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Jim Van Fleet vanfleet at
Sat Oct 8 00:30:22 AEDT 2016

We have seen on a number of benchmarks that the scheduler does not operate 
like you (I) would expect. One of the tools we have examined a trace file 
and showed that, for a "long" period of time, two processes were sharing a 
single cpu.  That, in itself, is not startling, but there were a number of 
idle processors when this was happening.

I found the article below searching for others who saw the same sort of 
problem. What made me keep reading was:
"With so many rules about when the load balancing does or does not occur, 
it becomes difficult to reason about how long an idle core would remain 
idle if there is work to do and how long a task might stay in a run queue 
waiting for its turn to run when there are idle cores in the system." 
Exactly the issue we were experiencing!

The authors were kind enough to provide patches in their article -- we 
built a kernel including the patches.  We have not had much time to test 
it, but we have seen a 30% gain on one benchmark..

The assistance I would like is to have an experienced scheduler person 
look at these changes to see how we can apply them to power.  We have 
smt8, but need to have all the threads working for smt8 to make a 
difference -- this fix addresses that problem and gives us an advantage.

Jim Van Fleet

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