bitcoin#17925: Improve UpdateTransactionsFromBlock with Epochs

src/txmempool.cpp

@@ -29,7 +29,7 @@ CTxMemPoolEntry::CTxMemPoolEntry(const CTransactionRef& _tx, const CAmount& _nFe
                                  int64_t _nTime, unsigned int _entryHeight,
                                  bool _spendsCoinbase, unsigned int _sigOps, LockPoints lp):
     tx(_tx), nFee(_nFee), nTime(_nTime), entryHeight(_entryHeight),
-    spendsCoinbase(_spendsCoinbase), sigOpCount(_sigOps), lockPoints(lp)
+    spendsCoinbase(_spendsCoinbase), sigOpCount(_sigOps), lockPoints(lp), m_epoch(0)
 {
     nTxSize = ::GetSerializeSize(*_tx, SER_NETWORK, PROTOCOL_VERSION);
     nUsageSize = RecursiveDynamicUsage(tx);
@@ -126,8 +126,6 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
     // setMemPoolChildren will be updated, an assumption made in
     // UpdateForDescendants.
     for (const uint256 &hash : reverse_iterate(vHashesToUpdate)) {
-        // we cache the in-mempool children to avoid duplicate updates
-        setEntries setChildren;
         // calculate children from mapNextTx
         txiter it = mapTx.find(hash);
         if (it == mapTx.end()) {
@@ -136,17 +134,21 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
         auto iter = mapNextTx.lower_bound(COutPoint(hash, 0));
         // First calculate the children, and update setMemPoolChildren to
         // include them, and update their setMemPoolParents to include this tx.
-        for (; iter != mapNextTx.end() && iter->first->hash == hash; ++iter) {
-            const uint256 &childHash = iter->second->GetHash();
-            txiter childIter = mapTx.find(childHash);
-            assert(childIter != mapTx.end());
-            // We can skip updating entries we've encountered before or that
-            // are in the block (which are already accounted for).
-            if (setChildren.insert(childIter).second && !setAlreadyIncluded.count(childHash)) {
-                UpdateChild(it, childIter, true);
-                UpdateParent(childIter, it, true);
+        // we cache the in-mempool children to avoid duplicate updates
+        {
+            const auto epoch = GetFreshEpoch();
+            for (; iter != mapNextTx.end() && iter->first->hash == hash; ++iter) {
+                const uint256 &childHash = iter->second->GetHash();
+                txiter childIter = mapTx.find(childHash);
+                assert(childIter != mapTx.end());
+                // We can skip updating entries we've encountered before or that
+                // are in the block (which are already accounted for).
+                if (!visited(childIter) && !setAlreadyIncluded.count(childHash)) {
+                    UpdateChild(it, childIter, true);
+                    UpdateParent(childIter, it, true);
+                }
             }
-        }
+        } // release epoch guard for UpdateForDescendants
         UpdateForDescendants(it, mapMemPoolDescendantsToUpdate, setAlreadyIncluded);
     }
 }
@@ -329,7 +331,7 @@ void CTxMemPoolEntry::UpdateAncestorState(int64_t modifySize, CAmount modifyFee,
 }
 
 CTxMemPool::CTxMemPool(CBlockPolicyEstimator* estimator) :
-    nTransactionsUpdated(0), minerPolicyEstimator(estimator)
+    nTransactionsUpdated(0), minerPolicyEstimator(estimator), m_epoch(0), m_has_epoch_guard(false)
 {
     _clear(); //lock free clear
 
@@ -1591,4 +1593,22 @@ void CTxMemPool::GetTransactionAncestry(const uint256& txid, size_t& ancestors,
     }
 }
 
