Rework how MB and GN handle concurrent links.

tools/mb/mb_config.pyl

 # Copyright 2015 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.
 
 {
   # This is a map of buildbot master names -> buildbot builder names ->
   # config names (where each config name is a key in the 'configs' dict,
   # below). MB uses this dict to look up which config to use for a given bot.
   'masters': {
     # Take care when changing any of these builders to ensure that you do not
@@ skipping 612 matching lines @@
       'win64-pgo': 'gyp_official_winpgo_x64',
     },
 
     'official.desktop.continuous': {
       'mac beta': 'gyp_official',
       'mac stable': 'gyp_official',
       'mac trunk': 'gyp_official',
       'precise64 beta': 'gn_official',
       'precise64 stable': 'gn_official',
       'precise64 trunk': 'gn_official',
-      'win beta': 'gyp_official',
-      'win stable': 'gyp_official',
-      'win trunk': 'gyp_official',
+      'win beta': 'gyp_official_six_concurrent_links',
+      'win stable': 'gyp_official_six_concurrent_links',
+      'win trunk': 'gyp_official_six_concurrent_links',
     },
 
     'tryserver.blink': {
       # Most tryservers should have '_trybot' in their config names, but
       # 'release_trybot' includes 'dcheck_always_on', and the blink
       # release trybots must *not* enable dchecks, because that could
       # cause them to produce different baselines than the release
       # waterfall bots, and run-webkit-tests can't handle that (by design).
       'linux_blink_compile_dbg': 'swarming_gn_debug_trybot',
       'linux_blink_compile_rel': 'swarming_gn_release_bot_minimal_symbols',
@@ skipping 670 matching lines @@
     ],
 
     'gyp_official_goma_minimal_symbols_x86': [
       'gyp', 'official', 'goma', 'minimal_symbols', 'x86',
     ],
 
     'gyp_official_syzyasan': [
       'gyp', 'official', 'syzyasan',
     ],
 
+    'gyp_official_six_concurrent_links': [
+      'gyp', 'official', 'six_concurrent_links',
+    ],
+
     # TODO(crbug.com/595947) - figure out how to handle PGO, which needs
     # to invoke GYP/GN twice, with two different sets of flags, apparently.
     'gyp_official_winpgo': [
       'gyp', 'error',
     ],
 
     'gyp_official_winpgo_x64': [
       'gyp', 'error', 'x64',
     ],
 
@@ skipping 869 matching lines @@
 
     'shared': {
       'gn_args': 'is_component_build=true',
       'gyp_defines': 'component=shared_library',
     },
 
     'shared_release_bot': {
       'mixins': ['shared', 'release', 'goma']
     },
 
+    'six_concurrent_links': {
+      # TODO(crbug.com/611491) Adjust the get_concurrent_links script
+      # to be more conservative so that we don't need this.

[Dirk Pranke, 2016/06/03 05:44:43]

@scottmg / @brucedawson - thoughts on what we should do w/ the windows link
defaults?

[brucedawson, 2016/06/03 19:42:17]

How much memory do the official builders have? The worst-case six links for
official builds would be six of the 64-bit test binaries which have a combined
linker working set of 82 GB. For smaller binaries the memory footprint can be an
order of magnitude less. Dynamic adjustment would be great but that is
definitely a separate bug (crbug.com/610459).

LTCG linking is multi-threaded in some stages which means that crazy parallelism
is less important on official builds when trying to maximize the hardware.

[Dirk Pranke, 2016/06/05 01:19:49]

looks like 256GB on linux, 128 on win, and 64 on mac.

+      'gn_args': 'concurrent_links=6',
+      'gyp_defines': 'gyp_link_concurrency=6',
+    },
+
     'static': {
       'gn_args': 'is_component_build=false',
       'gyp_defines': 'component=static_library',
     },
 
     'swarming': {
       'gn_args': '',
       'gyp_defines': 'test_isolation_mode=prepare',
     },
 
@@ skipping 62 matching lines @@
     },
 
     'drfuzz': { 'gn_args': 'use_drfuzz=true' },
 
     'drmemory': {
       'gn_args': 'is_component_build=true enable_iterator_debugging=false',
       'gyp_defines': 'component=shared_library build_for_tool=drmemory',
     },
   },
 }