From 78b82f4a16d8aad15ef397b1a1cd075b2efc8c16 Mon Sep 17 00:00:00 2001
From: Suhas Daftuar <sdaftuar@gmail.com>
Date: Thu, 24 Sep 2015 13:21:31 -0400
Subject: [PATCH 01/13] Reverse the sort on the mempool's feerate index

---
 src/test/mempool_tests.cpp | 46 +++++++++++++++++++-------------------
 src/txmempool.h            |  4 ++--
 2 files changed, 25 insertions(+), 25 deletions(-)

diff --git a/src/test/mempool_tests.cpp b/src/test/mempool_tests.cpp
index 5bf1e98e8..79d806bd2 100644
--- a/src/test/mempool_tests.cpp
+++ b/src/test/mempool_tests.cpp
@@ -153,11 +153,11 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
 
     std::vector<std::string> sortedOrder;
     sortedOrder.resize(5);
-    sortedOrder[0] = tx2.GetHash().ToString(); // 20000
-    sortedOrder[1] = tx4.GetHash().ToString(); // 15000
+    sortedOrder[0] = tx3.GetHash().ToString(); // 0
+    sortedOrder[1] = tx5.GetHash().ToString(); // 10000
     sortedOrder[2] = tx1.GetHash().ToString(); // 10000
-    sortedOrder[3] = tx5.GetHash().ToString(); // 10000
-    sortedOrder[4] = tx3.GetHash().ToString(); // 0
+    sortedOrder[3] = tx4.GetHash().ToString(); // 15000
+    sortedOrder[4] = tx2.GetHash().ToString(); // 20000
     CheckSort(pool, sortedOrder);
 
     /* low fee but with high fee child */
@@ -169,7 +169,7 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     pool.addUnchecked(tx6.GetHash(), CTxMemPoolEntry(tx6, 0LL, 1, 10.0, 1, true));
     BOOST_CHECK_EQUAL(pool.size(), 6);
     // Check that at this point, tx6 is sorted low
-    sortedOrder.push_back(tx6.GetHash().ToString());
+    sortedOrder.insert(sortedOrder.begin(), tx6.GetHash().ToString());
     CheckSort(pool, sortedOrder);
 
     CTxMemPool::setEntries setAncestors;
@@ -194,9 +194,9 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     BOOST_CHECK_EQUAL(pool.size(), 7);
 
     // Now tx6 should be sorted higher (high fee child): tx7, tx6, tx2, ...
-    sortedOrder.erase(sortedOrder.end()-1);
-    sortedOrder.insert(sortedOrder.begin(), tx6.GetHash().ToString());
-    sortedOrder.insert(sortedOrder.begin(), tx7.GetHash().ToString());
+    sortedOrder.erase(sortedOrder.begin());
+    sortedOrder.push_back(tx6.GetHash().ToString());
+    sortedOrder.push_back(tx7.GetHash().ToString());
     CheckSort(pool, sortedOrder);
 
     /* low fee child of tx7 */
@@ -211,7 +211,7 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     pool.addUnchecked(tx8.GetHash(), CTxMemPoolEntry(tx8, 0LL, 2, 10.0, 1, true), setAncestors);
 
     // Now tx8 should be sorted low, but tx6/tx both high
-    sortedOrder.push_back(tx8.GetHash().ToString());
+    sortedOrder.insert(sortedOrder.begin(), tx8.GetHash().ToString());
     CheckSort(pool, sortedOrder);
 
     /* low fee child of tx7 */
@@ -226,7 +226,7 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
 
     // tx9 should be sorted low
     BOOST_CHECK_EQUAL(pool.size(), 9);
-    sortedOrder.push_back(tx9.GetHash().ToString());
+    sortedOrder.insert(sortedOrder.begin(), tx9.GetHash().ToString());
     CheckSort(pool, sortedOrder);
 
     std::vector<std::string> snapshotOrder = sortedOrder;
@@ -255,21 +255,21 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
      *  tx8 and tx9 should both now be sorted higher
      *  Final order after tx10 is added:
      *
-     *  tx7 = 2.2M (4 txs)
-     *  tx6 = 2.2M (5 txs)
-     *  tx10 = 200k (1 tx)
-     *  tx8 = 200k (2 txs)
-     *  tx9 = 200k (2 txs)
-     *  tx2 = 20000 (1)
-     *  tx4 = 15000 (1)
-     *  tx1 = 10000 (1)
-     *  tx5 = 10000 (1)
      *  tx3 = 0 (1)
+     *  tx5 = 10000 (1)
+     *  tx1 = 10000 (1)
+     *  tx4 = 15000 (1)
+     *  tx2 = 20000 (1)
+     *  tx9 = 200k (2 txs)
+     *  tx8 = 200k (2 txs)
+     *  tx10 = 200k (1 tx)
+     *  tx6 = 2.2M (5 txs)
+     *  tx7 = 2.2M (4 txs)
      */
-    sortedOrder.erase(sortedOrder.end()-2, sortedOrder.end()); // take out tx8, tx9 from the end
-    sortedOrder.insert(sortedOrder.begin()+2, tx10.GetHash().ToString()); // tx10 is after tx6
-    sortedOrder.insert(sortedOrder.begin()+3, tx9.GetHash().ToString());
-    sortedOrder.insert(sortedOrder.begin()+3, tx8.GetHash().ToString());
+    sortedOrder.erase(sortedOrder.begin(), sortedOrder.begin()+2); // take out tx9, tx8 from the beginning
+    sortedOrder.insert(sortedOrder.begin()+5, tx9.GetHash().ToString());
+    sortedOrder.insert(sortedOrder.begin()+6, tx8.GetHash().ToString());
+    sortedOrder.insert(sortedOrder.begin()+7, tx10.GetHash().ToString()); // tx10 is just before tx6
     CheckSort(pool, sortedOrder);
 
