[boost] boost thread slow with only one thread

Subject: [boost] boost thread slow with only one thread
From: Natacha Fort (natachafrt_at_[hidden])
Date: 2010-05-20 06:36:59

hi!
here is my problem
i am multithreading my program with boost but i don't get what i expect, and
i can't understand why!
i just run one thread and wait for it, and the fps are divide by 3.
basically with no thread i have 30fps, and with 1 thread i have 10fps. here
is the code:

 Code:

boost::thread_group forces_threads;
forces_threads.create_thread(boost::bind(&ElasticRods::computeBendingForces,this,edges,kb,edges_initial));
forces_threads.join_all();

 i use thread_group because i want to run 7 thread but it is already too
slow!
do you have an idea of why this is so slow?
thank you for your help!