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

third_party/sqlite/BUILD.gn

 # Copyright (c) 2013 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.
 
 declare_args() {
   # Controls whether the build should uses the version of sqlite3 library
   # shipped with the system (currently only supported on iOS) or the one
   # shipped with Chromium source.
   use_system_sqlite = is_ios
 }
@@ skipping 49 matching lines @@
       # 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
@@ skipping 154 matching lines @@
         "//third_party/icu",
       ]
       if (is_clang) {
         # src/ext/icu/icu.c uses assert(!"string") which causes warnings about
         # conversion from string literal to bool.
         configs -= [ "//build/config/clang:extra_warnings" ]
       }
     }
   }
 }