Start jemalloc background threads manually after hugifyText

Summary:
We cannot have threads running when remapping hottext, so don't create background threads automatically.

The background threads are only created for server mode, after hugefyText.  Script mode won't have it, which I think is fine.

When running against older versions of jemalloc (in some OSS builds), the mallctl will fail, and we log a line, but nothing bad happens.

Reviewed By: markw65

Differential Revision: D7054160

fbshipit-source-id: 3e7e8d2f093d683270959cd240a9fa21d4f7df53

hphp/runtime/base/program-functions.cpp

@@ -76,6 +76,7 @@
 #include "hphp/runtime/vm/treadmill.h"
 
 #include "hphp/util/abi-cxx.h"
+#include "hphp/util/alloc.h"
 #include "hphp/util/arch.h"
 #include "hphp/util/boot-stats.h"
 #include "hphp/util/build-info.h"
@@ -2263,6 +2264,13 @@ void hphp_process_init() {
     hphp_process_exit();
     exit(0);
   }
+
+#if defined(USE_JEMALLOC) && (JEMALLOC_VERSION_MAJOR >= 5)
+  // jemalloc 5 has background threads, which handle purging asynchronously.
+  if (mallctlWrite("background_thread", true, /* errorOK */ true)) {
+    Logger::Warning("Failed to enable jemalloc background threads");
+  }
+#endif
 }
 
 static void handle_exception(bool& ret, ExecutionContext* context,

hphp/util/alloc.cpp

@@ -782,9 +782,10 @@ void thread_huge_tcache_destroy() {
 extern "C" {
   const char* malloc_conf = "narenas:1,lg_tcache_max:16"
 #if (JEMALLOC_VERSION_MAJOR >= 5)
-// FB-only as this feature does not exist in any jemalloc release as of
-// 2017-11-02 (latest: 5.0.1)
+    ",background_thread:false"
 #ifdef FACEBOOK
+    // FB-only as this feature does not exist in any jemalloc release as of
+    // 2017-11-02 (latest: 5.0.1)
     ",metadata_thp:disabled"
 #endif
 #endif