[Pw_forum] Memory question

[Paolo Giannozzi]

On Tuesday 14 February 2006 11:25, Miguel Martinez wrote:

> I'm running parallel espresso (v2.1.5) in an Itanium2 cluster [...]
> Most nodes use 4Gb, though there are some with 16Gb RAM.
>
> The thing is that, when I send a phonon multiprocessor run, while it
> doesn't use a lot of memory (some 250Mb per thread), it swaps out.
> I've run two processors in a 4 proc node and, to make sure other 
> jobs don't interfere, using all 4 procs for myself.

if you really use 2*250Mb out of 4Gb, you will not swap, unless your 
machine is horribly loaded. If you are swapping, you are using more 
memory than you think. How did you get that number? it is not easy
to know how much memory a process is really using. Also note that 
some recent versions of the code contain rather large statically 
allocated arrays that are not distributed across processors (i.e. each
process has its own copy). Check with command "size ph.x" how big
is the so-called "bss". Most of that memory should have been 
eliminated by now.

> I personally haven't seen this on my laptop (gfortran instead of ifc)...

you mean g95, or really gfortran? these are two distinct projects

> 2) Will this affect performance badly?

if you swap all the time, it will, for sure

> 3) This is a bit off topic. Is it OK to use -npools while using
> processors in the same node or is it dumb?

it is ok, but it won't save any memory: each process will allocate
the entire memory 

Paolo

-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
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