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).

1 files changed, 65 insertions, 0 deletions

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index e8d76dd2f1..7563c07880 100644
--- a/src/txmempool.cpp
+++ b/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());
+}