     // there should be 10 transactions in the mempool
diff --git a/src/txmempool.h b/src/txmempool.h
index c0eef0dd2..2085b718e 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -160,9 +160,9 @@ public:
         double f2 = aSize * bFees;
 
         if (f1 == f2) {
-            return a.GetTime() < b.GetTime();
+            return a.GetTime() >= b.GetTime();
         }
-        return f1 > f2;
+        return f1 < f2;
     }
 
     // Calculate which feerate to use for an entry (avoiding division).

From 49b6fd5663dfe081d127cd1eb11407c4d3eaf93d Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter.wuille@gmail.com>
Date: Fri, 2 Oct 2015 14:43:30 -0700
Subject: [PATCH 02/13] Add Mempool Expire function to remove old transactions

(note the 9x multiplier on (void*)'s for CTxMemPool::DynamicMemoryUsage
 was accidentally introduced in 5add7a7 but should have waited for this
 commit which adds the extra index)
---
 src/init.cpp      |  1 +
 src/main.h        |  2 ++
 src/txmempool.cpp | 16 ++++++++++++++++
 src/txmempool.h   | 11 ++++++++++-
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/init.cpp b/src/init.cpp
index a079dce5b..22f0525b3 100644
--- a/src/init.cpp
+++ b/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("-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));
 #ifndef WIN32
diff --git a/src/main.h b/src/main.h
index a6001eed8..ec7cc2fdc 100644
--- a/src/main.h
+++ b/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 -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) */
 static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; // 128 MiB
 /** The pre-allocation chunk size for blk?????.dat files (since 0.8) */
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 1370cab0c..57bb28460 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -792,6 +792,22 @@ void CTxMemPool::RemoveStaged(setEntries &stage) {
     }
 }
 
+int CTxMemPool::Expire(int64_t time) {
+    LOCK(cs);
+    indexed_transaction_set::nth_index<2>::type::iterator it = mapTx.get<2>().begin();
+    setEntries toremove;
+    while (it != mapTx.get<2>().end() && it->GetTime() < time) {
+        toremove.insert(mapTx.project<0>(it));
+        it++;
+    }
+    setEntries stage;
+    BOOST_FOREACH(txiter removeit, toremove) {
+        CalculateDescendants(removeit, stage);
+    }
+    RemoveStaged(stage);
+    return stage.size();
+}
+
 bool CTxMemPool::addUnchecked(const uint256&hash, const CTxMemPoolEntry &entry, bool fCurrentEstimate)
 {
     LOCK(cs);
diff --git a/src/txmempool.h b/src/txmempool.h
index 2085b718e..635b66fb8 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -211,9 +211,10 @@ public:
  *
  * CTxMemPool::mapTx, and CTxMemPoolEntry bookkeeping:
  *
- * mapTx is a boost::multi_index that sorts the mempool on 2 criteria:
+ * mapTx is a boost::multi_index that sorts the mempool on 3 criteria:
  * - transaction hash
  * - feerate [we use max(feerate of tx, feerate of tx with all descendants)]
+ * - time in mempool
  *
  * Note: the term "descendant" refers to in-mempool transactions that depend on
  * this one, while "ancestor" refers to in-mempool transactions that a given
@@ -294,6 +295,11 @@ public:
             boost::multi_index::ordered_non_unique<
                 boost::multi_index::identity<CTxMemPoolEntry>,
                 CompareTxMemPoolEntryByFee
+            >,
+            // sorted by entry time
+            boost::multi_index::ordered_non_unique<
+                boost::multi_index::identity<CTxMemPoolEntry>,
+                CompareTxMemPoolEntryByEntryTime
             >
         >
     > indexed_transaction_set;
@@ -397,6 +403,9 @@ 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);
 
+    /** Expire all transaction (and their dependencies) in the mempool older than time. Return the number of removed transactions. */
+    int Expire(int64_t time);
+
     unsigned long size()
     {
         LOCK(cs);

From 9c9b66f771ad904cd665f7f5f68e3279ebb2fa7e Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Fri, 2 Oct 2015 14:17:27 -0700
Subject: [PATCH 03/13] Fix calling mempool directly, instead of pool, in ATMP

---
 src/main.cpp      | 21 +++++++++------------
 src/txmempool.cpp |  4 ++--
 src/txmempool.h   |  4 ++--
 3 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index baad7fc05..613eeff22 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -740,17 +740,14 @@ bool CheckTransaction(const CTransaction& tx, CValidationState &state)
     return true;
 }
 
