[sql] Rewrite Connection::Preload() in terms of file reading.

r204860 rewrote sqlite3_preload() to read the file without placing the
data in the SQLite cache to improve memory footprint.  No reports of
performance degradation ensued.

This CL rewrites Connection::Preload() to use the SQLite file handle to
read the data, and backs out the preload-cache.patch from third_party/sqlite/.

BUG=243949

Review URL: https://codereview.chromium.org/116443004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242225 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 10 insertions, 12 deletions

diff --git a/sql/connection.h b/sql/connection.h
index 54c3f8b..5f11e44 100644
--- a/sql/connection.h
+++ b/sql/connection.h
@@ -187,19 +187,17 @@ class SQL_EXPORT Connection {
   // an uninitialized or already-closed database.
   void Close();
 
-  // Pre-loads the first <cache-size> pages into the cache from the file.
-  // If you expect to soon use a substantial portion of the database, this
-  // is much more efficient than allowing the pages to be populated organically
-  // since there is no per-page hard drive seeking. If the file is larger than
-  // the cache, the last part that doesn't fit in the cache will be brought in
-  // organically.
+  // Reads the first <cache-size>*<page-size> bytes of the file to prime the
+  // filesystem cache.  This can be more efficient than faulting pages
+  // individually.  Since this involves blocking I/O, it should only be used if
+  // the caller will immediately read a substantial amount of data from the
+  // database.
   //
-  // This function assumes your class is using a meta table on the current
-  // database, as it openes a transaction on the meta table to force the
-  // database to be initialized. You should feel free to initialize the meta
-  // table after calling preload since the meta table will already be in the
-  // database if it exists, and if it doesn't exist, the database won't
-  // generally exist either.
+  // TODO(shess): Design a set of histograms or an experiment to inform this
+  // 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