[Beowulf] Computation on the head node

Jeffrey B. Layton laytonjb at charter.net
Mon May 19 07:40:47 EDT 2008


Joe Landman wrote:
> Perry E. Metzger wrote:
>> checking. A very fast RAID array may be in order -- or it may be
>> completely unnecessary. One can't know without understanding one's
>> application intimately, and that requires testing.
>
> Of course.  But there are quite a few people/groups on this list with 
> decades of HPC experience that might have an inkling if a USB or 
> similar connected drive "is a good idea" for an app, even prior to 
> running it.  Benchmarking is important, but it is important that the 
> benchmark represent real runs.  Experience can provide a rough guide 
> in the case of no benchmark data availability.  With clusters, you run 
> into the very real problem of IO resource contention, quite quickly.  
> Putting lower end IO devices in there rarely makes sense.  Sure, you 
> can benchmark it, and you should if possible.  But it is also not a 
> bad idea to listen to people whom have been working on this stuff for 
> a while, they might have a clue about these things.

Here comes the $64 question - how do you benchmark the IO portion of your
code so you can understand whether you need a parallel file system, what 
kind
of connection do you need from a client to the storage, etc. This is a 
difficult
problem and one in which I have an interest.

The best way I've found is to look a the IO pattern of your code(s). The 
best
I've found to do this is to run an strace against the code. I've written 
an strace
analyzer that gives you a higher-level view of what's going on with the IO.
I'm also working on a tool that can take the strace output and create a 
"simulator"
that will run in a similar manner to the original code but actually 
perform the
IO of the original code using dummy data. This allows you to "give" away
a simple dummy code to various HPC storage vendors and test your 
application.
This code is taking a little longer than I'd hoped to develop :(

Jeff

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