Mempool Update Cut-Through Optimization

Often when we're updating mempool entries we update entries that we
ultimately end up removing the updated entries shortly thereafter. This
patch makes it so that we filter for such entries a bit earlier in
processing, which yields a mild improvement for these cases, and is
negligible overhead otherwise.

Author: JeremyRubin

src/txmempool.cpp

@@ -126,10 +126,9 @@ size_t CTxMemPoolEntry::GetTxSize() const
     return GetVirtualTransactionSize(nTxWeight, sigOpCost);
 }
 
-// Update the given tx for any in-mempool descendants.
-// Assumes that CTxMemPool::m_children is correct for the given tx and all
-// descendants.
-void CTxMemPool::UpdateForDescendants(txiter updateIt, cacheMap &cachedDescendants, const std::set<uint256> &setExclude)
+void CTxMemPool::UpdateForDescendants(txiter updateIt, cacheMap& cachedDescendants,
+                                      const std::set<uint256>& setExclude, std::set<uint256>& descendants_to_remove,
+                                      uint64_t ancestor_size_limit, uint64_t ancestor_count_limit)
 {
     CTxMemPoolEntry::Children stageEntries, descendants;
     stageEntries = updateIt->GetMemPoolChildrenConst();
@@ -166,17 +165,18 @@ void CTxMemPool::UpdateForDescendants(txiter updateIt, cacheMap &cachedDescendan
             cachedDescendants[updateIt].insert(mapTx.iterator_to(descendant));
             // Update ancestor state for each descendant
             mapTx.modify(mapTx.iterator_to(descendant), update_ancestor_state(updateIt->GetTxSize(), updateIt->GetModifiedFee(), 1, updateIt->GetSigOpCost()));
+            // Don't directly remove the transaction here -- doing so would
+            // invalidate iterators in cachedDescendants. Mark it for removal
+            // by inserting into descendants_to_remove.
+            if (descendant.GetCountWithAncestors() > ancestor_count_limit || descendant.GetSizeWithAncestors() > ancestor_size_limit) {
+                descendants_to_remove.insert(descendant.GetTx().GetHash());
+            }
         }
     }
     mapTx.modify(updateIt, update_descendant_state(modifySize, modifyFee, modifyCount));
 }
 
-// vHashesToUpdate is the set of transaction hashes from a disconnected block
-// which has been re-added to the mempool.
-// for each entry, look for descendants that are outside vHashesToUpdate, and
-// add fee/size information for such descendants to the parent.
-// for each such descendant, also update the ancestor state to include the parent.
-void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashesToUpdate)
+void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashesToUpdate, uint64_t ancestor_size_limit, uint64_t ancestor_count_limit)
 {
     AssertLockHeld(cs);
     // For each entry in vHashesToUpdate, store the set of in-mempool, but not
@@ -188,6 +188,8 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
     // accounted for in the state of their ancestors)
     std::set<uint256> setAlreadyIncluded(vHashesToUpdate.begin(), vHashesToUpdate.end());
 
+    std::set<uint256> descendants_to_remove;
+
     // Iterate in reverse, so that whenever we are looking at a transaction
     // we are sure that all in-mempool descendants have already been processed.
     // This maximizes the benefit of the descendant cache and guarantees that
@@ -217,7 +219,15 @@ void CTxMemPool::UpdateTransactionsFromBlock(const std::vector<uint256> &vHashes
                 }
             }
         } // release epoch guard for UpdateForDescendants
-        UpdateForDescendants(it, mapMemPoolDescendantsToUpdate, setAlreadyIncluded);
+        UpdateForDescendants(it, mapMemPoolDescendantsToUpdate, setAlreadyIncluded, descendants_to_remove, ancestor_size_limit, ancestor_count_limit);
+    }
+
+    for (const auto& txid : descendants_to_remove) {
+        // This txid may have been removed already in a prior call to removeRecursive.
+        // Therefore we ensure it is not yet removed already.
+        if (const std::optional<txiter> txiter = GetIter(txid)) {
+            removeRecursive((*txiter)->GetTx(), MemPoolRemovalReason::SIZELIMIT);
+        }
     }
 }
 

src/txmempool.h

@@ -632,16 +632,25 @@ class CTxMemPool
      */
     void RemoveStaged(setEntries& stage, bool updateDescendants, MemPoolRemovalReason reason) EXCLUSIVE_LOCKS_REQUIRED(cs);
 