-CAmount GetMinRelayFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree)
+CAmount GetMinRelayFee(const CTransaction& tx, const CTxMemPool& pool, unsigned int nBytes, bool fAllowFree)
 {
-    {
-        LOCK(mempool.cs);
-        uint256 hash = tx.GetHash();
-        double dPriorityDelta = 0;
-        CAmount nFeeDelta = 0;
-        mempool.ApplyDeltas(hash, dPriorityDelta, nFeeDelta);
-        if (dPriorityDelta > 0 || nFeeDelta > 0)
-            return 0;
-    }
+    uint256 hash = tx.GetHash();
+    double dPriorityDelta = 0;
+    CAmount nFeeDelta = 0;
+    pool.ApplyDeltas(hash, dPriorityDelta, nFeeDelta);
+    if (dPriorityDelta > 0 || nFeeDelta > 0)
+        return 0;
 
     CAmount nMinFee = ::minRelayTxFee.GetFee(nBytes);
 
@@ -879,11 +876,11 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         CAmount nFees = nValueIn-nValueOut;
         double dPriority = view.GetPriority(tx, chainActive.Height());
 
-        CTxMemPoolEntry entry(tx, nFees, GetTime(), dPriority, chainActive.Height(), mempool.HasNoInputsOf(tx));
+        CTxMemPoolEntry entry(tx, nFees, GetTime(), dPriority, chainActive.Height(), pool.HasNoInputsOf(tx));
         unsigned int nSize = entry.GetTxSize();
 
         // Don't accept it if it can't get into a block
-        CAmount txMinFee = GetMinRelayFee(tx, nSize, true);
+        CAmount txMinFee = GetMinRelayFee(tx, pool, nSize, true);
         if (fLimitFree && nFees < txMinFee)
             return state.DoS(0, false, REJECT_INSUFFICIENTFEE, "insufficient fee", false,
                 strprintf("%d < %d", nFees, txMinFee));
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 57bb28460..aa5aec055 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -735,10 +735,10 @@ void CTxMemPool::PrioritiseTransaction(const uint256 hash, const string strHash,
     LogPrintf("PrioritiseTransaction: %s priority += %f, fee += %d\n", strHash, dPriorityDelta, FormatMoney(nFeeDelta));
 }
 
-void CTxMemPool::ApplyDeltas(const uint256 hash, double &dPriorityDelta, CAmount &nFeeDelta)
+void CTxMemPool::ApplyDeltas(const uint256 hash, double &dPriorityDelta, CAmount &nFeeDelta) const
 {
     LOCK(cs);
-    std::map<uint256, std::pair<double, CAmount> >::iterator pos = mapDeltas.find(hash);
+    std::map<uint256, std::pair<double, CAmount> >::const_iterator pos = mapDeltas.find(hash);
     if (pos == mapDeltas.end())
         return;
     const std::pair<double, CAmount> &deltas = pos->second;
diff --git a/src/txmempool.h b/src/txmempool.h
index 635b66fb8..cee1a146d 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -83,7 +83,7 @@ public:
 
     const CTransaction& GetTx() const { return this->tx; }
     double GetPriority(unsigned int currentHeight) const;
-    CAmount GetFee() const { return nFee; }
+    const CAmount& GetFee() const { return nFee; }
     size_t GetTxSize() const { return nTxSize; }
     int64_t GetTime() const { return nTime; }
     unsigned int GetHeight() const { return nHeight; }
@@ -371,7 +371,7 @@ public:
 
     /** Affect CreateNewBlock prioritisation of transactions */
     void PrioritiseTransaction(const uint256 hash, const std::string strHash, double dPriorityDelta, const CAmount& nFeeDelta);
-    void ApplyDeltas(const uint256 hash, double &dPriorityDelta, CAmount &nFeeDelta);
+    void ApplyDeltas(const uint256 hash, double &dPriorityDelta, CAmount &nFeeDelta) const;
     void ClearPrioritisation(const uint256 hash);
 
 public:

From e8bcdce8a245af235f8be9853c8f81c9bda56412 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Tue, 13 Oct 2015 00:57:41 -0700
Subject: [PATCH 04/13] Track (and define) ::minRelayTxFee in CTxMemPool

---
 src/txmempool.cpp | 5 +++--
 src/txmempool.h   | 9 ++++++++-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index aa5aec055..e8d76dd2f 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -305,7 +305,7 @@ void CTxMemPoolEntry::UpdateState(int64_t modifySize, CAmount modifyFee, int64_t
     }
 }
 
-CTxMemPool::CTxMemPool(const CFeeRate& _minRelayFee) :
+CTxMemPool::CTxMemPool(const CFeeRate& _minReasonableRelayFee) :
     nTransactionsUpdated(0)
 {
     // Sanity checks off by default for performance, because otherwise
@@ -313,7 +313,8 @@ CTxMemPool::CTxMemPool(const CFeeRate& _minRelayFee) :
     // of transactions in the pool
     fSanityCheck = false;
 
-    minerPolicyEstimator = new CBlockPolicyEstimator(_minRelayFee);
+    minerPolicyEstimator = new CBlockPolicyEstimator(_minReasonableRelayFee);
+    minReasonableRelayFee = _minReasonableRelayFee;
 }
 
 CTxMemPool::~CTxMemPool()
diff --git a/src/txmempool.h b/src/txmempool.h
index cee1a146d..e45867f71 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -285,6 +285,8 @@ private:
     uint64_t totalTxSize; //! sum of all mempool tx' byte sizes
     uint64_t cachedInnerUsage; //! sum of dynamic memory usage of all the map elements (NOT the maps themselves)
 
