Simpler variant on multicast propagation algorithm seems to perform better by being less deterministic. May also be faster.

node/Multicaster.hpp

@@ -119,19 +119,6 @@ public:
 		return id.verifySignature(digest,signature,siglen);
 	}
 
-	/**
-	 * Update the most recent LIKE time for an address in a given multicast group on a given network
-	 *
-	 * @param nwid Network ID
-	 * @param mg Multicast group
-	 * @param addr Address that likes group on given network
-	 * @param now Current timestamp
-	 */
-	inline void likesMulticastGroup(const uint64_t nwid,const MulticastGroup &mg,const Address &addr,const uint64_t now)
-	{
-		_multicastMemberships[MulticastChannel(nwid,mg)][addr] = now;
-	}
-
 	/**
 	 * Compute the CRC64 code for multicast deduplication
 	 *
@@ -194,6 +181,27 @@ public:
 		_multicastHistory[mhi][1] = now;
 	}
 
+	/**
+	 * Update the most recent LIKE time for an address in a given multicast group on a given network
+	 *
+	 * @param nwid Network ID
+	 * @param mg Multicast group
+	 * @param addr Address that likes group on given network
+	 * @param now Current timestamp
+	 */
+	inline void likesMulticastGroup(const uint64_t nwid,const MulticastGroup &mg,const Address &addr,const uint64_t now)
+	{
+		Mutex::Lock _l(_multicastMemberships_m);
+		std::vector<MulticastMembership> &memberships = _multicastMemberships[MulticastChannel(nwid,mg)];
+		for(std::vector<MulticastMembership>::iterator mm(memberships.begin());mm!=memberships.end();++mm) {
+			if (mm->first == addr) {
+				mm->second = now;
+				return;
+			}
+		}
+		memberships.push_back(MulticastMembership(addr,now));
+	}
+
 	/**
 	 * Choose peers to send a propagating multicast to
 	 *
@@ -223,100 +231,50 @@ public:
 		P *peers,
 		uint64_t now)
 	{
-		P toConsider[ZT_MULTICAST_PICK_MAX_SAMPLE_SIZE];
-		unsigned int sampleSize = 0;
-
-		// Decay a few random bits in bloom filter to probabilistically eliminate
-		// false positives as we go. The odds of decaying an already-set bit
-		// increases as the bloom filter saturates, so in the early hops of
-		// propagation this likely won't have any effect. This allows peers with
-		// bloom filter collisions to be reconsidered, but at positions on the
-		// network graph likely to be hops away from the original origin of the
+		typename std::set< P,_PeerPropagationPrioritySortOrder<P> > toConsider;
+
+		// Pick up to ZT_MULTICAST_PICK_MAX_SAMPLE_SIZE peers that have
+		// subscribed to this channel and that are not in bloom filter.
+		// Pick randomly from subscribers, but place into a set that is
+		// sorted in descending order of time of most recent unicast
+		// frame transfer. (Implicit social ordering.) Also ignore original
+		// submitter and upstream, since we know these have seen this
 		// message.
-		for(unsigned int i=0;i<ZT_MULTICAST_BLOOM_FILTER_DECAY_RATE;++i)
-			bf.decay((unsigned int)prng.next32());
-
 		{
 			Mutex::Lock _l(_multicastMemberships_m);
-
-			// Sample a random subset of peers that we know have LIKEd this multicast
-			// group on this network.
-			std::map< MulticastChannel,std::map<Address,uint64_t> >::iterator channelMembers(_multicastMemberships.find(MulticastChannel(nwid,mg)));
-			if ((channelMembers != _multicastMemberships.end())&&(!channelMembers->second.empty())) {
-				unsigned long numEntriesPermittedToSkip = (channelMembers->second.size() > ZT_MULTICAST_PICK_MAX_SAMPLE_SIZE) ? (unsigned long)(channelMembers->second.size() - ZT_MULTICAST_PICK_MAX_SAMPLE_SIZE) : (unsigned long)0;
-				double skipWhatFraction = (double)numEntriesPermittedToSkip / (double)channelMembers->second.size();
-
-				std::map<Address,uint64_t>::iterator channelMemberEntry(channelMembers->second.begin());
-
-				while (channelMemberEntry != channelMembers->second.end()) {
-					// Auto-clean the channel members map if their LIKEs are expired. This will
-					// technically skew the random distribution of chosen members just a little, but
-					// it's unlikely that enough will expire in any single pick to make much of a
-					// difference overall.
-					if ((now - channelMemberEntry->second) > ZT_MULTICAST_LIKE_EXPIRE) {
-						channelMembers->second.erase(channelMemberEntry++);
-						continue;
-					}
-
-					// Skip some fraction of entries so that our sampling will be randomly distributed,
-					// since there is no other good way to sample randomly from a map.
-					if (numEntriesPermittedToSkip) {
-						if (prng.nextDouble() <= skipWhatFraction) {
-							--numEntriesPermittedToSkip;
-							++channelMemberEntry;
-							continue;
-						}
-					}
-
-					// If it's not expired and it's from our random sample, add it to the set of peers
-					// to consider. Exclude immediate upstream and original submitter, since we know for
-					// a fact they've already seen this. Also exclude things in the bloom filter.
-					if ((channelMemberEntry->first != originalSubmitter)&&(channelMemberEntry->first != upstream)) {
-						if (!bf.contains(channelMemberEntry->first.sum())) {
-							P peer = topology.getPeer(channelMemberEntry->first);
-							if ((peer)&&(peer->hasActiveDirectPath(now))) {
-								toConsider[sampleSize++] = peer;
-								if (sampleSize >= ZT_MULTICAST_PICK_MAX_SAMPLE_SIZE)
-									break; // abort if we have enough candidates
-							}
-						}
+			std::map< MulticastChannel,std::vector<MulticastMembership> >::iterator mm(_multicastMemberships.find(MulticastChannel(nwid,mg)));
+			if ((mm != _multicastMemberships.end())&&(!mm->second.empty())) {
+				for(unsigned int stries=0;stries<ZT_MULTICAST_PICK_MAX_SAMPLE_SIZE;++stries) {
+					MulticastMembership &m = mm->second[prng.next32() % mm->second.size()];
+					if (((now - m.second) < ZT_MULTICAST_LIKE_EXPIRE)&&(!bf.contains(m.first.sum()))&&(m.first != originalSubmitter)&&(m.first != upstream)) {
+						P peer(topology.getPeer(m.first));
+						if (peer)
+							toConsider.insert(peer);
 					}
-					++channelMemberEntry;
 				}
-
-				// Auto-clean: erase whole map if there are no more LIKEs for this channel
-				if (channelMembers->second.empty())
-					_multicastMemberships.erase(channelMembers);
 			}
 		}
 
