Re: Scheduler fairness problem on 2.6 series

From: Bill Davidsen
Date: Thu Aug 12 2004 - 13:16:11 EST


Con Kolivas wrote:
Prakash K. Cheemplavam wrote:

|
| I don't think it is the overhead. I rather think the way the kernel
| schedulers gives mpich and the cpu bound program resources is unfair.

Well, I don't know whether it helps, but I ran a profiler and these are
the functions which cause so much wasted CPU cycles when running 16
processes of my example with mpich:

124910 9.8170 vmlinux tcp_poll
123356 9.6949 vmlinux sys_select
85634 6.7302 vmlinux do_select
71858 5.6475 vmlinux sysenter_past_esp
62093 4.8801 vmlinux kfree
51658 4.0600 vmlinux __copy_to_user_ll
37495 2.9468 vmlinux max_select_fd
36949 2.9039 vmlinux __kmalloc
22700 1.7841 vmlinux __copy_from_user_ll
14587 1.1464 vmlinux do_gettimeofday

Is anything scheduler related?


No

It looks like your select timeouts are too short and when the cpu load goes up they repeatedly timeout wasting cpu cycles.
I quote from `man select_tut` under the section SELECT LAW:

1. You should always try use select without a timeout. Your program
should have nothing to do if there is no data available. Code
that depends on timeouts is not usually portable and difficult
to debug.

There's a generalization which should confuse novice users... correctly used a timeout IS a debugging technique. Useful to detect when a peer has gone walkabout, as a common example.

Sounds as if the timeout is way too low here, however. Perhaps they are using it as poorly-done polling? In any case, not kernel misbehaviour.

--
-bill davidsen (davidsen@xxxxxxx)
"The secret to procrastination is to put things off until the
last possible moment - but no longer" -me
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