Clusters Vs Grids

Shin shin at solarider.org
Sun Jul 20 18:02:11 EDT 2003


Hi,

I got a few queries about the exact differences between clusters and
grids and as I couldn't really find a general purpose grid list to
post on and because this list is normally a fountain of knowledge I
thought I'd ask here. However if there is somewhere more appropriate
to ask then please push me in that direction.

Broadly (very broadly) as I understand it a cluster is a collection
of machines that will run parallel jobs for codes that require high
performance - they might be connected by a high speed interconnect
(ie Myrinet, SCI, etc) or via a normal ethernet type connections.
The former are described as closely or tightly coupled and the
latter as loosely coupled? Hopefully I'm correct so far. 

A cluster will normally (always?) be located at one specific location.

A grid is also a collection of computing resources (cpu's, storage)
that will run parallel jobs for codes that also require high
performance (or perhaps very long run times?). However these
resources might be distributed over a department, campus or even
further afield in other organisations, in different parts of the
world?

As such a grid cam not be closely coupled and any codes that are
developed for a grid will have to take the very high latency
overheads of a grid into consideration. Not sure what the bandwidth
of a grid would be like?

On the other hand, a grid potentially makes more raw computing power
available to a user who does not have a local adequately specced
cluster available.

So I was wondering just how all those coders out there who are
developing codes on clusters connected with fast interconnects are
going to convert their codes to use on a grid - or is there even the
concept of a highly coupled grid - ie grid components that are
connected via fast interconnections (10Gb ethernet perhaps?) or is
that still very low in terms of what closely coupled clusters are
capable of.

Or are people making their clusters available as components of a
grid, call it a ClusterGrid and in the same way that a grid app
would specify certain resoure  requirements - it could specify that
it should look for an available cluster on a grid.

However I can't see why establishments who have spent a lot of money
developing their clusters would then make them available on a grid
for others to use - when they could just create an account for the
user on their cluster to run their code on.

I could understand the use of single machines that are mostly idle
being made available for a grid - but presumably most clusters are
in constant demand and use from users.

So I was just looking to see if I have my terminology above correct
for grids and clusters and whether there was any concept of a
tightly coupled grid or even a ClusterGrid. And if there was any
useful cross over between clusters and grids - or are the two so
completely different architecurally that they will never meet; or
not for the near future at least.

I was also curious about all these codes that use MPI across tightly
coupled systems and how they would adapt to use on loosely coupled
grid. 

I'm having a hard time marrying the 2 concept of a cluster and a
grid together; but I'm sure much finer brains than mine have already
considered all this and ruled it out/in/not-yet.

Thanks for any clarity and information you can provide. Oh and if
anyone has any comments on the following comment from a colleague
I'd appreciate that as well; "grids - hmmm - there're just the
latest computing fad - real high performance scientists won't use
them and grids will be just so much hype for many years to come".

Thanks
Shin

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