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

sql/connection.h

Index: sql/connection.h
diff --git a/sql/connection.h b/sql/connection.h
index 19592d9f0e44ed68764b9d25a88d9a65424ba9b7..0263bb1cedce5c3a220e643d8b786536f7214ec5 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -262,6 +262,7 @@ class SQL_EXPORT Connection {
   // 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
@@ -638,6 +639,12 @@ class SQL_EXPORT Connection {
     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_;
@@ -679,6 +686,14 @@ class SQL_EXPORT Connection {
   // 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.