Implement on-the-fly mempool size limitation.

After each transaction which is added to mempool, we first call
Expire() to remove old transactions, then throwing away the
lowest-feerate transactions.

After throwing away transactions by feerate, we set the minimum
relay fee to the maximum fee transaction-and-dependant-set we
removed, plus the default minimum relay fee.

After the next block is received, the minimum relay fee is allowed
to decrease exponentially. Its halflife defaults to 12 hours, but
is decreased to 6 hours if the mempool is smaller than half its
maximum size, and 3 hours if the mempool is smaller than a quarter
its maximum size.

The minimum -maxmempool size is 40*-limitdescendantsize, as it is
easy for an attacker to play games with the cheapest
-limitdescendantsize transactions. -maxmempool defaults to 300MB.

This disables high-priority transaction relay when the min relay
fee adjustment is >0 (ie when the mempool is full). When the relay
fee adjustment drops below the default minimum relay fee / 2 it is
set to 0 (re-enabling priority-based free relay).

src/init.cpp

@@ -320,6 +320,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-dbcache=<n>", strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache));
     strUsage += HelpMessageOpt("-loadblock=<file>", _("Imports blocks from external blk000??.dat file") + " " + _("on startup"));
     strUsage += HelpMessageOpt("-maxorphantx=<n>", strprintf(_("Keep at most <n> unconnectable transactions in memory (default: %u)"), DEFAULT_MAX_ORPHAN_TRANSACTIONS));
+    strUsage += HelpMessageOpt("-maxmempool=<n>", strprintf(_("Keep the transaction memory pool below <n> megabytes (default: %u)"), DEFAULT_MAX_MEMPOOL_SIZE));
     strUsage += HelpMessageOpt("-mempoolexpiry=<n>", strprintf(_("Do not keep transactions in the mempool longer than <n> hours (default: %u)"), DEFAULT_MEMPOOL_EXPIRY));
     strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
         -GetNumCores(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
@@ -842,6 +843,12 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     fCheckBlockIndex = GetBoolArg("-checkblockindex", chainparams.DefaultConsistencyChecks());
     fCheckpointsEnabled = GetBoolArg("-checkpoints", true);
 
+    // -mempoollimit limits
+    int64_t nMempoolSizeLimit = GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
+    int64_t nMempoolDescendantSizeLimit = GetArg("-limitdescendantsize", DEFAULT_DESCENDANT_SIZE_LIMIT) * 1000;
+    if (nMempoolSizeLimit < 0 || nMempoolSizeLimit < nMempoolDescendantSizeLimit * 40)
+        return InitError(strprintf(_("Error: -maxmempool must be at least %d MB"), GetArg("-limitdescendantsize", DEFAULT_DESCENDANT_SIZE_LIMIT) / 25));
+
     // -par=0 means autodetect, but nScriptCheckThreads==0 means no concurrency
     nScriptCheckThreads = GetArg("-par", DEFAULT_SCRIPTCHECK_THREADS);
     if (nScriptCheckThreads <= 0)

src/main.cpp

@@ -885,8 +885,11 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient fee", false,
                 strprintf("%d < %d", nFees, txMinFee));
 
-        // Require that free transactions have sufficient priority to be mined in the next block.
-        if (GetBoolArg("-relaypriority", true) && nFees < ::minRelayTxFee.GetFee(nSize) && !AllowFree(view.GetPriority(tx, chainActive.Height() + 1))) {
+        CAmount mempoolRejectFee = pool.GetMinFee(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000).GetFee(nSize);
+        if (mempoolRejectFee > 0 && nFees < mempoolRejectFee) {
+            return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "mempool min fee not met", false, strprintf("%d < %d", nFees, mempoolRejectFee));
+        } else if (GetBoolArg("-relaypriority", true) && nFees < ::minRelayTxFee.GetFee(nSize) && !AllowFree(view.GetPriority(tx, chainActive.Height() + 1))) {
+            // Require that free transactions have sufficient priority to be mined in the next block.
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient priority");
         }
 
@@ -951,6 +954,15 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
 
         // Store transaction in memory
         pool.addUnchecked(hash, entry, setAncestors, !IsInitialBlockDownload());
+
+        // trim mempool and check if tx was trimmed
+        int expired = pool.Expire(GetTime() - GetArg("-mempoolexpiry", DEFAULT_MEMPOOL_EXPIRY) * 60 * 60);
+        if (expired != 0)
+            LogPrint("mempool", "Expired %i transactions from the memory pool\n", expired);
+
+        pool.TrimToSize(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+        if (!pool.exists(tx.GetHash()))
+            return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "mempool full");
     }
 
     SyncWithWallets(tx, NULL);

src/main.h

@@ -51,6 +51,8 @@ static const unsigned int DEFAULT_ANCESTOR_SIZE_LIMIT = 900;
 static const unsigned int DEFAULT_DESCENDANT_LIMIT = 1000;
 /** Default for -limitdescendantsize, maximum kilobytes of in-mempool descendants */
 static const unsigned int DEFAULT_DESCENDANT_SIZE_LIMIT = 2500;
+/** Default for -maxmempool, maximum megabytes of mempool memory usage */
+static const unsigned int DEFAULT_MAX_MEMPOOL_SIZE = 300;
 /** Default for -mempoolexpiry, expiration time for mempool transactions in hours */
 static const unsigned int DEFAULT_MEMPOOL_EXPIRY = 72;
 /** The maximum size of a blk?????.dat file (since 0.8) */

src/txmempool.cpp

@@ -13,6 +13,7 @@
 #include "streams.h"
 #include "util.h"
 #include "utilmoneystr.h"
+#include "utiltime.h"
 #include "version.h"
 
