[IndexedDB] Pool and evict leveldb iterators, to save memory

content/browser/indexed_db/leveldb/leveldb_database.cc

Index: content/browser/indexed_db/leveldb/leveldb_database.cc
diff --git a/content/browser/indexed_db/leveldb/leveldb_database.cc b/content/browser/indexed_db/leveldb/leveldb_database.cc
index 7eaa89405cb23782b5c070ca97947a64edb9f3ef..64ff33ab46c4d22e36e6c255255df92e14a574cf 100644
--- a/content/browser/indexed_db/leveldb/leveldb_database.cc
+++ b/content/browser/indexed_db/leveldb/leveldb_database.cc
@@ -92,9 +92,13 @@ LevelDBSnapshot::LevelDBSnapshot(LevelDBDatabase* db)
 
 LevelDBSnapshot::~LevelDBSnapshot() { db_->ReleaseSnapshot(snapshot_); }
 
-LevelDBDatabase::LevelDBDatabase() {}
+LevelDBDatabase::LevelDBDatabase(size_t max_open_iterators)
+    : eviction_enabled_(max_open_iterators != NO_CURSOR_EVICTION),
+      iterator_lru_(max_open_iterators) {}

[pwnall, 2017/03/24 09:16:02]

Shouldn't the MRUCache be initialized with NO_AUTO_EVICT? I think we'd leak if
it would do its own eviction.

[dmurph, 2017/03/24 23:33:39]

Creating our own struct so we do correct eviction. let me know what you think.

 
 LevelDBDatabase::~LevelDBDatabase() {
+  LOCAL_HISTOGRAM_COUNTS_10000("Storage.IndexedDB.LevelDB.MaxIterators",
+                               max_iterators_);
   base::trace_event::MemoryDumpManager::GetInstance()->UnregisterDumpProvider(
       this);
   // db_'s destructor uses comparator_adapter_; order of deletion is important.
@@ -288,6 +292,7 @@ static void HistogramLevelDBError(const std::string& histogram_name,
 leveldb::Status LevelDBDatabase::Open(const base::FilePath& file_name,
                                       const LevelDBComparator* comparator,
                                       std::unique_ptr<LevelDBDatabase>* result,
+                                      size_t max_open_cursors,
                                       bool* is_disk_full) {
   IDB_TRACE("LevelDBDatabase::Open");
   base::TimeTicks begin_time = base::TimeTicks::Now();
@@ -318,7 +323,7 @@ leveldb::Status LevelDBDatabase::Open(const base::FilePath& file_name,
 
   CheckFreeSpace("Success", file_name);
 
-  (*result) = base::WrapUnique(new LevelDBDatabase());
+  (*result) = base::WrapUnique(new LevelDBDatabase(max_open_cursors));
   (*result)->db_ = std::move(db);
   (*result)->comparator_adapter_ = std::move(comparator_adapter);
   (*result)->comparator_ = comparator;
@@ -348,8 +353,9 @@ std::unique_ptr<LevelDBDatabase> LevelDBDatabase::OpenInMemory(
     return std::unique_ptr<LevelDBDatabase>();
   }
 
+  // Since the database is in-memory, cursor pooling is pointless.
   std::unique_ptr<LevelDBDatabase> result =
-      base::WrapUnique(new LevelDBDatabase());
+      base::WrapUnique(new LevelDBDatabase(NO_CURSOR_EVICTION));
   result->env_ = std::move(in_memory_env);
   result->db_ = std::move(db);
   result->comparator_adapter_ = std::move(comparator_adapter);
@@ -433,9 +439,12 @@ std::unique_ptr<LevelDBIterator> LevelDBDatabase::CreateIterator(
                                          // performance impact is too great.
   read_options.snapshot = snapshot ? snapshot->snapshot_ : 0;
 
+  num_iterators_++;
+  max_iterators_ = std::max(max_iterators_, num_iterators_);
   std::unique_ptr<leveldb::Iterator> i(db_->NewIterator(read_options));
   return std::unique_ptr<LevelDBIterator>(
-      IndexedDBClassFactory::Get()->CreateIteratorImpl(std::move(i)));
+      IndexedDBClassFactory::Get()->CreateIteratorImpl(std::move(i), this,
+                                                       read_options.snapshot));

[pwnall, 2017/03/24 09:16:02]

Why doesn't the newly created iterator get added to the LRU here?

[dmurph, 2017/03/24 23:33:39]

We don't actually allocated memory in the iterator until we do a Seek operation.
So I don't add it until it's actually doing a costly operation. Do you think I
should change that?

[pwnall, 2017/03/27 17:08:03]

I would at the very least document the assumption. I guess the main downside is
that the system is a tad more difficult to reason about ("we have at most 50
seeked iterators", vs "we have at most 50 iterators") but, as long as you're
sure about the resource usage, I suppose this is fine.

[dmurph, 2017/03/27 21:32:04]

Done.

 }
 
 const LevelDBComparator* LevelDBDatabase::Comparator() const {
@@ -479,4 +488,35 @@ bool LevelDBDatabase::OnMemoryDump(
   return true;
 }
 
+std::unique_ptr<leveldb::Iterator> LevelDBDatabase::CreateLevelDBIterator(
+    const leveldb::Snapshot* snapshot) {
+  leveldb::ReadOptions read_options;
+  read_options.verify_checksums = true;
+  read_options.snapshot = snapshot;
+  return std::unique_ptr<leveldb::Iterator>(db_->NewIterator(read_options));
+}
+
+void LevelDBDatabase::NotifyIteratorUsed(LevelDBIterator* iter) {
+  // This line updates the LRU if the item exists.
+  if (!eviction_enabled_ || iterator_lru_.Get(iter) != iterator_lru_.end())
+    return;
+  if (iterator_lru_.size() == iterator_lru_.max_size()) {
+    auto to_evict_iter = iterator_lru_.rbegin();
+    LevelDBIterator* to_evict = to_evict_iter->first;
+    to_evict->PurgeMemory();
+    iterator_lru_.Erase(to_evict_iter);
+  }
+  iterator_lru_.Put(iter, iter);
+}
+
+void LevelDBDatabase::NotifyIteratorDestroyed(LevelDBIterator* iter) {
+  DCHECK_GT(num_iterators_, 0u);
+  --num_iterators_;
+  if (!eviction_enabled_)
+    return;
+  auto it = iterator_lru_.Peek(iter);
+  if (it == iterator_lru_.end())
+    return;
+  iterator_lru_.Erase(it);
+}
 }  // namespace content