src: override v8 thread defaults using cli options

Based on the conversation in #4243 this implements a way to increase
and decrease the size of the thread pool used in v8.

Currently v8 restricts the thread pool size to `kMaxThreadPoolSize`
which at this commit is (4). So it is only possible to
decrease the thread pool size at the time of this commit. However with
changes upstream this could change at a later date.
If set to 0 then v8 would choose an appropriate size of the thread pool
based on the number of online processors.

PR-URL: #4344
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/node.1

@@ -99,6 +99,13 @@ Process v8 profiler output generated using the v8 option \fB\-\-prof\fR
 .BR \-\-v8\-options
 Print v8 command line options.
 
+.TP
+.BR \-\-v8\-pool\-size =\fInum\fR
+Set v8's thread pool size which will be used to allocate background jobs.
+If set to 0 then v8 will choose an appropriate size of the thread pool based
+on the number of online processors. If the value provided is larger than v8's
+max then the largest value will be chosen.
+
 .TP
 .BR \-\-tls\-cipher\-list =\fIlist\fR
 Specify an alternative default TLS cipher list. (Requires Node.js to be built with crypto support. (Default))
@@ -115,7 +122,6 @@ Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (
 .BR \-\-icu\-data\-dir =\fIfile\fR
 Specify ICU data load path. (overrides \fBNODE_ICU_DATA\fR)
 
-
 .SH ENVIRONMENT VARIABLES
 
 .TP

src/node.cc

@@ -148,6 +148,8 @@ static const char** preload_modules = nullptr;
 static bool use_debug_agent = false;
 static bool debug_wait_connect = false;
 static int debug_port = 5858;
+static const int v8_default_thread_pool_size = 4;
+static int v8_thread_pool_size = v8_default_thread_pool_size;
 static bool prof_process = false;
 static bool v8_is_profiling = false;
 static bool node_is_initialized = false;
@@ -178,7 +180,6 @@ static uv_async_t dispatch_debug_messages_async;
 static node::atomic<Isolate*> node_isolate;
 static v8::Platform* default_platform;
 
-
 static void PrintErrorString(const char* format, ...) {
   va_list ap;
   va_start(ap, format);
@@ -3319,6 +3320,7 @@ static void PrintHelp() {
          "  --zero-fill-buffers   automatically zero-fill all newly allocated\n"
          "                        Buffer and SlowBuffer instances\n"
          "  --v8-options          print v8 command line options\n"
+         "  --v8-pool-size=num    set v8's thread pool size\n"
 #if HAVE_OPENSSL
          "  --tls-cipher-list=val use an alternative default TLS cipher list\n"
 #if NODE_FIPS_MODE
@@ -3466,6 +3468,8 @@ static void ParseArgs(int* argc,
     } else if (strcmp(arg, "--v8-options") == 0) {
       new_v8_argv[new_v8_argc] = "--help";
       new_v8_argc += 1;
+    } else if (strncmp(arg, "--v8-pool-size=", 15) == 0) {
+      v8_thread_pool_size = atoi(arg + 15);
 #if HAVE_OPENSSL
     } else if (strncmp(arg, "--tls-cipher-list=", 18) == 0) {
       default_cipher_list = arg + 18;
@@ -4285,8 +4289,7 @@ int Start(int argc, char** argv) {
   V8::SetEntropySource(crypto::EntropySource);
 #endif
 
-  const int thread_pool_size = 4;
-  default_platform = v8::platform::CreateDefaultPlatform(thread_pool_size);
+  default_platform = v8::platform::CreateDefaultPlatform(v8_thread_pool_size);
   V8::InitializePlatform(default_platform);
   V8::Initialize();