I noticed on RHEL 6 that when a large amount of disk I/O is happening that CPU bound tasks "slow down". I have been able to reproduce it in Fedora 21 as well and here are the instructions of how I can reproduce it with a simple test:

1) Build the disk_test.cc (the "CPU bound task") and run it.
2) Create a large file to copy ( fallocate -l 10G junk ).
3) Copy that file with a one minute delay between copies ( while true; do cp junk junk2; sleep 60; done )

If you direct the output of disk_test.cc to a file, then you can plot the results in gnuplot with the following commands to see the change in the mean time between "finishing the work cycle" when the file is being copied:
set xdata time
set timefmt "%s"
plot "out.txt" using 1:3 with lines

You can also notice that the load average is also going up, so it seems like something in the kernel/scheduler is getting some sort of exclusive lock in the disk I/O process and that's causing the CPU bound task to not be able to execute when it should. Any ideas?

Thanks,
Dave