[Beowulf] Capacity / Capability Computing

Robert G. Brown rgb at phy.duke.edu
Tue May 20 08:37:53 EDT 2008

On Tue, 20 May 2008, andrew holway wrote:

> Definitions please.

"Capacity" is usually used to refer to ability to hold a quantity of
something.  In the context of computing the something is usually "data".
It can, and usually does, refer to disk, but with suitable modifiers it
can refer to other capacities, e.g. RAM capacity (quantity of RAM).

The term "can" be used to describe rates as well (in good English).  For
example the "capacity" of my ethernet connection is around 10
megabytes per second.  The mental picture is of a wire holding a
quantity of 10 megabytes of data (the capacity part) being delivered
every second.  No ambiguity, everybody understands.  Similarly other
rates like "compute capacity" in MFLOPs or the like are permitted usage
and with suitable modifiers or context are well-understood, even though
using the term with rates stretches the literal meaning of the word to
where it is synonymous with "capability".

Which it is, in ordinary English.  I have the capacity to do good means
that I am capable of doing good.  Again, the conveyed picture is
different; hence one would use the two sentence fragments in different
contexts.  Capacity means that I contain within me mental room to hold
good impulses and a good nature, and one would actually often use the
sentence to imply that while I do have the room and may have the
impulses, I don't do it as often as the speaker would like.  Capable
just means that I can do good

"Capabilities", then, are just abilities.  In computing, they are pretty
much any of the rates and capacities that describe a system, AND are a
listing of particular things a system can do.  My system is capable of
running a Monte Carlo simulation on a lattice up to 100x100x100 in size,
but I don't think it is capable of running one 1000x1000x1000.  This is
because it hasn't the memory capacity to hold the entire lattice image,
and if I run it in virtual memory using swap, it would take the rest of
my life to complete.  My system and network are capable of serving a
dozen slave systems in a parallel calculation at home.  This means that
they have the capacity to do this, of course, but if I said that you'd
be visualizing it slightly differently.

Hope this helps.  The above is more or less according to the dictionary
definition of capacity as a "stationary" or "passive" capability, which
is why using it to describe a rate is second order in time; the
stationary capacity of a line to transmit data PER SECOND.  Capability
is a bit broader and includes rates.


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Robert G. Brown                            Phone(cell): 1-919-280-8443
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