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

third_party/sqlite/sqlite.gyp

 # 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.
 
 {
   'variables': {
     'use_system_sqlite%': 0,
     'required_sqlite_version': '3.6.1',
   },
   'target_defaults': {
@@ skipping 10 matching lines @@
       # TODO(shess): This shouldn't use faux-SQLite naming.
       'SQLITE_SEPARATE_CACHE_POOLS',
       # TODO(shess): SQLite adds mutexes to protect structures which cross
       # threads.  In theory Chromium should be able to turn this off for a
       # slight speed boost.
       'THREADSAFE',
       # SQLite can spawn threads to sort in parallel if configured
       # appropriately.  Chromium doesn't configure SQLite for that, and would
       # prefer to control distribution to worker threads.
       'SQLITE_MAX_WORKER_THREADS=0',
+      # Allow 256MB mmap footprint per connection.  Should not be too open-ended
+      # as that could cause memory fragmentation.  50MB encompasses the 99th
+      # percentile of Chrome databases in the wild.
+      # TODO(shess): A 64-bit-specific value could be 1G or more.
+      # TODO(shess): Figure out if exceeding this is costly.
+      'SQLITE_MAX_MMAP_SIZE=268435456',
       # Use a read-only memory map when mmap'ed I/O is enabled to prevent memory
       # stompers from directly corrupting the database.
       # TODO(shess): Upstream the ability to use this define.
       'SQLITE_MMAP_READ_ONLY=1',
       # NOTE(shess): Some defines can affect the amalgamation.  Those should be
       # added to google_generate_amalgamation.sh, and the amalgamation
       # re-generated.  Usually this involves disabling features which include
       # keywords or syntax, for instance SQLITE_OMIT_VIRTUALTABLE omits the
       # virtual table syntax entirely.  Missing an item usually results in
       # syntax working but execution failing.  Review:
@@ skipping 202 matching lines @@
             'SQLITE_CORE',
           ],
           'sources': [
             'src/ext/icu/icu.c',
           ],
         },
       ],
     }],
   ],
 }