[sql] Use memory-mapped I/O for sql::Connection.

sql/connection.cc

Index: sql/connection.cc
diff --git a/sql/connection.cc b/sql/connection.cc
index 300e5e6eaaba53f177224139dfc7d131065f0c32..20f0f075cbafb0dd6c09f4afe5f4a021f5522b36 100644
--- a/sql/connection.cc
+++ b/sql/connection.cc
@@ -282,6 +282,9 @@ Connection::Connection()
       needs_rollback_(false),
       in_memory_(false),
       poisoned_(false),
+      mmap_disabled_(false),
+      mmap_enabled_(false),
+      total_changes_at_last_release_(0),
       stats_histogram_(NULL),
       commit_time_histogram_(NULL),
       autocommit_time_histogram_(NULL),
@@ -466,6 +469,75 @@ void Connection::Preload() {
   }
 }
 
+// SQLite keeps unused pages associated with a connection in a cache.  It asks
+// the cache for pages by an id, and if the page is present and the database is
+// unchanged, it considers the content of the page valid and doesn't read it
+// from disk.  When memory-mapped I/O is enabled, on read SQLite uses page
+// structures created from the memory map data before consulting the cache.  On
+// write SQLite creates a new in-memory page structure, copies the data from the
+// memory map, and later writes it, releasing the updated page back to the
+// cache.
+//
+// This means that in memory-mapped mode, the contents of the cached pages are
+// not re-used for reads, but they are re-used for writes if the re-written page
+// is still in the cache. The implementation of sqlite3_db_release_memory() as
+// of SQLite 3.8.7.4 frees all pages from pcaches associated with the
+// connection, so it should free these pages.
+//
+// Unfortunately, the zero page is also freed.  That page is never accessed
+// using memory-mapped I/O, and the cached copy can be re-used after verifying
+// the file change counter on disk.  Also, fresh pages from cache receive some
+// pager-level initialization before they can be used.  Since the information
+// involved will immediately be accessed in various ways, it is unclear if the
+// additional overhead is material, or just moving processor cache effects
+// around.
+//
+// TODO(shess): It would be better to release the pages immediately when they
+// are no longer needed.  This would basically happen after SQLite commits a
+// transaction.  I had implemented a pcache wrapper to do this, but it involved
+// layering violations, and it had to be setup before any other sqlite call,
+// which was brittle.  Also, for large files it would actually make sense to
+// maintain the existing pcache behavior for blocks past the memory-mapped
+// segment.  I think drh would accept a reasonable implementation of the overall
+// concept for upstreaming to SQLite core.
+//
+// TODO(shess): Another possibility would be to set the cache size small, which
+// would keep the zero page around, plus some pre-initialized pages, and SQLite
+// can manage things.  The downside is that updates larger than the cache would
+// spill to the journal.  That could be compensated by setting cache_spill to
+// false.  The downside then is that it allows open-ended use of memory for
+// large transactions.
+//
+// TODO(shess): The TrimMemory() trick of bouncing the cache size would also
+// work.  There could be two prepared statements, one for cache_size=1 one for
+// cache_size=goal.
+void Connection::ReleaseCacheMemoryIfNeeded(bool implicit_change_performed) {
+  DCHECK(is_open());
+
+  // If memory-mapping is not enabled, the page cache helps performance.
+  if (!mmap_enabled_)
+    return;
+
+  // On caller request, force the change comparison to fail.  Done before the
+  // transaction-nesting test so that the signal can carry to transaction
+  // commit.
+  if (implicit_change_performed)
+    --total_changes_at_last_release_;
+
+  // Cached pages may be re-used within the same transaction.
+  if (transaction_nesting())
+    return;
+
+  // If no changes have been made, skip flushing.  This allows the first page of
+  // the database to remain in cache across multiple reads.
+  const int total_changes = sqlite3_total_changes(db_);
+  if (total_changes == total_changes_at_last_release_)
+    return;
+
+  total_changes_at_last_release_ = total_changes;
+  sqlite3_db_release_memory(db_);
+}
+
 void Connection::TrimMemory(bool aggressively) {
   if (!db_)
     return;
@@ -776,6 +848,9 @@ bool Connection::CommitTransaction() {
   RecordCommitTime(delta);
   RecordOneEvent(EVENT_COMMIT);
 
+  // Release dirty cache pages after the transaction closes.
+  ReleaseCacheMemoryIfNeeded(false);
+
   return ret;
 }
 
@@ -866,6 +941,12 @@ int Connection::ExecuteAndReturnErrorCode(const char* sql) {
     const base::TimeDelta delta = Now() - before;
     RecordTimeAndChanges(delta, read_only);
   }
+
+  // Most calls to Execute() modify the database.  The main exceptions would be
+  // calls such as CREATE TABLE IF NOT EXISTS which could modify the database
+  // but sometimes don't.
+  ReleaseCacheMemoryIfNeeded(true);
+
   return rc;
 }
 
@@ -1230,6 +1311,18 @@ bool Connection::OpenInternal(const std::string& file_name,
   // secure_delete.
   ignore_result(Execute("PRAGMA journal_mode = TRUNCATE"));
 
+  // Enable memory-mapped access.  This value will be capped by
+  // SQLITE_MAX_MMAP_SIZE, which could be different between 32-bit and 64-bit
+  // platforms.
+  mmap_enabled_ = false;
+  if (!mmap_disabled_)
+    ignore_result(Execute("PRAGMA mmap_size = 268435456"));  // 256MB.
+  {
+    Statement s(GetUniqueStatement("PRAGMA mmap_size"));
+    if (s.Step() && s.ColumnInt64(0) > 0)
+      mmap_enabled_ = true;
+  }
+
   const base::TimeDelta kBusyTimeout =
     base::TimeDelta::FromSeconds(kBusyTimeoutSeconds);
 
@@ -1273,6 +1366,11 @@ void Connection::DoRollback() {
   RecordUpdateTime(delta);
   RecordOneEvent(EVENT_ROLLBACK);
 
+  // The cache may have been accumulating dirty pages for commit.  Note that in
+  // some cases sql::Transaction can fire rollback after a database is closed.

[Scott Hess - ex-Googler, 2015/10/02 23:42:44]

This test _could_ be at the top of this method, but the ROLLBACK itself could
cause the error handler to close things.

+  if (is_open())
+    ReleaseCacheMemoryIfNeeded(false);
+
   needs_rollback_ = false;
 }