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SubjectRe: busted CFS group load balancer?
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On Mon, 2008-11-17 at 15:19 -0600, Chris Friesen wrote:
> Ken Chen wrote:
> > On Mon, Nov 17, 2008 at 7:37 AM, Chris Friesen wrote:
> >>> It appears that the fair-group load balancer in 2.6.27 does not work
> >>> properly.
> >> There was an issue fixed post 2.6.27 where the load balancer didn't work
> >> properly if there was one task per group per cpu. You might try
> >> backporting commit 38736f4 and see if that helps.
> >
> > Tested git commit 38736f4, it doesn't fix the problem I'm seeing.
> >
>
> I plugged in the same weights into my test app (groups 1 and 2 instead
> of ant/bee) and got the results below for a 10-sec run. The "actual"
> numbers give the overall average and then the values for each hog
> separately. In this case we see that both tasks in group 2 ended up
> sharing a cpu with one of the tasks from group 1.
>
> group actual(%) expected(%) ctx switches max_latency(ms)
> 1 99.69(99.38/99.99) 99.81 160/262 4/0
> 2 0.31( 0.31/0.31) 0.19 32/33 391/375
>
> I've only got a 2-way system. If the results really are that much worse
> on larger systems, then that's going to cause problems for us as well.
> I'll see if I can get some time on a bigger machine.

Note that with larger cpu count and/or lower group weight we'll quickly
run into numerical trouble...

I would recommend trying this with the minimum weight in the order of
8-16 times number of cpus on your system.

There is only so much one can do with 10 bit fixed precision math :/




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