Looking around in the docs, I found this regarding thread_concurrency:
On Solaris, mysqld will call thr_setconcurrency() with this value.
thr_setconcurrency() permits the application to give the threads system a
hint for the desired number of threads that should be run at the same time.
I had noticed that it is set to 10 on my system, though I only have 2 CPUs
(I've also seen that the recommended value is #CPUS*2. So, I was
considering dropping it to 4. But, after reading the above, I wonder if it
makes difference on a Linux system (or any other non-Solaris system).