Reland of "[platform] Change default platform max thread pool size 4 -> 8"

src/libplatform/default-platform.cc

 // Copyright 2013 the V8 project 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 "src/libplatform/default-platform.h"
 
 #include <algorithm>
 #include <queue>
 
 #include "src/base/logging.h"
@@ skipping 11 matching lines @@
   platform->SetThreadPoolSize(thread_pool_size);
   platform->EnsureInitialized();
   return platform;
 }
 
 
 bool PumpMessageLoop(v8::Platform* platform, v8::Isolate* isolate) {
   return reinterpret_cast<DefaultPlatform*>(platform)->PumpMessageLoop(isolate);
 }
 
-
-const int DefaultPlatform::kMaxThreadPoolSize = 4;
-
+const int DefaultPlatform::kMaxThreadPoolSize = 8;
 
 DefaultPlatform::DefaultPlatform()
     : initialized_(false), thread_pool_size_(0) {}
 
 
 DefaultPlatform::~DefaultPlatform() {
   base::LockGuard<base::Mutex> guard(&lock_);
   queue_.Terminate();
   if (initialized_) {
     for (auto i = thread_pool_.begin(); i != thread_pool_.end(); ++i) {
@@ skipping 14 matching lines @@
       i->second.pop();
     }
   }
 }
 
 
 void DefaultPlatform::SetThreadPoolSize(int thread_pool_size) {
   base::LockGuard<base::Mutex> guard(&lock_);
   DCHECK(thread_pool_size >= 0);
   if (thread_pool_size < 1) {
-    thread_pool_size = base::SysInfo::NumberOfProcessors();
+    thread_pool_size = base::SysInfo::NumberOfProcessors() - 1;
   }
   thread_pool_size_ =
       std::max(std::min(thread_pool_size, kMaxThreadPoolSize), 1);
 }
 
 
 void DefaultPlatform::EnsureInitialized() {
   base::LockGuard<base::Mutex> guard(&lock_);
   if (initialized_) return;
   initialized_ = true;
@@ skipping 115 matching lines @@
   return dummy;
 }
 
 
 size_t DefaultPlatform::NumberOfAvailableBackgroundThreads() {
   return static_cast<size_t>(thread_pool_size_);
 }
 
 }  // namespace platform
 }  // namespace v8