SDSC Thread Graphic Issue 4, February 2006





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User Services Director:
Anke Kamrath

Editor:
Subhashini Sivagnanam

Graphics Designer:
Diana Diehl

Application Designer:
Fariba Fana


Help Desk: User Questions

Frequently asked questions from our users

—Kyle Rollin

Dear SDSC Consulting,
Is there a way to convert between little and big endian on Datastar?

Answer:
The key to converting between little and big endian, and vice-versa, is in the data formatting of the file (for example: 4 double-precision followed by 4 integers). Once this is understood it is pretty straightforward to switch the bit order. Unfortunately, this must be done manually.
This tar file with C and FORTRAN libraries will help you to accomplish this task.

Dear SDSC Consulting,
What exactly is meant by wall clock time, and am I charged for the amount of time between when my job ends and the wall clock limit is reached?

Answer:
Thanks for asking. What is referred to as "walltime" on SDSC Teragrid and "wall_clock_limit" on Datastar often sounds confusing. Fortunately, it's not. Put simply, these are time values set by the user in order to protect against accidentally having a program run for too long, which could potentially result in both faulty data as well as wasted SUs. Furthermore, having an 18 hour limit on wallclock time on Datastar and SDSC Teragrid (12 hours on Bluegene) allows for all users to have a fair shot at running their programs in a relatively time efficient manner.

In certain situations it is even possible to use the wallclock to expedite your wait in the queue. For example: If you want to run a serial job and you know your program is only going to take 10 minutes, then set your wall clock limit to 15 minutes. This will cause the backfill to recognize the job as a prime candidate and will most likely squeeze it in sooner than later (anywhere from a matter of minutes to a few hours). The same job, however, with a wall clock limit of 18 hours, would sit in the queue for much longer (perhaps a matter of days). This is just one way you can use the wallclock along with your powers of estimation to get the job done fast.

Finally, to answer your second question, you are charged up until your program stops running. If this is before the wall clock time is reached, then you are only charged up to that point. Conversely, if your program is only supposed to run for one hour but ends up unexpectedly running up to the wallclock limit of 18 hours, then you will be charged the full 18.

Kyle Rollin is reachable via e-mail at krollin@sdsc.edu

Did you know ..?

that SDSC has limited the core file size to 32MB.
To make good use of the core file size it is recommneded using the MP_COREFILE_FORMAT environment variable (or its associated command-line flag -corefile_format) to set the format of corefiles to lightweight corefiles.
See Thread article Issue 4, February 2006 for more details.