Stop purging arenas in MemoryIdler

Summary:
jemalloc has background threads to purge unused pages, so stop purging
arenas used by idle threads since those arenas may be shared with active
threads which must page fault new pages in after the purge

Reviewed By: ot, davidtgoldblatt

Differential Revision: D26793042

fbshipit-source-id: 13617bfb0ace1c5b7aae52dfdc428b19ff99f7bf

folly/detail/MemoryIdler.cpp

@@ -27,10 +27,16 @@
 #include <folly/concurrency/CacheLocality.h>
 #include <folly/memory/MallctlHelper.h>
 #include <folly/memory/Malloc.h>
+#include <folly/portability/GFlags.h>
 #include <folly/portability/PThread.h>
 #include <folly/portability/SysMman.h>
 #include <folly/portability/Unistd.h>
 
+DEFINE_bool(
+    folly_memory_idler_purge_arenas,
+    false,
+    "if enabled, folly memory-idler purges jemalloc arenas on thread idle");
+
 namespace folly {
 namespace detail {
 
@@ -46,10 +52,11 @@ void MemoryIdler::flushLocalMallocCaches() {
     return;
   }
 
-  try {
   // Not using mallctlCall as this will fail if tcache is disabled.
   mallctl("thread.tcache.flush", nullptr, nullptr, nullptr, 0);
 
+  if (!FLAGS_folly_memory_idler_purge_arenas) {
+    try {
       // By default jemalloc has 4 arenas per cpu, and then assigns each
       // thread to one of those arenas.  This means that in any service
       // that doesn't perform a lot of context switching, the chances that
@@ -75,6 +82,7 @@ void MemoryIdler::flushLocalMallocCaches() {
     } catch (const std::runtime_error& ex) {
       FB_LOG_EVERY_MS(WARNING, 10000) << ex.what();
     }
+  }
 }
 
 // Stack madvise isn't Linux or glibc specific, but the system calls