Merge pull request #4835 from clemahieu/fanout_log

Change fanout to scale by std::log rather than std::sqrt

nano/core_test/peer_container.cpp

@@ -174,8 +174,8 @@ TEST (peer_container, DISABLED_list_fanout)
 	nano::test::system system{ 1 };
 	auto node = system.nodes[0];
 	ASSERT_EQ (0, node->network.size ());
-	ASSERT_EQ (0.0, node->network.size_sqrt ());
-	ASSERT_EQ (0, node->network.fanout ());
+	ASSERT_EQ (0.0f, node->network.size_log ());
+	ASSERT_EQ (1, node->network.fanout ());
 	ASSERT_TRUE (node->network.list (node->network.fanout ()).empty ());
 
 	auto add_peer = [&node, &system] () {
@@ -185,26 +185,27 @@ TEST (peer_container, DISABLED_list_fanout)
 
 	add_peer ();
 	ASSERT_TIMELY_EQ (5s, 1, node->network.size ());
-	ASSERT_EQ (1.f, node->network.size_sqrt ());
+	ASSERT_EQ (0.0f, node->network.size_log ());
 	ASSERT_EQ (1, node->network.fanout ());
 	ASSERT_EQ (1, node->network.list (node->network.fanout ()).size ());
 
 	add_peer ();
-	ASSERT_TIMELY_EQ (5s, 2, node->network.size ());
-	ASSERT_EQ (std::sqrt (2.f), node->network.size_sqrt ());
+	add_peer ();
+	ASSERT_TIMELY_EQ (5s, 3, node->network.size ());
+	ASSERT_EQ (std::log (3.0f), node->network.size_log ());
 	ASSERT_EQ (2, node->network.fanout ());
 	ASSERT_EQ (2, node->network.list (node->network.fanout ()).size ());
 
 	unsigned number_of_peers = 10;
-	for (unsigned i = 2; i < number_of_peers; ++i)
+	for (unsigned i = 3; i < number_of_peers; ++i)
 	{
 		add_peer ();
 	}
 
 	ASSERT_TIMELY_EQ (5s, number_of_peers, node->network.size ());
-	ASSERT_EQ (std::sqrt (float (number_of_peers)), node->network.size_sqrt ());
-	ASSERT_EQ (4, node->network.fanout ());
-	ASSERT_EQ (4, node->network.list (node->network.fanout ()).size ());
+	ASSERT_EQ (std::log (float (number_of_peers)), node->network.size_log ());
+	ASSERT_EQ (3, node->network.fanout ());
+	ASSERT_EQ (3, node->network.list (node->network.fanout ()).size ());
 }
 
 // Test to make sure we don't repeatedly send keepalive messages to nodes that aren't responding

nano/node/network.cpp

@@ -430,7 +430,8 @@ std::deque<std::shared_ptr<nano::transport::channel>> nano::network::list_non_pr
 // Simulating with sqrt_broadcast_simulate shows we only need to broadcast to sqrt(total_peers) random peers in order to successfully publish to everyone with high probability
 std::size_t nano::network::fanout (float scale) const
 {
-	return static_cast<std::size_t> (std::ceil (scale * size_sqrt ()));
+	auto fanout_l = std::max (2.0f, size_log ());
+	return static_cast<std::size_t> (std::ceil (scale * fanout_l));
 }
 
 std::unordered_set<std::shared_ptr<nano::transport::channel>> nano::network::random_set (std::size_t max_count, uint8_t minimum_version) const
@@ -519,9 +520,10 @@ std::size_t nano::network::size () const
 	return tcp_channels.size ();
 }
 
-float nano::network::size_sqrt () const
+float nano::network::size_log () const
 {
-	return static_cast<float> (std::sqrt (size ()));
+	auto size_l = std::max (static_cast<size_t> (1u), size ()); // Clamp size to domain of std::log
+	return static_cast<float> (std::log (size_l));
 }
 
 bool nano::network::empty () const
@@ -735,4 +737,4 @@ nano::container_info nano::syn_cookies::container_info () const
 	info.put ("syn_cookies", cookies.size ());
 	info.put ("syn_cookies_per_ip", cookies_per_ip.size ());
 	return info;
-}
+}

nano/node/network.hpp

@@ -134,7 +134,7 @@ class network final
 	nano::tcp_endpoint bootstrap_peer ();
 	void cleanup (std::chrono::steady_clock::time_point const & cutoff);
 	std::size_t size () const;
-	float size_sqrt () const;
+	float size_log () const;
 	bool empty () const;
 	void erase (nano::transport::channel const &);
 	/** Disconnects and adds peer to exclusion list */