On Tue, 2009-09-08 at 11:13 +0200, Jens Axboe wrote:
> And here's a newer version.

I tinkered a bit with your proglet and finally found the problem.

You used a single pipe per child, this means the loop in run_child()
would consume what it just wrote out until it got force preempted by the
parent which would also get woken.

This results in the child spinning a while (its full quota) and only
reporting the last timestamp to the parent.

Since consumer (parent) is a single thread the program basically
measures the worst delay in a thundering herd wakeup of N children.

The below version yields:

idle

[root@opteron sched]# ./latt -c8 sleep 30
Entries: 664 (clients=8)

Averages:
------------------------------
        Max           128 usec
        Avg            26 usec
        Stdev          16 usec


make -j4

[root@opteron sched]# ./latt -c8 sleep 30
Entries: 648 (clients=8)                 

Averages:
------------------------------
        Max         20861 usec
        Avg          3763 usec
        Stdev        4637 usec


Mike's patch, make -j4

[root@opteron sched]# ./latt -c8 sleep 30
Entries: 648 (clients=8)

Averages:
------------------------------
        Max         17854 usec
        Avg          6298 usec
        Stdev        4735 usec