Do not keep fully spent but unwritten CCoins entries cached.

Instead of storing CCoins entries directly in CCoinsMap, store a CCoinsCacheEntry
which additionally keeps track of whether a particular entry is:
* dirty: potentially different from its parent view.
* fresh: the parent view is known to not have a non-pruned version.

This allows us to skip non-dirty cache entries when pushing batches of changes up,
and to remove CCoins entries about transactions that are fully spent before the
parent cache learns about them.

src/coins.cpp

@@ -71,18 +71,6 @@ CCoinsKeyHasher::CCoinsKeyHasher() : salt(GetRandHash()) {}
 
 CCoinsViewCache::CCoinsViewCache(CCoinsView &baseIn, bool fDummy) : CCoinsViewBacked(baseIn), hasModifier(false), hashBlock(0) { }
 
-bool CCoinsViewCache::GetCoins(const uint256 &txid, CCoins &coins) const {
-    if (cacheCoins.count(txid)) {
-        coins = cacheCoins[txid];
-        return true;
-    }
-    if (base->GetCoins(txid, coins)) {
-        cacheCoins[txid] = coins;
-        return true;
-    }
-    return false;
-}
-
 CCoinsViewCache::~CCoinsViewCache()
 {
     assert(!hasModifier);
@@ -93,21 +81,43 @@ CCoinsMap::const_iterator CCoinsViewCache::FetchCoins(const uint256 &txid) const
     if (it != cacheCoins.end())
         return it;
     CCoins tmp;
-    if (!base->GetCoins(txid,tmp))
+    if (!base->GetCoins(txid, tmp))
         return cacheCoins.end();
-    CCoinsMap::iterator ret = cacheCoins.insert(it, std::make_pair(txid, CCoins()));
+    CCoinsMap::iterator ret = cacheCoins.insert(std::make_pair(txid, CCoinsCacheEntry())).first;
-    tmp.swap(ret->second);
+    tmp.swap(ret->second.coins);
+    if (ret->second.coins.IsPruned()) {
+        // The parent only has an empty entry for this txid; we can consider our
+        // version as fresh.
+        ret->second.flags = CCoinsCacheEntry::FRESH;
+    }
     return ret;
 }
 
+bool CCoinsViewCache::GetCoins(const uint256 &txid, CCoins &coins) const {
+    CCoinsMap::const_iterator it = FetchCoins(txid);
+    if (it != cacheCoins.end()) {
+        coins = it->second.coins;
+        return true;
+    }
+    return false;
+}
+
 CCoinsModifier CCoinsViewCache::ModifyCoins(const uint256 &txid) {
     assert(!hasModifier);
     hasModifier = true;
-    std::pair<CCoinsMap::iterator, bool> ret = cacheCoins.insert(std::make_pair(txid, CCoins()));
+    std::pair<CCoinsMap::iterator, bool> ret = cacheCoins.insert(std::make_pair(txid, CCoinsCacheEntry()));
     if (ret.second) {
-        if (!base->GetCoins(txid, ret.first->second))
+        if (!base->GetCoins(txid, ret.first->second.coins)) {
-            ret.first->second.Clear();
+            // The parent view does not have this entry; mark it as fresh.
+            ret.first->second.coins.Clear();
+            ret.first->second.flags = CCoinsCacheEntry::FRESH;
+        } else if (ret.first->second.coins.IsPruned()) {
+            // The parent view only has a pruned entry for this; mark it as fresh.
+            ret.first->second.flags = CCoinsCacheEntry::FRESH;
         }
+    }
+    // Assume that whenever ModifyCoins is called, the entry will be modified.
+    ret.first->second.flags |= CCoinsCacheEntry::DIRTY;
     return CCoinsModifier(*this, ret.first);
 }
 
@@ -116,7 +126,7 @@ const CCoins* CCoinsViewCache::AccessCoins(const uint256 &txid) const {
     if (it == cacheCoins.end()) {
         return NULL;
     } else {
-        return &it->second;
+        return &it->second.coins;
     }
 }
 
@@ -126,7 +136,7 @@ bool CCoinsViewCache::HaveCoins(const uint256 &txid) const {
     // as we only care about the case where an transaction was replaced entirely
     // in a reorganization (which wipes vout entirely, as opposed to spending
     // which just cleans individual outputs).
-    return (it != cacheCoins.end() && !it->second.vout.empty());
+    return (it != cacheCoins.end() && !it->second.coins.vout.empty());
 }
 