+    CFeeRate minReasonableRelayFee;
+
 public:
     typedef boost::multi_index_container<
         CTxMemPoolEntry,
@@ -334,7 +336,12 @@ public:
     std::map<COutPoint, CInPoint> mapNextTx;
     std::map<uint256, std::pair<double, CAmount> > mapDeltas;
 
-    CTxMemPool(const CFeeRate& _minRelayFee);
+    /** Create a new CTxMemPool.
+     *  minReasonableRelayFee should be a feerate which is, roughly, somewhere
+     *  around what it "costs" to relay a transaction around the network and
+     *  below which we would reasonably say a transaction has 0-effective-fee.
+     */
+    CTxMemPool(const CFeeRate& _minReasonableRelayFee);
     ~CTxMemPool();
 
     /**

From 241d6078ba26db4d3a36227d3275be2ee34625a6 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Tue, 13 Oct 2015 00:53:19 -0700
Subject: [PATCH 05/13] Add CFeeRate += operator

---
 src/amount.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/amount.h b/src/amount.h
index 90e6b5aa8..a4c7764cd 100644
--- a/src/amount.h
+++ b/src/amount.h
@@ -51,6 +51,7 @@ public:
     friend bool operator==(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK == b.nSatoshisPerK; }
     friend bool operator<=(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK <= b.nSatoshisPerK; }
     friend bool operator>=(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK >= b.nSatoshisPerK; }
+    CFeeRate& operator+=(const CFeeRate& a) { nSatoshisPerK += a.nSatoshisPerK; return *this; }
     std::string ToString() const;
 
     ADD_SERIALIZE_METHODS;

From e6c7b362ab8915e2aac167fa519bd29836d482af Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Thu, 8 Oct 2015 00:46:57 -0700
Subject: [PATCH 06/13] Print mempool size in KB when adding txn

---
 src/main.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index 613eeff22..f01bb8ec9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -4287,10 +4287,10 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             RelayTransaction(tx);
             vWorkQueue.push_back(inv.hash);
 
-            LogPrint("mempool", "AcceptToMemoryPool: peer=%d: accepted %s (poolsz %u)\n",
+            LogPrint("mempool", "AcceptToMemoryPool: peer=%d: accepted %s (poolsz %u txn, %u kB)\n",
                 pfrom->id,
                 tx.GetHash().ToString(),
-                mempool.size());
+                mempool.size(), mempool.DynamicMemoryUsage() / 1000);
 
             // Recursively process any orphan transactions that depended on this one
             set<NodeId> setMisbehaving;

From 794a8cec5db84fde1cce82ada51740070ec188ac Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Fri, 2 Oct 2015 14:19:55 -0700
Subject: [PATCH 07/13] 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      |  7 +++++
 src/main.cpp      | 16 ++++++++++--
 src/main.h        |  2 ++
 src/txmempool.cpp | 65 +++++++++++++++++++++++++++++++++++++++++++++++
 src/txmempool.h   | 18 +++++++++++++
 5 files changed, 106 insertions(+), 2 deletions(-)

diff --git a/src/init.cpp b/src/init.cpp
index 22f0525b3..e3ad63a56 100644
--- a/src/init.cpp
+++ b/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)
diff --git a/src/main.cpp b/src/main.cpp
index f01bb8ec9..c1df9998a 100644
--- a/src/main.cpp
+++ b/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);
diff --git a/src/main.h b/src/main.h
index ec7cc2fdc..be0d2bf47 100644
--- a/src/main.h
+++ b/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) */
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index e8d76dd2f..7563c0788 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());
+}
diff --git a/src/txmempool.h b/src/txmempool.h
index e45867f71..e8572e7bd 100644
--- a/src/txmempool.h
+++ b/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);
 

From d355cf4420043a866e418c97778d999cd1958f61 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Fri, 2 Oct 2015 14:20:38 -0700
Subject: [PATCH 08/13] Only call TrimToSize once per reorg/blocks disconnect

---
 src/main.cpp                         | 27 ++++++++++++++++++---------
 src/main.h                           |  2 +-
 src/rpcrawtransaction.cpp            |  2 +-
 src/test/txvalidationcache_tests.cpp |  2 +-
 src/wallet/wallet.cpp                |  2 +-
 5 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/src/main.cpp b/src/main.cpp
index c1df9998a..f379ea46e 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -776,7 +776,7 @@ static std::string FormatStateMessage(const CValidationState &state)
 }
 
 bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransaction &tx, bool fLimitFree,
-                        bool* pfMissingInputs, bool fRejectAbsurdFee)
+                        bool* pfMissingInputs, bool fOverrideMempoolLimit, bool fRejectAbsurdFee)
 {
     AssertLockHeld(cs_main);
     if (pfMissingInputs)
@@ -956,13 +956,15 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         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);
+        if (!fOverrideMempoolLimit) {
+            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");
+            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);
@@ -2029,7 +2031,7 @@ void static UpdateTip(CBlockIndex *pindexNew) {
     }
 }
 
