Following your advice, it seems to work perfectly now:

$ ./matmul.th 500 1
Temp de calcul: utime 2.324145, stime 0.020001, rtime 2.325608
$ ./matmul.th 500 2
Temp de calcul: utime 1.780111, stime 0.000000, rtime 0.890797
$ ./matmul.th 500 3
Temp de calcul: utime 1.784111, stime 0.004000, rtime 0.608895
$ ./matmul.th 500 4
Temp de calcul: utime 1.764110, stime 0.004000, rtime 0.451214
$ ./matmul.th 500 5
Temp de calcul: utime 1.768111, stime 0.000000, rtime 0.393285
$ ./matmul.th 500 6
Temp de calcul: utime 1.924120, stime 0.004001, rtime 0.333215
$ ./matmul.th 500 7
Temp de calcul: utime 1.788112, stime 0.000000, rtime 0.302328
$ ./matmul.th 500 8
Temp de calcul: utime 1.992124, stime 0.000000, rtime 0.290383

Wow! 2.6x faster on 2 cores is good. ;-)

That's a really fantastic piece of work. I'll do my best to study it and write
literature about it. May I ask, can you give a rough overview of the design?
For example, is there a separate nursery per thread so each thread can
allocate a certain amount before incurring a global pause? Do you have any
ideas for libraries built on top of this, such as a task parallel library
using work-stealing deques?

Thanks very much!!!

--
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e