Subject: Re: sched_m2 is too unfair
To: None <,>
From: Mindaugas R. <>
List: netbsd-bugs
Date: 10/29/2007 17:51:37
> >Number:         37245
> >Category:       kern
> >Synopsis:       sched_m2 is too unfair
> <...>
> 	see the following test program and an output of top.
> 	there seems to be two problems, at least:
> 	- cpu-hogging threads never moves between cpus unless
> 	  there are idle cpus.

Yes, this case is known. The problem is with CPU-hogging threads, which
never sleeps - they dance only with sched_dequeue/sched_enqueue via
preempt/mi_switch, thus never gets sched_takecpu. There are few more cases:
 - Yielding;
 - STOPPED -> RUN or SUSPENDED -> RUN transitions;
For the first case, I am thinking to perform sched_takecpu() in setrunnable().

> 	- balance cpus periodically.

I was thinking about calling sched_takecpu() in preempt() when that is
necessary, according to the data collected by sched_balance().
Did you meant something else?

> 	- there are threads which get completely starved.
> 	  i guess they have never got run after fork and their sl_lrtime
> 	  are still 0.
> <...>
> 	- make sched_enqueue initialize l_lrtime properly for new lwps.

I think changing the else case in sched_enqueue to this would be correct:
	} else if (sil->sl_lrtime == 0)
		sil->sl_lrtime = hardclock_ticks;


Best regards,