-/** Disconnect chainActive's tip. */
+/** Disconnect chainActive's tip. You want to manually re-limit mempool size after this */
 bool static DisconnectTip(CValidationState &state) {
     CBlockIndex *pindexDelete = chainActive.Tip();
     assert(pindexDelete);
@@ -2056,7 +2058,7 @@ bool static DisconnectTip(CValidationState &state) {
         // ignore validation errors in resurrected transactions
         list<CTransaction> removed;
         CValidationState stateDummy;
-        if (tx.IsCoinBase() || !AcceptToMemoryPool(mempool, stateDummy, tx, false, NULL)) {
+        if (tx.IsCoinBase() || !AcceptToMemoryPool(mempool, stateDummy, tx, false, NULL, true)) {
             mempool.remove(tx, removed, true);
         } else if (mempool.exists(tx.GetHash())) {
             vHashUpdate.push_back(tx.GetHash());
@@ -2229,9 +2231,11 @@ static bool ActivateBestChainStep(CValidationState &state, CBlockIndex *pindexMo
     const CBlockIndex *pindexFork = chainActive.FindFork(pindexMostWork);
 
     // Disconnect active blocks which are no longer in the best chain.
+    bool fBlocksDisconnected = false;
     while (chainActive.Tip() && chainActive.Tip() != pindexFork) {
         if (!DisconnectTip(state))
             return false;
+        fBlocksDisconnected = true;
     }
 
     // Build list of new blocks to connect.
@@ -2277,6 +2281,9 @@ static bool ActivateBestChainStep(CValidationState &state, CBlockIndex *pindexMo
     }
     }
 
+    if (fBlocksDisconnected)
+        mempool.TrimToSize(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+
     // Callbacks/notifications for a new best chain.
     if (fInvalidFound)
         CheckForkWarningConditionsOnNewFork(vpindexToConnect.back());
@@ -2363,6 +2370,8 @@ bool InvalidateBlock(CValidationState& state, CBlockIndex *pindex) {
         }
     }
 
+    mempool.TrimToSize(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000);
+
     // The resulting new best tip may not be in setBlockIndexCandidates anymore, so
     // add it again.
     BlockMap::iterator it = mapBlockIndex.begin();
diff --git a/src/main.h b/src/main.h
index be0d2bf47..202d2c772 100644
--- a/src/main.h
+++ b/src/main.h
@@ -229,7 +229,7 @@ void PruneAndFlush();
 
 /** (try to) add transaction to memory pool **/
 bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransaction &tx, bool fLimitFree,
-                        bool* pfMissingInputs, bool fRejectAbsurdFee=false);
+                        bool* pfMissingInputs, bool fOverrideMempoolLimit=false, bool fRejectAbsurdFee=false);
 
 
 struct CNodeStateStats {
diff --git a/src/rpcrawtransaction.cpp b/src/rpcrawtransaction.cpp
index 4dec53396..4b9647350 100644
--- a/src/rpcrawtransaction.cpp
+++ b/src/rpcrawtransaction.cpp
@@ -809,7 +809,7 @@ UniValue sendrawtransaction(const UniValue& params, bool fHelp)
         // push to local node and sync with wallets
         CValidationState state;
         bool fMissingInputs;
-        if (!AcceptToMemoryPool(mempool, state, tx, false, &fMissingInputs, !fOverrideFees)) {
+        if (!AcceptToMemoryPool(mempool, state, tx, false, &fMissingInputs, false, !fOverrideFees)) {
             if (state.IsInvalid()) {
                 throw JSONRPCError(RPC_TRANSACTION_REJECTED, strprintf("%i: %s", state.GetRejectCode(), state.GetRejectReason()));
             } else {
diff --git a/src/test/txvalidationcache_tests.cpp b/src/test/txvalidationcache_tests.cpp
index edad18644..9b8e1c088 100644
--- a/src/test/txvalidationcache_tests.cpp
+++ b/src/test/txvalidationcache_tests.cpp
@@ -23,7 +23,7 @@ ToMemPool(CMutableTransaction& tx)
     LOCK(cs_main);
 
     CValidationState state;
-    return AcceptToMemoryPool(mempool, state, tx, false, NULL, false);
+    return AcceptToMemoryPool(mempool, state, tx, false, NULL, true, false);
 }
 
 BOOST_FIXTURE_TEST_CASE(tx_mempool_block_doublespend, TestChain100Setup)
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index bd3004061..3f2d5a05f 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -2863,6 +2863,6 @@ int CMerkleTx::GetBlocksToMaturity() const
 bool CMerkleTx::AcceptToMemoryPool(bool fLimitFree, bool fRejectAbsurdFee)
 {
     CValidationState state;
-    return ::AcceptToMemoryPool(mempool, state, *this, fLimitFree, NULL, fRejectAbsurdFee);
+    return ::AcceptToMemoryPool(mempool, state, *this, fLimitFree, NULL, false, fRejectAbsurdFee);
 }
 

From 074cb155c2f01ba6ddc555c01943fc20c46c0b46 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Tue, 13 Oct 2015 00:43:15 -0700
Subject: [PATCH 09/13] Add reasonable test case for mempool trimming

