How to view system services use ?

[Mikus Grinbergs]

I normally run with a (niced priority) computational task in the 
background.  The result is that 'gkrellm' normally shows the CPU_use 
panel all green (100% of CPU execution time is being spent in nice 
mode).  But from time to time, 'gkrellm' shows a vertical stripe of 
orange (that means - a noticeable percentage of the CPU execution 
time is being spent in system mode).  'Top' confirms this - for 
instance %ni shows 60 and %sy shows 40 or thereabouts (for a short 
while).

What I would like to know is __which__ system services are being 
called upon (when that orange stripe appears within 'gkrellm').  I'm 
not very familiar with Linux.  My question -- what facilities/tools 
are available for me to determine "what services is the system 
running at the instant that system mode is taking this noticeable 
percentage of the CPU" ?

Thanks for whatever guidance you can provide.  mikus