission(access, page_type);
   void* result = mmap(hint, size, prot, flags, kMmapFd, kMmapFdOffset);
   if (result == MAP_FAILED) return nullptr;
+
+#if V8_ENABLE_PRIVATE_MAPPING_FORK_OPTIMIZATION
+  // This is advisory, so we ignore errors.
+  madvise(result, size, MADV_DONTFORK);
+#endif
+
 #if ENABLE_HUGEPAGE
   if (result != nullptr && size >= kHugePageSize) {
     const uintptr_t huge_start =