---
 src/test/mempool_tests.cpp | 154 +++++++++++++++++++++++++++++++++++++
 src/txmempool.h            |   4 +-
 2 files changed, 157 insertions(+), 1 deletion(-)

diff --git a/src/test/mempool_tests.cpp b/src/test/mempool_tests.cpp
index 79d806bd2..dce278d99 100644
--- a/src/test/mempool_tests.cpp
+++ b/src/test/mempool_tests.cpp
@@ -281,4 +281,158 @@ BOOST_AUTO_TEST_CASE(MempoolIndexingTest)
     CheckSort(pool, snapshotOrder);
 }
 
+BOOST_AUTO_TEST_CASE(MempoolSizeLimitTest)
+{
+    CTxMemPool pool(CFeeRate(1000));
+
+    CMutableTransaction tx1 = CMutableTransaction();
+    tx1.vin.resize(1);
+    tx1.vin[0].scriptSig = CScript() << OP_1;
+    tx1.vout.resize(1);
+    tx1.vout[0].scriptPubKey = CScript() << OP_1 << OP_EQUAL;
+    tx1.vout[0].nValue = 10 * COIN;
+    pool.addUnchecked(tx1.GetHash(), CTxMemPoolEntry(tx1, 10000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx1)));
+
+    CMutableTransaction tx2 = CMutableTransaction();
+    tx2.vin.resize(1);
+    tx2.vin[0].scriptSig = CScript() << OP_2;
+    tx2.vout.resize(1);
+    tx2.vout[0].scriptPubKey = CScript() << OP_2 << OP_EQUAL;
+    tx2.vout[0].nValue = 10 * COIN;
+    pool.addUnchecked(tx2.GetHash(), CTxMemPoolEntry(tx2, 5000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx2)));
+
+    pool.TrimToSize(pool.DynamicMemoryUsage()); // should do nothing
+    BOOST_CHECK(pool.exists(tx1.GetHash()));
+    BOOST_CHECK(pool.exists(tx2.GetHash()));
+
+    pool.TrimToSize(pool.DynamicMemoryUsage() * 3 / 4); // should remove the lower-feerate transaction
+    BOOST_CHECK(pool.exists(tx1.GetHash()));
+    BOOST_CHECK(!pool.exists(tx2.GetHash()));
+
+    pool.addUnchecked(tx2.GetHash(), CTxMemPoolEntry(tx2, 5000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx2)));
+    CMutableTransaction tx3 = CMutableTransaction();
+    tx3.vin.resize(1);
+    tx3.vin[0].prevout = COutPoint(tx2.GetHash(), 0);
+    tx3.vin[0].scriptSig = CScript() << OP_2;
+    tx3.vout.resize(1);
+    tx3.vout[0].scriptPubKey = CScript() << OP_3 << OP_EQUAL;
+    tx3.vout[0].nValue = 10 * COIN;
+    pool.addUnchecked(tx3.GetHash(), CTxMemPoolEntry(tx3, 20000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx3)));
+
+    pool.TrimToSize(pool.DynamicMemoryUsage() * 3 / 4); // tx3 should pay for tx2 (CPFP)
+    BOOST_CHECK(!pool.exists(tx1.GetHash()));
+    BOOST_CHECK(pool.exists(tx2.GetHash()));
+    BOOST_CHECK(pool.exists(tx3.GetHash()));
+
+    pool.TrimToSize(::GetSerializeSize(CTransaction(tx1), SER_NETWORK, PROTOCOL_VERSION)); // mempool is limited to tx1's size in memory usage, so nothing fits
+    BOOST_CHECK(!pool.exists(tx1.GetHash()));
+    BOOST_CHECK(!pool.exists(tx2.GetHash()));
+    BOOST_CHECK(!pool.exists(tx3.GetHash()));
+
+    CFeeRate maxFeeRateRemoved(25000, ::GetSerializeSize(CTransaction(tx3), SER_NETWORK, PROTOCOL_VERSION) + ::GetSerializeSize(CTransaction(tx2), SER_NETWORK, PROTOCOL_VERSION));
+    BOOST_CHECK_EQUAL(pool.GetMinFee(1).GetFeePerK(), maxFeeRateRemoved.GetFeePerK() + 1000);
+
+    CMutableTransaction tx4 = CMutableTransaction();
+    tx4.vin.resize(2);
+    tx4.vin[0].prevout.SetNull();
+    tx4.vin[0].scriptSig = CScript() << OP_4;
+    tx4.vin[1].prevout.SetNull();
+    tx4.vin[1].scriptSig = CScript() << OP_4;
+    tx4.vout.resize(2);
+    tx4.vout[0].scriptPubKey = CScript() << OP_4 << OP_EQUAL;
+    tx4.vout[0].nValue = 10 * COIN;
+    tx4.vout[1].scriptPubKey = CScript() << OP_4 << OP_EQUAL;
+    tx4.vout[1].nValue = 10 * COIN;
+
+    CMutableTransaction tx5 = CMutableTransaction();
+    tx5.vin.resize(2);
+    tx5.vin[0].prevout = COutPoint(tx4.GetHash(), 0);
+    tx5.vin[0].scriptSig = CScript() << OP_4;
+    tx5.vin[1].prevout.SetNull();
+    tx5.vin[1].scriptSig = CScript() << OP_5;
+    tx5.vout.resize(2);
+    tx5.vout[0].scriptPubKey = CScript() << OP_5 << OP_EQUAL;
+    tx5.vout[0].nValue = 10 * COIN;
+    tx5.vout[1].scriptPubKey = CScript() << OP_5 << OP_EQUAL;
+    tx5.vout[1].nValue = 10 * COIN;
+
+    CMutableTransaction tx6 = CMutableTransaction();
+    tx6.vin.resize(2);
+    tx6.vin[0].prevout = COutPoint(tx4.GetHash(), 1);
+    tx6.vin[0].scriptSig = CScript() << OP_4;
+    tx6.vin[1].prevout.SetNull();
+    tx6.vin[1].scriptSig = CScript() << OP_6;
+    tx6.vout.resize(2);
+    tx6.vout[0].scriptPubKey = CScript() << OP_6 << OP_EQUAL;
+    tx6.vout[0].nValue = 10 * COIN;
+    tx6.vout[1].scriptPubKey = CScript() << OP_6 << OP_EQUAL;
