I have some highly floating point intensive processes doing very little I/O. One is called "xspec", which calculates a numerical model and returns a floating point result back to a master process every second (via stdout). It is niced at the 19 level. I have another simple process "cpufloattest" which just does numerical computations in a tight loop. It is not niced.

I have a 4-core i7 system with hyperthreading disabled. I have started 4 of each type of process. Why is the Linux scheduler (Linux 3.4.2) not properly limiting the CPU time taken up by the niced processes?

Cpu(s): 56.2%us,  1.0%sy, 41.8%ni,  0.0%id,  0.0%wa,  0.9%hi,  0.1%si,  0.0%st
Mem:  12297620k total, 12147472k used,   150148k free,   831564k buffers
Swap:  2104508k total,    71172k used,  2033336k free,  4753956k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                         
32399 jss       20   0 44728  32m  772 R 62.7  0.3   4:17.93 cpufloattest                                    
32400 jss       20   0 44728  32m  744 R 53.1  0.3   4:14.17 cpufloattest                                    
32402 jss       20   0 44728  32m  744 R 51.1  0.3   4:14.09 cpufloattest                                    
32398 jss       20   0 44728  32m  744 R 48.8  0.3   4:15.44 cpufloattest                                    
 3989 jss       39  19 1725m 690m 7744 R 44.1  5.8   1459:59 xspec                                           
 3981 jss       39  19 1725m 689m 7744 R 42.1  5.7   1459:34 xspec                                           
 3985 jss       39  19 1725m 689m 7744 R 42.1  5.7   1460:51 xspec                                           
 3993 jss       39  19 1725m 691m 7744 R 38.8  5.8   1458:24 xspec                                           

The scheduler does what I expect if I start 8 of the cpufloattest processes, with 4 of them niced (i.e. 4 with most of the CPU, and 4 with very little)