Messages in this thread | | | Subject | Re: busted CFS group load balancer? | From | Peter Zijlstra <> | Date | Tue, 18 Nov 2008 06:19:08 +0100 |
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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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