+    tx6.vout[1].nValue = 10 * COIN;
+
+    CMutableTransaction tx7 = CMutableTransaction();
+    tx7.vin.resize(2);
+    tx7.vin[0].prevout = COutPoint(tx5.GetHash(), 0);
+    tx7.vin[0].scriptSig = CScript() << OP_5;
+    tx7.vin[1].prevout = COutPoint(tx6.GetHash(), 0);
+    tx7.vin[1].scriptSig = CScript() << OP_6;
+    tx7.vout.resize(2);
+    tx7.vout[0].scriptPubKey = CScript() << OP_7 << OP_EQUAL;
+    tx7.vout[0].nValue = 10 * COIN;
+    tx7.vout[0].scriptPubKey = CScript() << OP_7 << OP_EQUAL;
+    tx7.vout[0].nValue = 10 * COIN;
+
+    pool.addUnchecked(tx4.GetHash(), CTxMemPoolEntry(tx4, 7000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx4)));
+    pool.addUnchecked(tx5.GetHash(), CTxMemPoolEntry(tx5, 1000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx5)));
+    pool.addUnchecked(tx6.GetHash(), CTxMemPoolEntry(tx6, 1100LL, 0, 10.0, 1, pool.HasNoInputsOf(tx6)));
+    pool.addUnchecked(tx7.GetHash(), CTxMemPoolEntry(tx7, 9000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx7)));
+
+    // we only require this remove, at max, 2 txn, because its not clear what we're really optimizing for aside from that
+    pool.TrimToSize(pool.DynamicMemoryUsage() - 1);
+    BOOST_CHECK(pool.exists(tx4.GetHash()));
+    BOOST_CHECK(pool.exists(tx6.GetHash()));
+    BOOST_CHECK(!pool.exists(tx7.GetHash()));
+
+    if (!pool.exists(tx5.GetHash()))
+        pool.addUnchecked(tx5.GetHash(), CTxMemPoolEntry(tx5, 1000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx5)));
+    pool.addUnchecked(tx7.GetHash(), CTxMemPoolEntry(tx7, 9000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx7)));
+
+    pool.TrimToSize(pool.DynamicMemoryUsage() / 2); // should maximize mempool size by only removing 5/7
+    BOOST_CHECK(pool.exists(tx4.GetHash()));
+    BOOST_CHECK(!pool.exists(tx5.GetHash()));
+    BOOST_CHECK(pool.exists(tx6.GetHash()));
+    BOOST_CHECK(!pool.exists(tx7.GetHash()));
+
+    pool.addUnchecked(tx5.GetHash(), CTxMemPoolEntry(tx5, 1000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx5)));
+    pool.addUnchecked(tx7.GetHash(), CTxMemPoolEntry(tx7, 9000LL, 0, 10.0, 1, pool.HasNoInputsOf(tx7)));
+
+    std::vector<CTransaction> vtx;
+    std::list<CTransaction> conflicts;
+    SetMockTime(42);
+    SetMockTime(42 + CTxMemPool::ROLLING_FEE_HALFLIFE);
+    BOOST_CHECK_EQUAL(pool.GetMinFee(1).GetFeePerK(), maxFeeRateRemoved.GetFeePerK() + 1000);
+    // ... we should keep the same min fee until we get a block
+    pool.removeForBlock(vtx, 1, conflicts);
+    SetMockTime(42 + 2*CTxMemPool::ROLLING_FEE_HALFLIFE);
+    BOOST_CHECK_EQUAL(pool.GetMinFee(1).GetFeePerK(), (maxFeeRateRemoved.GetFeePerK() + 1000)/2);
+    // ... then feerate should drop 1/2 each halflife
+
+    SetMockTime(42 + 2*CTxMemPool::ROLLING_FEE_HALFLIFE + CTxMemPool::ROLLING_FEE_HALFLIFE/2);
+    BOOST_CHECK_EQUAL(pool.GetMinFee(pool.DynamicMemoryUsage() * 5 / 2).GetFeePerK(), (maxFeeRateRemoved.GetFeePerK() + 1000)/4);
+    // ... with a 1/2 halflife when mempool is < 1/2 its target size
+
+    SetMockTime(42 + 2*CTxMemPool::ROLLING_FEE_HALFLIFE + CTxMemPool::ROLLING_FEE_HALFLIFE/2 + CTxMemPool::ROLLING_FEE_HALFLIFE/4);
+    BOOST_CHECK_EQUAL(pool.GetMinFee(pool.DynamicMemoryUsage() * 9 / 2).GetFeePerK(), (maxFeeRateRemoved.GetFeePerK() + 1000)/8);
+    // ... with a 1/4 halflife when mempool is < 1/4 its target size
+
+    SetMockTime(42 + 7*CTxMemPool::ROLLING_FEE_HALFLIFE + CTxMemPool::ROLLING_FEE_HALFLIFE/2 + CTxMemPool::ROLLING_FEE_HALFLIFE/4);
+    BOOST_CHECK_EQUAL(pool.GetMinFee(1).GetFeePerK(), 1000);
+    // ... but feerate should never drop below 1000
+
+    SetMockTime(42 + 8*CTxMemPool::ROLLING_FEE_HALFLIFE + CTxMemPool::ROLLING_FEE_HALFLIFE/2 + CTxMemPool::ROLLING_FEE_HALFLIFE/4);
+    pool.GetMinFee(1);
+    BOOST_CHECK_EQUAL(pool.GetMinFee(1).GetFeePerK(), 0);
+    // ... unless it has gone all the way to 0 (after getting past 1000/2)
+
+    SetMockTime(0);
+}
+
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/txmempool.h b/src/txmempool.h
index e8572e7bd..319b972c3 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -290,11 +290,13 @@ private:
     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:
+
+    static const int ROLLING_FEE_HALFLIFE = 60 * 60 * 12; // public only for testing
+
     typedef boost::multi_index_container<
         CTxMemPoolEntry,
         boost::multi_index::indexed_by<

