add -invertmempool option

src/init.cpp

@@ -509,6 +509,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-loadblock=<file>", "Imports blocks from external file on startup", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-lowmem=<n>", strprintf("If system available memory falls below <n> MiB, flush caches (0 to disable, default: %s)", g_low_memory_threshold / 1024 / 1024), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-maxmempool=<n>", strprintf("Keep the transaction memory pool below <n> megabytes (default: %u)", DEFAULT_MAX_MEMPOOL_SIZE_MB), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+    argsman.AddArg("-invertmempool", strprintf("Start pruning high fees txs when mempool get full (default: %u)", DEFAULT_INVERT_MEMPOOL), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-maxorphantx=<n>", strprintf("Keep at most <n> unconnectable transactions in memory (default: %u)", DEFAULT_MAX_ORPHAN_TRANSACTIONS), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-mempoolexpiry=<n>", strprintf("Do not keep transactions in the mempool longer than <n> hours (default: %u)", DEFAULT_MEMPOOL_EXPIRY_HOURS), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-minimumchainwork=<hex>", strprintf("Minimum work assumed to exist on a valid chain in hex (default: %s, testnet3: %s, testnet4: %s, signet: %s)", defaultChainParams->GetConsensus().nMinimumChainWork.GetHex(), testnetChainParams->GetConsensus().nMinimumChainWork.GetHex(), testnet4ChainParams->GetConsensus().nMinimumChainWork.GetHex(), signetChainParams->GetConsensus().nMinimumChainWork.GetHex()), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::OPTIONS);

src/kernel/mempool_options.h

@@ -22,6 +22,8 @@ enum class TRUCPolicy { Reject, Accept, Enforce };
 
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
+/** Default for -invertmempool */
+static constexpr unsigned int DEFAULT_INVERT_MEMPOOL{false};
 /** Default for -maxmempool when blocksonly is set */
 static constexpr unsigned int DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB{5};
 /** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
@@ -52,6 +54,7 @@ struct MemPoolOptions {
     /* The ratio used to determine how often sanity checks will run.  */
     int check_ratio{0};
     int64_t max_size_bytes{DEFAULT_MAX_MEMPOOL_SIZE_MB * 1'000'000};
+    bool invert_mempool{DEFAULT_INVERT_MEMPOOL};
     std::chrono::seconds expiry{std::chrono::hours{DEFAULT_MEMPOOL_EXPIRY_HOURS}};
     CFeeRate incremental_relay_feerate{DEFAULT_INCREMENTAL_RELAY_FEE};
     /** A fee rate smaller than this is considered zero fee (for relaying, mining and transaction creation) */

src/node/mempool_args.cpp

@@ -96,6 +96,8 @@ util::Result<void> ApplyArgsManOptions(const ArgsManager& argsman, const CChainP
     mempool_opts.check_ratio = argsman.GetIntArg("-checkmempool", mempool_opts.check_ratio);
 
     if (auto mb = argsman.GetIntArg("-maxmempool")) mempool_opts.max_size_bytes = *mb * 1'000'000;
+
+    if (auto invert = argsman.GetBoolArg("-invertmempool")) mempool_opts.invert_mempool = *invert;
 
     if (auto hours = argsman.GetIntArg("-mempoolexpiry")) mempool_opts.expiry = std::chrono::hours{*hours};
 

src/rpc/mempool.cpp

@@ -888,7 +888,11 @@ UniValue MempoolInfoToJSON(const CTxMemPool& pool, const std::optional<MempoolHi
     ret.pushKV("usage", (int64_t)pool.DynamicMemoryUsage());
     ret.pushKV("total_fee", ValueFromAmount(pool.GetTotalFee()));
     ret.pushKV("maxmempool", pool.m_opts.max_size_bytes);
-    ret.pushKV("mempoolminfee", ValueFromAmount(std::max(pool.GetMinFee(), pool.m_opts.min_relay_feerate).GetFeePerK()));
+    if (pool.m_opts.invert_mempool) {
+        ret.pushKV("mempoolmaxfee", ValueFromAmount(pool.GetMaxFee().GetFeePerK()));
+    } else {
+        ret.pushKV("mempoolminfee", ValueFromAmount(std::max(pool.GetMinFee(), pool.m_opts.min_relay_feerate).GetFeePerK()));
+    }
     ret.pushKV("minrelaytxfee", ValueFromAmount(pool.m_opts.min_relay_feerate.GetFeePerK()));
     ret.pushKV("incrementalrelayfee", ValueFromAmount(pool.m_opts.incremental_relay_feerate.GetFeePerK()));
     ret.pushKV("dustrelayfee", ValueFromAmount(pool.m_opts.dust_relay_feerate.GetFeePerK()));