 uint256 CCoinsViewCache::GetBestBlock() const {
@@ -142,7 +152,32 @@ void CCoinsViewCache::SetBestBlock(const uint256 &hashBlockIn) {
 bool CCoinsViewCache::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlockIn) {
     assert(!hasModifier);
     for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end();) {
-        cacheCoins[it->first].swap(it->second);
+        if (it->second.flags & CCoinsCacheEntry::DIRTY) { // Ignore non-dirty entries (optimization).
+            CCoinsMap::iterator itUs = cacheCoins.find(it->first);
+            if (itUs == cacheCoins.end()) {
+                if (!it->second.coins.IsPruned()) {
+                    // The parent cache does not have an entry, while the child
+                    // cache does have (a non-pruned) one. Move the data up, and
+                    // mark it as fresh (if the grandparent did have it, we
+                    // would have pulled it in at first GetCoins).
+                    assert(it->second.flags & CCoinsCacheEntry::FRESH);
+                    CCoinsCacheEntry& entry = cacheCoins[it->first];
+                    entry.coins.swap(it->second.coins);
+                    entry.flags = CCoinsCacheEntry::DIRTY | CCoinsCacheEntry::FRESH;
+                }
+            } else {
+                if ((itUs->second.flags & CCoinsCacheEntry::FRESH) && it->second.coins.IsPruned()) {
+                    // The grandparent does not have an entry, and the child is
+                    // modified and being pruned. This means we can just delete
+                    // it from the parent.
+                    cacheCoins.erase(itUs);
+                } else {
+                    // A normal modification.
+                    itUs->second.coins.swap(it->second.coins);
+                    itUs->second.flags |= CCoinsCacheEntry::DIRTY;
+                }
+            }
+        }
         CCoinsMap::iterator itOld = it++;
         mapCoins.erase(itOld);
     }
@@ -212,8 +247,12 @@ double CCoinsViewCache::GetPriority(const CTransaction &tx, int nHeight) const
 
 CCoinsModifier::CCoinsModifier(CCoinsViewCache& cache_, CCoinsMap::iterator it_) : cache(cache_), it(it_) {}
 
-CCoinsModifier::~CCoinsModifier() {
+CCoinsModifier::~CCoinsModifier()
+{
     assert(cache.hasModifier);
     cache.hasModifier = false;
-    it->second.Cleanup();
+    it->second.coins.Cleanup();
+    if ((it->second.flags & CCoinsCacheEntry::FRESH) && it->second.coins.IsPruned()) {
+        cache.cacheCoins.erase(it);
+    }
 }

src/coins.h

@@ -271,7 +271,20 @@ public:
     }
 };
 
-typedef boost::unordered_map<uint256, CCoins, CCoinsKeyHasher> CCoinsMap;
+struct CCoinsCacheEntry
+{
+    CCoins coins; // The actual cached data.
+    unsigned char flags;
+
+    enum Flags {
+        DIRTY = (1 << 0), // This cache entry is potentially different from the version in the parent view.
+        FRESH = (1 << 1), // The parent view does not have this entry (or it is pruned).
+    };
+
+    CCoinsCacheEntry() : coins(), flags(0) {}
+};
+
+typedef boost::unordered_map<uint256, CCoinsCacheEntry, CCoinsKeyHasher> CCoinsMap;
 
 struct CCoinsStats
 {
@@ -343,8 +356,8 @@ private:
     CCoinsModifier(CCoinsViewCache& cache_, CCoinsMap::iterator it_);
 
 public:
-    CCoins* operator->() { return &it->second; }
+    CCoins* operator->() { return &it->second.coins; }
-    CCoins& operator*() { return it->second; }
+    CCoins& operator*() { return it->second.coins; }
     ~CCoinsModifier();
     friend class CCoinsViewCache;
 };

src/main.cpp

@@ -1797,10 +1797,10 @@ void static UpdateTip(CBlockIndex *pindexNew) {
     nTimeBestReceived = GetTime();
     mempool.AddTransactionsUpdated(1);
 
-    LogPrintf("UpdateTip: new best=%s  height=%d  log2_work=%.8g  tx=%lu  date=%s progress=%f\n",
+    LogPrintf("UpdateTip: new best=%s  height=%d  log2_work=%.8g  tx=%lu  date=%s progress=%f  cache=%u\n",
       chainActive.Tip()->GetBlockHash().ToString(), chainActive.Height(), log(chainActive.Tip()->nChainWork.getdouble())/log(2.0), (unsigned long)chainActive.Tip()->nChainTx,
       DateTimeStrFormat("%Y-%m-%d %H:%M:%S", chainActive.Tip()->GetBlockTime()),
-      Checkpoints::GuessVerificationProgress(chainActive.Tip()));
+      Checkpoints::GuessVerificationProgress(chainActive.Tip()), (unsigned int)pcoinsTip->GetCacheSize());
 
     cvBlockChange.notify_all();
 

src/txdb.cpp

@@ -45,17 +45,22 @@ uint256 CCoinsViewDB::GetBestBlock() const {
 }
 
 bool CCoinsViewDB::BatchWrite(CCoinsMap &mapCoins, const uint256 &hashBlock) {
-    LogPrint("coindb", "Committing %u changed transactions to coin database...\n", (unsigned int)mapCoins.size());
-
     CLevelDBBatch batch;
+    size_t count = 0;
+    size_t changed = 0;
     for (CCoinsMap::iterator it = mapCoins.begin(); it != mapCoins.end();) {
-        BatchWriteCoins(batch, it->first, it->second);
+        if (it->second.flags & CCoinsCacheEntry::DIRTY) {
+            BatchWriteCoins(batch, it->first, it->second.coins);
+            changed++;
+        }
+        count++;
         CCoinsMap::iterator itOld = it++;
         mapCoins.erase(itOld);
     }
     if (hashBlock != uint256(0))
         BatchWriteHashBestChain(batch, hashBlock);
 
+    LogPrint("coindb", "Committing %u changed transactions (out of %u) to coin database...\n", (unsigned int)changed, (unsigned int)count);
     return db.WriteBatch(batch);
 }