High priority threads causing severe CPU load imbalances

From: Suresh Jayaraman
Date: Tue Apr 06 2010 - 09:13:04 EST

I have a simple test program that accepts number of threads(pthreads) to
be created as a input. Each of these threads that gets created invokes a
function which is just a infinite while loop. The main function after
creating those threads goes in a infinite loop itself

My test machine is a Dual Core AMD Opteron(tm) 860 with 8
sockets(non-HT), I run this test program with number of threads ==
number of CPUs:

./loadcpu -t 16

I see 100% CPU utilization on almost all CPUs (via mpstat/htop/vmstat).

When the above threads are running, if I introduce a few high priority
threads by doing:

nice -n -13 ./loadcpu -t 3

After a short while, I see a few CPUs becoming idle at ~0% utilization
(the number of CPUs becoming idle equals roughly the number of high
priority threads i.e. 3). When I stop the high priority threads, the CPU
utilization comes back to normal i.e. ~100%.

This is reproducible on stable kernel with all the recent all
SMT fixes (I hope) and I think it would be reproducible in current
upstream as well.

sched_mc_power_savings has been always set to 0.

I spent a while staring at the load balancing and the thread migration
code, but could not figure out why this is happening. Would appreciate
any pointers.


Suresh Jayaraman
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