Need help with Box2d and Threading

Hi all any one know about the difference in cocos2dx ? dose calling CC_SCHEDULE_SELECTOR
create a separate thread or dose it run in the main thread of cocosa2dx as I know that most hardware now has more then one CPU.

I’m using this code to create a schedule its just I keep getting jitters in simulation
this->schedule(CC_SCHEDULE_SELECTOR(PhysicsClass::Box2dTimer), UPDATE_INTERVAL);

however if create my a standard thread and up the rate to 1 / 120 its smooth. using this should have noting to do with cocos2dx .
std::thread t1( &PhysicsClass::Box2dTimer , this );
t1.detach();

Doing it this way will there be any gains or am I just over thinking it too much.
and should just leave it with 1 / 60 and just chuck I in the standard “scheduleUpdate()” loop and just let it run and don’t worry about it because the scheduleUpdate() is smooth too but I’m worried where updating too much and wasting CPU time.

Many thanks in advance.