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

sql/connection.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SQL_CONNECTION_H_
 #define SQL_CONNECTION_H_
 
 #include <stdint.h>
 #include <map>
 #include <set>
@@ skipping 244 matching lines @@
   // decision.  Preloading should almost always improve later performance
   // numbers for this database simply because it pulls operations forward, but
   // if the data isn't actually used soon then preloading just slows down
   // everything else.
   void Preload();
 
   // Try to trim the cache memory used by the database.  If |aggressively| is
   // true, this function will try to free all of the cache memory it can. If
   // |aggressively| is false, this function will try to cut cache memory
   // usage by half.
+  // NOTE(shess): See also ReleaseCacheMemory().

[rmcilroy, 2015/09/17 17:17:44]

Not sure this comment is needed given ReleaseCacheMemory is private. Are there
any situations where it would be worth having TrimMemory call
ReleaseCacheMemory()?

[Scott Hess - ex-Googler, 2015/09/17 21:14:37]

I believe sqlite3_db_release_memory() should match TrimMemory() when run in
aggressive mode, so if the mmap stuff works as intended, then for those
platforms TrimMemory() is no longer useful.

I'll back out those comments, though.  Mostly I'm thinking about whether
TrimMemory() should be removed entirely, or implementation as always-aggressive
using sqlite3_db_release_memory(), or if even the non-mmap case should be
converted to always releasing the cache and relying on the filesystem buffers to
make things fast (mod syscall overhead).  None of that thinking-out-loud needs
to be in this CL though!

   void TrimMemory(bool aggressively);
 
   // Raze the database to the ground.  This approximates creating a
   // fresh database from scratch, within the constraints of SQLite's
   // locking protocol (locks and open handles can make doing this with
   // filesystem operations problematic).  Returns true if the database
   // was razed.
   //
   // false is returned if the database is locked by some other
   // process.  RazeWithTimeout() may be used if appropriate.
@@ skipping 356 matching lines @@
   // true, autocommit time if the database is not in a transaction, or update
   // time if the database is in a transaction.  Also records change count to
   // EVENT_CHANGES_AUTOCOMMIT or EVENT_CHANGES_COMMIT.
   void RecordTimeAndChanges(const base::TimeDelta& delta, bool read_only);
 
   // Helper to return the current time from the time source.
   base::TimeTicks Now() {
     return clock_->Now();
   }
 
+  // Release page-cache memory if memory-mapped I/O is enabled and the database
+  // was changed.  Passing true for |assume_changed| allows overriding the
+  // change detection for cases like DDL (CREATE, DROP, etc), which do not
+  // participate in the total-rows-changed tracking.
+  void ReleaseCacheMemoryIfNeeded(bool assume_changed);
+
   // The actual sqlite database. Will be NULL before Init has been called or if
   // Init resulted in an error.
   sqlite3* db_;
 
   // Parameters we'll configure in sqlite before doing anything else. Zero means
   // use the default value.
   int page_size_;
   int cache_size_;
   bool exclusive_locking_;
   bool restrict_to_user_;
@@ skipping 21 matching lines @@
   // True if database is open with OpenInMemory(), False if database is open
   // with Open().
   bool in_memory_;
 
   // |true| if the connection was closed using RazeAndClose().  Used
   // to enable diagnostics to distinguish calls to never-opened
   // databases (incorrect use of the API) from calls to once-valid
   // databases.
   bool poisoned_;
 
+  // |true| if SQLite memory-mapped I/O is enabled for this connection.
+  // Used by ReleaseCacheMemoryIfNeeded().
+  bool mmap_enabled_;
+
+  // Used by ReleaseCacheMemoryIfNeeded() to track if new changes have happened
+  // since memory was last released.
+  int total_changes_;

[Scott Hess - ex-Googler, 2015/09/17 21:14:37]

I changed this to |total_changes_last_release_| to discourage anyone from
assuming this is a generally-useful cache of sqlite3_total_changes().

[rmcilroy, 2015/09/18 11:16:29]

How about total_changes_since_last_release_ for clarity?

[Scott Hess - ex-Googler, 2015/09/21 18:08:57]

total_changes_at_last_release_, as this is an absolute value.  If I make it even
longer, I guess that gives me incentive to get rid of it eventually!

+
   ErrorCallback error_callback_;
 
   // Tag for auxiliary histograms.
   std::string histogram_tag_;
 
   // Linear histogram for RecordEvent().
   base::HistogramBase* stats_histogram_;
 
   // Histogram for tracking time taken in commit.
   base::HistogramBase* commit_time_histogram_;
@@ skipping 11 matching lines @@
   // Source for timing information, provided to allow tests to inject time
   // changes.
   scoped_ptr<TimeSource> clock_;
 
   DISALLOW_COPY_AND_ASSIGN(Connection);
 };
 
 }  // namespace sql
 
 #endif  // SQL_CONNECTION_H_