GN: Don't import a file more than once.

tools/gn/scheduler.cc

 // 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.
 
 #include "tools/gn/scheduler.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 36 matching lines @@
     return result;
 
   // Base the default number of worker threads on number of cores in the
   // system. When building large projects, the speed can be limited by how fast
   // the main thread can dispatch work and connect the dependency graph. If
   // there are too many worker threads, the main thread can be starved and it
   // will run slower overall.
   //
   // One less worker thread than the number of physical CPUs seems to be a
   // good value, both theoretically and experimentally. But always use at
-  // least three workers to prevent us from being too sensitive to I/O latency
+  // least some workers to prevent us from being too sensitive to I/O latency
   // on low-end systems.
+  //
+  // The minimum thread count is based on measuring the optimal threads for the
+  // Chrome build on a several-year-old 4-core MacBook.
   int num_cores = GetCPUCount() / 2;  // Almost all CPUs now are hyperthreaded.
-  return std::max(num_cores - 1, 3);
+  return std::max(num_cores - 1, 8);
 }
 
 }  // namespace
 
 Scheduler::Scheduler()
     : pool_(new base::SequencedWorkerPool(GetThreadCount(), "worker_")),
       input_file_manager_(new InputFileManager),
       verbose_logging_(false),
       work_count_(0),
       is_failed_(false),
@@ skipping 157 matching lines @@
 void Scheduler::DoWork(const base::Closure& closure) {
   closure.Run();
   DecrementWorkCount();
 }
 
 void Scheduler::OnComplete() {
   // Should be called on the main thread.
   DCHECK(base::MessageLoop::current() == main_loop());
   runner_.Quit();
 }