src/txmempool.cpp

@@ -1222,6 +1222,18 @@ CFeeRate CTxMemPool::GetMinFee(size_t sizelimit) const {
     return std::max(CFeeRate(llround(rollingMinimumFeeRate)), m_opts.incremental_relay_feerate);
 }
 
+CFeeRate CTxMemPool::GetMaxFee(size_t sizelimit) const {
+    LOCK(cs);
+    CFeeRate maxFeeRate(0);
+    for (const auto& entry : mapTx) {
+        CFeeRate entryFeeRate(entry.GetModifiedFee(), entry.GetTxSize());
+        if (entryFeeRate > maxFeeRate) {
+            maxFeeRate = entryFeeRate;
+        }
+    }
+    return maxFeeRate;
+}
+
 void CTxMemPool::trackPackageRemoved(const CFeeRate& rate) {
     AssertLockHeld(cs);
     if (rate.GetFeePerK() > rollingMinimumFeeRate) {
@@ -1236,12 +1248,15 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<COutPoint>* pvNoSpends
     unsigned nTxnRemoved = 0;
     CFeeRate maxFeeRateRemoved(0);
     while (!mapTx.empty() && DynamicMemoryUsage() > sizelimit) {
-        indexed_transaction_set::index<descendant_score>::type::iterator it = mapTx.get<descendant_score>().begin();
+        indexed_transaction_set::index<descendant_score>::type& index = mapTx.get<descendant_score>();
+        indexed_transaction_set::index<descendant_score>::type::iterator it;
+
+        if (m_opts.invert_mempool) {
+            it = std::prev(index.end());
+        } else {
+            it = index.begin();
+        }
 
-        // We set the new mempool min fee to the feerate of the removed set, plus the
-        // "minimum reasonable fee rate" (ie some value under which we consider txn
-        // to have 0 fee). This way, we don't allow txn to enter mempool with feerate
-        // equal to txn which were removed with no block in between.
         CFeeRate removed(it->GetModFeesWithDescendants(), it->GetSizeWithDescendants());
         removed += m_opts.incremental_relay_feerate;
         trackPackageRemoved(removed);
@@ -1268,9 +1283,13 @@ void CTxMemPool::TrimToSize(size_t sizelimit, std::vector<COutPoint>* pvNoSpends
         }
     }
 
-    if (maxFeeRateRemoved > CFeeRate(0)) {
+    if (maxFeeRateRemoved > CFeeRate(0) && !m_opts.invert_mempool) {
         LogPrint(BCLog::MEMPOOL, "Removed %u txn, rolling minimum fee bumped to %s\n", nTxnRemoved, maxFeeRateRemoved.ToString());
     }
+
+    if (m_opts.invert_mempool) {
+        LogPrint(BCLog::MEMPOOL, "Removed %u txn, rolling maxmimum fee bumped to %s\n", nTxnRemoved, GetMaxFee().ToString());
+    }
 }
 
 uint64_t CTxMemPool::CalculateDescendantMaximum(txiter entry) const {

src/txmempool.h

@@ -336,6 +336,8 @@ class CTxMemPool
     bool m_load_tried GUARDED_BY(cs){false};
 
     CFeeRate GetMinFee(size_t sizelimit) const;
+
+    CFeeRate GetMaxFee(size_t sizelimit) const;
 
 public:
 
@@ -639,6 +641,10 @@ class CTxMemPool
     CFeeRate GetMinFee() const {
         return GetMinFee(m_opts.max_size_bytes);
     }
+
+    CFeeRate GetMaxFee() const {
+        return GetMaxFee(m_opts.max_size_bytes);
+    }
 
     /** Remove transactions from the mempool until its dynamic size is <= sizelimit.
       *  pvNoSpendsRemaining, if set, will be populated with the list of outpoints