Cache fanout candidates to optimize txreconciliation

src/node/txreconciliation.cpp

@@ -30,6 +30,10 @@ constexpr size_t OUTBOUND_FANOUT_DESTINATIONS = 1;
  * reconciliation sets and short ids mappings, and CPU used for sketch computation.
  */
 constexpr size_t MAX_SET_SIZE = 3000;
+/**
+ * Maximum number of transactions for which we store assigned fanout targets.
+ */
+constexpr size_t FANOUT_TARGETS_PER_TX_CACHE_SIZE = 3000;
 /**
  * Salt (specified by BIP-330) constructed from contributions from both peers. It is used
  * to compute transaction short IDs, which are then used to construct a sketch representing a set
@@ -90,6 +94,19 @@ class TxReconciliationTracker::Impl
      */
     std::unordered_map<NodeId, std::variant<uint64_t, TxReconciliationState>> m_states GUARDED_BY(m_txreconciliation_mutex);
 
+    /*
+     * A least-recently-added cache tracking which peers we should fanout a transaction to.
+     *
+     * Since the time between cache accesses is on the order of seconds, returning an outdated
+     * set of peers is not a concern (especially since we fanout to outbound peers, which should
+     * be hard to manipulate).
+     *
+     * No need to use LRU (bump transaction order upon access) because in most cases
+     * transactions are processed almost-sequentially.
+     */
+    std::deque<Wtxid> tx_fanout_targets_cache_order;
+    std::map<Wtxid, std::set<NodeId>> tx_fanout_targets_cache_data GUARDED_BY(m_txreconciliation_mutex);
+
 public:
     explicit Impl(uint32_t recon_version) : m_recon_version(recon_version) {}
 
@@ -204,10 +221,19 @@ class TxReconciliationTracker::Impl
         return IsPeerRegistered(peer_id);
     }
 
-    bool IsFanoutTarget(const CSipHasher& deterministic_randomizer_with_wtxid,
+    bool IsFanoutTarget(CSipHasher&& deterministic_randomizer,
                         bool we_initiate, double limit,
-                        NodeId peer_id) const EXCLUSIVE_LOCKS_REQUIRED(m_txreconciliation_mutex)
+                        NodeId peer_id, const Wtxid& wtxid) EXCLUSIVE_LOCKS_REQUIRED(m_txreconciliation_mutex)
     {
+        auto fanout_candidates = tx_fanout_targets_cache_data.find(wtxid);
+        if (fanout_candidates != tx_fanout_targets_cache_data.end()) {
+            return fanout_candidates->second.find(peer_id) != fanout_candidates->second.end();
+        }
+
+        // We use the pre-determined randomness to give a consistent result per transaction,
+        // thus making sure that no transaction gets "unlucky" if every per-peer roll fails.
+        deterministic_randomizer.Write(wtxid.ToUint256());
+
         // The algorithm works as follows. We iterate through the peers (of a given direction)
         // hashing them with the given wtxid, and sort them by this hash.
         // We then consider top `limit` peers to be low-fanout flood targets.
@@ -217,15 +243,15 @@ class TxReconciliationTracker::Impl
         // The fractional part of `limit` is stored in the lower 32 bits, and then we check
         // whether adding a random lower 32-bit value (first element) would end up modifying
         // the higher bits.
-        const size_t targets_size = ((deterministic_randomizer_with_wtxid.Finalize() & 0xFFFFFFFF) + uint64_t(limit * 0x100000000)) >> 32;
+        const size_t targets_size = ((deterministic_randomizer.Finalize() & 0xFFFFFFFF) + uint64_t(limit * 0x100000000)) >> 32;
 
         std::vector<std::pair<uint64_t, NodeId>> best_peers;
         best_peers.reserve(m_states.size());
 
         for (const auto& indexed_state : m_states) {
             const auto cur_state = std::get_if<TxReconciliationState>(&indexed_state.second);
             if (cur_state && cur_state->m_we_initiate == we_initiate) {
-                uint64_t hash_key = CSipHasher(deterministic_randomizer_with_wtxid).Write(cur_state->m_k0).Finalize();
+                uint64_t hash_key = CSipHasher(deterministic_randomizer).Write(cur_state->m_k0).Finalize();
                 best_peers.emplace_back(hash_key, indexed_state.first);
             }
         }
@@ -235,16 +261,27 @@ class TxReconciliationTracker::Impl
         };
         std::sort(best_peers.begin(), best_peers.end(), cmp_by_key);
 