+CTxMemPool::EpochGuard CTxMemPool::GetFreshEpoch() const
+{
+    return EpochGuard(*this);
+}
+CTxMemPool::EpochGuard::EpochGuard(const CTxMemPool& in) : pool(in)
+{
+    assert(!pool.m_has_epoch_guard);
+    ++pool.m_epoch;
+    pool.m_has_epoch_guard = true;
+}
+
+CTxMemPool::EpochGuard::~EpochGuard()
+{
+    // prevents stale results being used
+    ++pool.m_epoch;
+    pool.m_has_epoch_guard = false;
+}
+
 SaltedTxidHasher::SaltedTxidHasher() : k0(GetRand(std::numeric_limits<uint64_t>::max())), k1(GetRand(std::numeric_limits<uint64_t>::max())) {}

src/txmempool.h

@@ -136,6 +136,7 @@ public:
     // If this is a proTx, this will be the hash of the key for which this ProTx was valid
     mutable uint256 validForProTxKey;
     mutable bool isKeyChangeProTx{false};
+    mutable uint64_t m_epoch; //!< epoch when last touched, useful for graph algorithms
 };
 
 // Helpers for modifying CTxMemPool::mapTx, which is a boost multi_index.
@@ -456,6 +457,8 @@ private:
     mutable int64_t lastRollingFeeUpdate;
     mutable bool blockSinceLastRollingFeeBump;
     mutable double rollingMinimumFeeRate; //!< minimum fee to get into the pool, decreases exponentially
+    mutable uint64_t m_epoch;
+    mutable bool m_has_epoch_guard;
 
     void trackPackageRemoved(const CFeeRate& rate) EXCLUSIVE_LOCKS_REQUIRED(cs);
 
@@ -731,6 +734,55 @@ private:
      *  removal.
      */
     void removeUnchecked(txiter entry, MemPoolRemovalReason reason = MemPoolRemovalReason::UNKNOWN) EXCLUSIVE_LOCKS_REQUIRED(cs);
+public:
+    /** EpochGuard: RAII-style guard for using epoch-based graph traversal algorithms.
+     *     When walking ancestors or descendants, we generally want to avoid
+     * visiting the same transactions twice. Some traversal algorithms use
+     * std::set (or setEntries) to deduplicate the transaction we visit.
+     * However, use of std::set is algorithmically undesirable because it both
+     * adds an asymptotic factor of O(log n) to traverals cost and triggers O(n)
+     * more dynamic memory allocations.
+     *     In many algorithms we can replace std::set with an internal mempool
+     * counter to track the time (or, "epoch") that we began a traversal, and
+     * check + update a per-transaction epoch for each transaction we look at to
+     * determine if that transaction has not yet been visited during the current
+     * traversal's epoch.
+     *     Algorithms using std::set can be replaced on a one by one basis.
+     * Both techniques are not fundamentally incomaptible across the codebase.
+     * Generally speaking, however, the remaining use of std::set for mempool
+     * traversal should be viewed as a TODO for replacement with an epoch based
+     * traversal, rather than a preference for std::set over epochs in that
+     * algorithm.
+     */
+    class EpochGuard {
+        const CTxMemPool& pool;
+        public:
+        EpochGuard(const CTxMemPool& in);
+        ~EpochGuard();
+    };
+    // N.B. GetFreshEpoch modifies mutable state via the EpochGuard construction
+    // (and later destruction)
+    EpochGuard GetFreshEpoch() const EXCLUSIVE_LOCKS_REQUIRED(cs);
+
+    /** visited marks a CTxMemPoolEntry as having been traversed
+     * during the lifetime of the most recently created EpochGuard
+     * and returns false if we are the first visitor, true otherwise.
+     *
+     * An EpochGuard must be held when visited is called or an assert will be
+     * triggered.
+     *
+     */
+    bool visited(txiter it) const EXCLUSIVE_LOCKS_REQUIRED(cs) {
+        assert(m_has_epoch_guard);
+        bool ret = it->m_epoch >= m_epoch;
+        it->m_epoch = std::max(it->m_epoch, m_epoch);
+        return ret;
+    }
+
+    bool visited(boost::optional<txiter> it) const EXCLUSIVE_LOCKS_REQUIRED(cs) {
+        assert(m_has_epoch_guard);
+        return !it || visited(*it);
+    }
 };
 
 /**