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.

1 files changed, 21 insertions, 11 deletions

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index fcfc27d38e..225e5652b6 100644
--- a/src/txmempool.cpp
+++ b/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);
+        }
     }
 }