From eb6f3f86e5c64a14c06e24894c0ff64b4b99a462 Mon Sep 17 00:00:00 2001
From: Eric Liang <ekhliang@gmail.com>
Date: Thu, 12 Dec 2019 21:41:19 -0800
Subject: [PATCH] Seed using multiple samples (#6471)

---
 src/ray/util/util.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/ray/util/util.h b/src/ray/util/util.h
index 4e4f71ccd..7f7e864b4 100644
--- a/src/ray/util/util.h
+++ b/src/ray/util/util.h
@@ -8,6 +8,7 @@
 #include <random>
 #include <sstream>
 #include <string>
+#include <thread>
 #include <unordered_map>
 
 #include "ray/common/status.h"
@@ -95,6 +96,13 @@ void FillRandom(T *data) {
   RAY_CHECK(data != nullptr);
   auto randomly_seeded_mersenne_twister = []() {
     auto seed = std::chrono::high_resolution_clock::now().time_since_epoch().count();
+    // To increase the entropy, mix in a number of time samples instead of a single one.
+    // This avoids the possibility of duplicate seeds for many workers that start in
+    // close succession.
+    for (int i = 0; i < 128; i++) {
+      std::this_thread::sleep_for(std::chrono::microseconds(10));
+      seed += std::chrono::high_resolution_clock::now().time_since_epoch().count();
+    }
     std::mt19937 seeded_engine(seed);
     return seeded_engine;
   };