 using namespace std;
@@ -308,6 +309,8 @@ void CTxMemPoolEntry::UpdateState(int64_t modifySize, CAmount modifyFee, int64_t
 CTxMemPool::CTxMemPool(const CFeeRate& _minReasonableRelayFee) :
     nTransactionsUpdated(0)
 {
+    clear();
+
     // Sanity checks off by default for performance, because otherwise
     // accepting transactions becomes O(N^2) where N is the number
     // of transactions in the pool
@@ -539,6 +542,8 @@ void CTxMemPool::removeForBlock(const std::vector<CTransaction>& vtx, unsigned i
     }
     // After the txs in the new block have been removed from the mempool, update policy estimates
     minerPolicyEstimator->processBlock(nBlockHeight, entries, fCurrentEstimate);
+    lastRollingFeeUpdate = GetTime();
+    blockSinceLastRollingFeeBump = true;
 }
 
 void CTxMemPool::clear()
@@ -549,6 +554,9 @@ void CTxMemPool::clear()
     mapNextTx.clear();
     totalTxSize = 0;
     cachedInnerUsage = 0;
+    lastRollingFeeUpdate = GetTime();
+    blockSinceLastRollingFeeBump = false;
+    rollingMinimumFeeRate = 0;
     ++nTransactionsUpdated;
 }
 
@@ -854,3 +862,60 @@ const CTxMemPool::setEntries & CTxMemPool::GetMemPoolChildren(txiter entry) cons
     assert(it != mapLinks.end());
     return it->second.children;
 }
+
+CFeeRate CTxMemPool::GetMinFee(size_t sizelimit) const {
+    LOCK(cs);
+    if (!blockSinceLastRollingFeeBump || rollingMinimumFeeRate == 0)
+        return CFeeRate(rollingMinimumFeeRate);
+
+    int64_t time = GetTime();
+    if (time > lastRollingFeeUpdate + 10) {
+        double halflife = ROLLING_FEE_HALFLIFE;
+        if (DynamicMemoryUsage() < sizelimit / 4)
+            halflife /= 4;
+        else if (DynamicMemoryUsage() < sizelimit / 2)
+            halflife /= 2;
+
+        rollingMinimumFeeRate = rollingMinimumFeeRate / pow(2.0, (time - lastRollingFeeUpdate) / halflife);
+        lastRollingFeeUpdate = time;
+
+        if (rollingMinimumFeeRate < minReasonableRelayFee.GetFeePerK() / 2)
+            rollingMinimumFeeRate = 0;
+    }
+    return std::max(CFeeRate(rollingMinimumFeeRate), minReasonableRelayFee);
+}
+
+void CTxMemPool::trackPackageRemoved(const CFeeRate& rate) {
+    AssertLockHeld(cs);
+    if (rate.GetFeePerK() > rollingMinimumFeeRate) {
+        rollingMinimumFeeRate = rate.GetFeePerK();
+        blockSinceLastRollingFeeBump = false;
+    }
+}
+
+void CTxMemPool::TrimToSize(size_t sizelimit) {
+    LOCK(cs);
+
+    unsigned nTxnRemoved = 0;
+    CFeeRate maxFeeRateRemoved(0);
+    while (DynamicMemoryUsage() > sizelimit) {
+        indexed_transaction_set::nth_index<1>::type::iterator it = mapTx.get<1>().begin();
+
+        // We set the new mempool min fee to either the feerate of the removed set,
+        // or the "minimum reasonable fee rate" (ie some value under which we consider
+        // txn to have 0 fee). This way, if the mempool reaches its full size on free
+        // txn, we will simply disable free txn until there is a block, and some time.
+        CFeeRate removed(it->GetFeesWithDescendants(), it->GetSizeWithDescendants());
+        removed += minReasonableRelayFee;
+        trackPackageRemoved(removed);
+        maxFeeRateRemoved = std::max(maxFeeRateRemoved, removed);
+
+        setEntries stage;
+        CalculateDescendants(mapTx.project<0>(it), stage);
+        RemoveStaged(stage);
+        nTxnRemoved += stage.size();
+    }
+
+    if (maxFeeRateRemoved > CFeeRate(0))
+        LogPrint("mempool", "Removed %u txn, rolling minimum fee bumped to %s\n", nTxnRemoved, maxFeeRateRemoved.ToString());
+}

src/txmempool.h

@@ -287,6 +287,13 @@ private:
 
     CFeeRate minReasonableRelayFee;
 
+    mutable int64_t lastRollingFeeUpdate;
+    mutable bool blockSinceLastRollingFeeBump;
+    mutable double rollingMinimumFeeRate; //! minimum fee to get into the pool, decreases exponentially
+    static const int ROLLING_FEE_HALFLIFE = 60 * 60 * 12;
+
+    void trackPackageRemoved(const CFeeRate& rate);
+
 public:
     typedef boost::multi_index_container<
         CTxMemPoolEntry,
@@ -410,6 +417,17 @@ public:
      */
     bool CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntries &setAncestors, uint64_t limitAncestorCount, uint64_t limitAncestorSize, uint64_t limitDescendantCount, uint64_t limitDescendantSize, std::string &errString, bool fSearchForParents = true);
 
+    /** The minimum fee to get into the mempool, which may itself not be enough
+	 *  for larger-sized transactions.
+	 *  The minReasonableRelayFee constructor arg is used to bound the time it
+     *  takes the fee rate to go back down all the way to 0. When the feerate
+     *  would otherwise be half of this, it is set to 0 instead.
+	 */
+    CFeeRate GetMinFee(size_t sizelimit) const;
+
+    /** Remove transactions from the mempool until its dynamic size is <= sizelimit. */
+    void TrimToSize(size_t sizelimit);
+
     /** Expire all transaction (and their dependencies) in the mempool older than time. Return the number of removed transactions. */
     int Expire(int64_t time);