From 9e93640be6c49fa1505ba5c5df8c89210da5a6e4 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Tue, 13 Oct 2015 01:06:59 -0700
Subject: [PATCH 10/13] Drop minRelayTxFee to 1000

There is no exact science to setting this parameter, but 5000
(just over 1 US cent at the time of writing) is higher than the
cost to relay a transaction around the network (the new benchmark
due to mempool limiting).
---
 src/main.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/main.cpp b/src/main.cpp
index f379ea46e..31b3c2114 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -75,7 +75,7 @@ uint64_t nPruneTarget = 0;
 bool fAlerts = DEFAULT_ALERTS;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
-CFeeRate minRelayTxFee = CFeeRate(5000);
+CFeeRate minRelayTxFee = CFeeRate(1000);
 
 CTxMemPool mempool(::minRelayTxFee);
 

From 8abe0f56584ff49ad250115eb1f0a9ac8f9cf0ca Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Wed, 14 Oct 2015 12:44:18 -0700
Subject: [PATCH 11/13] Undo GetMinFee-requires-extra-call-to-hit-0

---
 src/test/mempool_tests.cpp | 1 -
 src/txmempool.cpp          | 4 +++-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/test/mempool_tests.cpp b/src/test/mempool_tests.cpp
index dce278d99..0cf906a25 100644
--- a/src/test/mempool_tests.cpp
+++ b/src/test/mempool_tests.cpp
@@ -428,7 +428,6 @@ BOOST_AUTO_TEST_CASE(MempoolSizeLimitTest)
     // ... but feerate should never drop below 1000
 
     SetMockTime(42 + 8*CTxMemPool::ROLLING_FEE_HALFLIFE + CTxMemPool::ROLLING_FEE_HALFLIFE/2 + CTxMemPool::ROLLING_FEE_HALFLIFE/4);
-    pool.GetMinFee(1);
     BOOST_CHECK_EQUAL(pool.GetMinFee(1).GetFeePerK(), 0);
     // ... unless it has gone all the way to 0 (after getting past 1000/2)
 
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 7563c0788..9a651049d 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -879,8 +879,10 @@ CFeeRate CTxMemPool::GetMinFee(size_t sizelimit) const {
         rollingMinimumFeeRate = rollingMinimumFeeRate / pow(2.0, (time - lastRollingFeeUpdate) / halflife);
         lastRollingFeeUpdate = time;
 
-        if (rollingMinimumFeeRate < minReasonableRelayFee.GetFeePerK() / 2)
+        if (rollingMinimumFeeRate < minReasonableRelayFee.GetFeePerK() / 2) {
             rollingMinimumFeeRate = 0;
+            return CFeeRate(0);
+        }
     }
     return std::max(CFeeRate(rollingMinimumFeeRate), minReasonableRelayFee);
 }

From 2bc50187ee6b6a3e4dfaa23bf292d63ad2915945 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Wed, 14 Oct 2015 12:46:20 -0700
Subject: [PATCH 12/13] Fix comment formatting tabs

---
 src/txmempool.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/txmempool.h b/src/txmempool.h
index 319b972c3..d44995eef 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -420,11 +420,11 @@ 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.
-	 */
+      *  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. */

From 58254aa3bc2e92840679183cc884eb76670af525 Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Mon, 19 Oct 2015 02:40:25 -0700
Subject: [PATCH 13/13] Fix stale comment in CTxMemPool::TrimToSize.

---
 src/txmempool.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 9a651049d..bb148005c 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -903,10 +903,10 @@ void CTxMemPool::TrimToSize(size_t sizelimit) {
     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.
+        // 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->GetFeesWithDescendants(), it->GetSizeWithDescendants());
         removed += minReasonableRelayFee;
         trackPackageRemoved(removed);