Autotasking considered harmful (was Re: "vi" & Supercomputer Performance)

[Martin Fouts]

Alan Klietz replies to an earlier groan about the difficulty of
autotasking on an interactive system:

Granted, autotasking complicates things somewhat because it makes a context
switch much more painful where stragglers can slow down striped loops. 
I don't know how UNICOS 6 does things, but as you suggested it ought to
dedicate processors to autotasking or greatly penalize switches of active 
autotasked jobs.  Swapping can be avoided by setting your sched parameters
appropriately.

The problem with {multi,micro,auto}tasking in a multiuser (even batch)
workload is the assumption that all those processors are MINE, ALL
MINE... 

It takes overhead to implement autotasking and it costs to run more
threads than there are CPUS.  If you autotask and I autotask, and we
both run at once, we both take the same time in our code as if we
didn't, plus the overhead we've introduced to manage the threads, so
we slow the system down, get worse turnaround time and degrade
throughput.

(In economics, this would be a microeconomic gain leading to a
macroeconomic loss, since it is good for one, but not good for more
than one.)

If you've got enough users to keep all of the processors on the
machine busy without *tasking, than you are better off not doing it.

Marty

--
Martin Fouts

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