-    /** When adding transactions from a disconnected block back to the mempool,
-     *  new mempool entries may have children in the mempool (which is generally
-     *  not the case when otherwise adding transactions).
-     *  UpdateTransactionsFromBlock() will find child transactions and update the
-     *  descendant state for each transaction in vHashesToUpdate (excluding any
-     *  child transactions present in vHashesToUpdate, which are already accounted
-     *  for).  Note: vHashesToUpdate should be the set of transactions from the
-     *  disconnected block that have been accepted back into the mempool.
+    /** UpdateTransactionsFromBlock is called when adding transactions from a
+     * disconnected block back to the mempool, new mempool entries may have
+     * children in the mempool (which is generally not the case when otherwise
+     * adding transactions).
+     *  @post updated descendant state for descendants of each transaction in
+     *        vHashesToUpdate (excluding any child transactions present in
+     *        vHashesToUpdate, which are already accounted for). Updated state
+     *        includes add fee/size information for such descendants to the
+     *        parent and updated ancestor state to include the parent.
+     *
+     * @param[in] vHashesToUpdate          The set of txids from the
+     *     disconnected block that have been accepted back into the mempool.
+     * @param[in] ancestor_size_limit      The maximum allowed size in virtual
+     *     bytes of an entry and its ancestors
+     * @param[in] ancestor_count_limit     The maximum allowed number of
+     *     transactions including the entry and its ancestors.
      */
-    void UpdateTransactionsFromBlock(const std::vector<uint256>& vHashesToUpdate) EXCLUSIVE_LOCKS_REQUIRED(cs, cs_main) LOCKS_EXCLUDED(m_epoch);
+    void UpdateTransactionsFromBlock(const std::vector<uint256>& vHashesToUpdate,
+            uint64_t ancestor_size_limit, uint64_t ancestor_count_limit) EXCLUSIVE_LOCKS_REQUIRED(cs, cs_main) LOCKS_EXCLUDED(m_epoch);
 
     /** Try to calculate all in-mempool ancestors of entry.
      *  (these are all calculated including the tx itself)
@@ -790,19 +799,38 @@ class CTxMemPool
     /** UpdateForDescendants is used by UpdateTransactionsFromBlock to update
      *  the descendants for a single transaction that has been added to the
      *  mempool but may have child transactions in the mempool, eg during a
-     *  chain reorg.  setExclude is the set of descendant transactions in the
-     *  mempool that must not be accounted for (because any descendants in
-     *  setExclude were added to the mempool after the transaction being
-     *  updated and hence their state is already reflected in the parent
-     *  state).
+     *  chain reorg.
+     *
+     * @pre CTxMemPool::m_children is correct for the given tx and all
+     *      descendants.
+     * @pre cachedDescendants is an accurate cache where each entry has all
+     *      descendants of the corresponding key, including those that should
+     *      be removed for violation of ancestor limits.
+     * @post if updateIt has any non-excluded descendants, cachedDescendants has
+     *       a new cache line for updateIt.
+     * @post descendants_to_remove has a new entry for any descendant which exceeded
+     *       ancestor limits relative to updateIt.
      *
-     *  cachedDescendants will be updated with the descendants of the transaction
-     *  being updated, so that future invocations don't need to walk the
-     *  same transaction again, if encountered in another transaction chain.
+     * @param[in] updateIt the entry to update for its descendants
+     * @param[in,out] cachedDescendants a cache where each line corresponds to all
+     *     descendants. It will be updated with the descendants of the transaction
+     *     being updated, so that future invocations don't need to walk the same
+     *     transaction again, if encountered in another transaction chain.
+     * @param[in] setExclude the set of descendant transactions in the mempool
+     *     that must not be accounted for (because any descendants in setExclude
+     *     were added to the mempool after the transaction being updated and hence
+     *     their state is already reflected in the parent state).
+     * @param[out] descendants_to_remove Populated with the txids of entries that
+     *     exceed ancestor limits. It's the responsibility of the caller to
+     *     removeRecursive them.
+     * @param[in] ancestor_size_limit the max number of ancestral bytes allowed
+     *     for any descendant
+     * @param[in] ancestor_count_limit the max number of ancestor transactions
+     *     allowed for any descendant
      */
-    void UpdateForDescendants(txiter updateIt,
-            cacheMap &cachedDescendants,
-            const std::set<uint256> &setExclude) EXCLUSIVE_LOCKS_REQUIRED(cs);
+    void UpdateForDescendants(txiter updateIt, cacheMap& cachedDescendants,
+                              const std::set<uint256>& setExclude, std::set<uint256>& descendants_to_remove,
+                              uint64_t ancestor_size_limit, uint64_t ancestor_count_limit) EXCLUSIVE_LOCKS_REQUIRED(cs);
     /** Update ancestors of hash to add/remove it as a descendant transaction. */
     void UpdateAncestorsOf(bool add, txiter hash, setEntries &setAncestors) EXCLUSIVE_LOCKS_REQUIRED(cs);
     /** Set ancestor state for an entry */

src/validation.cpp

@@ -348,7 +348,9 @@ void CChainState::MaybeUpdateMempoolForReorg(
     // previously-confirmed transactions back to the mempool.
     // UpdateTransactionsFromBlock finds descendants of any transactions in
     // the disconnectpool that were added back and cleans up the mempool state.
-    m_mempool->UpdateTransactionsFromBlock(vHashUpdate);
+    const uint64_t ancestor_count_limit = gArgs.GetIntArg("-limitancestorcount", DEFAULT_ANCESTOR_LIMIT);
+    const uint64_t ancestor_size_limit = gArgs.GetIntArg("-limitancestorsize", DEFAULT_ANCESTOR_SIZE_LIMIT) * 1000;
+    m_mempool->UpdateTransactionsFromBlock(vHashUpdate, ancestor_size_limit, ancestor_count_limit);
 
     const auto check_final_and_mature = [this, flags=STANDARD_LOCKTIME_VERIFY_FLAGS](CTxMemPool::txiter it)
         EXCLUSIVE_LOCKS_REQUIRED(m_mempool->cs, ::cs_main) {