+        std::set<NodeId> new_fanout_candidates;
         auto it = best_peers.begin();
         for (size_t i = 0; i < targets_size && it != best_peers.end(); ++i, ++it) {
-            if (it->second == peer_id) return true;
+            new_fanout_candidates.insert(it->second);
         }
-        return false;
+
+        tx_fanout_targets_cache_data.emplace(wtxid, new_fanout_candidates);
+
+        // If the cache is full, make room for the new entry.
+        if (tx_fanout_targets_cache_order.size () == FANOUT_TARGETS_PER_TX_CACHE_SIZE) {
+            auto expired_tx = tx_fanout_targets_cache_order.front();
+            tx_fanout_targets_cache_data.erase(expired_tx);
+            tx_fanout_targets_cache_order.pop_front();
+        }
+        tx_fanout_targets_cache_order.push_back(wtxid);
+        return new_fanout_candidates.find(peer_id) != new_fanout_candidates.end();
     }
 
     bool ShouldFanoutTo(const Wtxid& wtxid, CSipHasher&& deterministic_randomizer, NodeId peer_id,
                         size_t inbounds_nonrcncl_tx_relay, size_t outbounds_nonrcncl_tx_relay)
-        const EXCLUSIVE_LOCKS_REQUIRED(!m_txreconciliation_mutex)
+        EXCLUSIVE_LOCKS_REQUIRED(!m_txreconciliation_mutex)
     {
         AssertLockNotHeld(m_txreconciliation_mutex);
         LOCK(m_txreconciliation_mutex);
@@ -281,10 +318,7 @@ class TxReconciliationTracker::Impl
             return false;
         }
 
-        // We use the pre-determined randomness to give a consistent result per transaction,
-        // thus making sure that no transaction gets "unlucky" if every per-peer roll fails.
-        deterministic_randomizer.Write(wtxid.ToUint256());
-        return IsFanoutTarget(std::move(deterministic_randomizer), recon_state.m_we_initiate, destinations, peer_id);
+        return IsFanoutTarget(std::move(deterministic_randomizer), recon_state.m_we_initiate, destinations, peer_id, wtxid);
     }
 };
 
@@ -323,8 +357,8 @@ bool TxReconciliationTracker::IsPeerRegistered(NodeId peer_id) const
     return m_impl->IsPeerRegisteredExternal(peer_id);
 }
 
-bool TxReconciliationTracker::ShouldFanoutTo(const Wtxid& wtxid, CSipHasher&& deterministic_randomizer, NodeId peer_id,
-                                             size_t inbounds_nonrcncl_tx_relay, size_t outbounds_nonrcncl_tx_relay) const
+bool TxReconciliationTracker::ShouldFanoutTo(const Wtxid& wtxid, CSipHasher deterministic_randomizer, NodeId peer_id,
+                                             size_t inbounds_nonrcncl_tx_relay, size_t outbounds_nonrcncl_tx_relay)
 {
     return m_impl->ShouldFanoutTo(wtxid, std::move(deterministic_randomizer), peer_id,
                                   inbounds_nonrcncl_tx_relay, outbounds_nonrcncl_tx_relay);

src/node/txreconciliation.h

@@ -102,8 +102,8 @@ class TxReconciliationTracker
     /**
      * Returns whether the peer is chosen as a low-fanout destination for a given tx.
      */
-    bool ShouldFanoutTo(const Wtxid& wtxid, CSipHasher&& deterministic_randomizer, NodeId peer_id,
-                        size_t inbounds_nonrcncl_tx_relay, size_t outbounds_nonrcncl_tx_relay) const;
+    bool ShouldFanoutTo(const Wtxid& wtxid, CSipHasher deterministic_randomizer, NodeId peer_id,
+                        size_t inbounds_nonrcncl_tx_relay, size_t outbounds_nonrcncl_tx_relay);
 };
 
 #endif // BITCOIN_NODE_TXRECONCILIATION_H