[Beowulf] How to restrict CPU usage (kindly solve this)

Mark Hahn hahn at physics.mcmaster.ca
Mon Aug 2 10:56:44 PDT 2004

> >>i have made a 4 node cluster now how to restrict CPU usage of a particular
> >>node i.e. if i want node 2's CPU not to exceed 70% usage.........  

Chris is right.  I read this very strictly (CPU meaning a particular 
physical processor).  but the author probably just meant "how do I configure
my queueing system to keep the load at under .7".  the answer is that you 
read the fine documentation of your queueing system, of course!

this is actually somewhat relevant, since anyone who talks about
loadaverage is not doing parallel-type HPC clustering.  any parallel 
program which has any collective operations is quite sensitive to 
preemption, and so cannot tolerate competition for access to CPUs.
this sensitivity an very steep function of the size of jobs (ncpus)
and the frequency of collectives.

people who are mostly used to serial farms find this a hard concept
to grasp, somehow.  we've actually thought of dynamically designating
some nodes to be parallel-only, since serial jobs can tolerate CPU
overcommittment fairly nicely.

interestingly, there are three main categories of jobs I see that would 
sometimes benefit from overcommittment:
	- serial jobs.  they don't actually benefit, but can tolerate 
	overcommittment without serious degredation.  and serial people
	sometimes like to see that they've got jobs at least started, 
	even if they're not getting 100% CPU.

	- gaussian jobs, even running in sysv-marallel mode.  here the 
	issue is that gaussian is often waiting for IO to its obscenely
	large scratch files.  it's not unusual to see a 4-way gaussian
	job *average* around 26% utilization of a 4-way node.

	- interactive or other IO-intensive jobs.  we have some people 
	who want to run interactive matlab sessions, or other programs
	that have guis.  mostly, they're OK if they just do this on a
	head node, but that sometimes causes trouble, too.

regards, mark hahn.

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