-		// Sort in descending order of most recent direct unicast frame, picking
-		// peers with whom we have recently communicated. This is "implicit social
-		// switching."
-		std::sort(toConsider,toConsider + sampleSize,PeerPropagationPrioritySortOrder<P>());
-
-		// Pick the best N peers
-		unsigned int picked = 0;
-		for(unsigned int i=0;((i<sampleSize)&&(picked < max));++i) {
-			peers[picked++] = toConsider[i];
-			bf.set(toConsider[i]->address().sum());
-		}
+		// The first peers in toConsider will be the 'best'
+		unsigned int chosen = 0;
+		for(typename std::set< P,_PeerPropagationPrioritySortOrder<P> >::iterator i(toConsider.begin());((i!=toConsider.end())&&(chosen < max));++i)
+			bf.set((peers[chosen++] = *i)->address().sum());
 
-		// Add a supernode if there's nowhere else to go. Supernodes know of all multicast
-		// LIKEs and so can act to bridge sparse multicast groups.
-		if (!picked) {
+		// Add a supernode if there are fewer than the desired
+		// number of recipients.
+		if (chosen < max) {
 			P peer = topology.getBestSupernode(&originalSubmitter,1,true);
 			if (peer)
-				peers[picked++] = peer;
+				peers[chosen++] = peer;
 		}
 
-		return picked;
+		return chosen;
 	}
 
 private:
 	// Sort order for chosen propagation peers
 	template<typename P>
-	struct PeerPropagationPrioritySortOrder
+	struct _PeerPropagationPrioritySortOrder
 	{
 		inline bool operator()(const P &p1,const P &p2) const
 		{
@@ -355,8 +313,10 @@ private:
 	// network ID and a multicast group within that network.
 	typedef std::pair<uint64_t,MulticastGroup> MulticastChannel;
 
-	// Address and time of last LIKE, by network ID and multicast group
-	std::map< MulticastChannel,std::map<Address,uint64_t> > _multicastMemberships;
+	// Address and time of last LIKE
+	typedef std::pair<Address,uint64_t> MulticastMembership;
+
+	std::map< MulticastChannel,std::vector<MulticastMembership> > _multicastMemberships;
 	Mutex _multicastMemberships_m;
 };