[Omp] First Touch initialization

[Francisco Jesús Martínez Serrano]

Many thanks for the prompt answer. We believe solaris has better NUMA
support and will be giving it a try in short. I tried solaris express one
moth
ago, but it failed to use our Silicon Image SATA controller. From what I've
read, support is coming soon if it's not here yet.



On 3/8/06, Ruud van der Pas  wrote:
>
> Hi Francisco,
>
> > Initializing shared arrays in parallel at the very beginning of the
> program
> > will distribute the contents of each array according to the access
> pattern
> > hence, in NUMA machines access will be much faster since it's
> local-node.
> >
> > We have tried it and it works indeed (Intel Fortran compiler v9 on 4-way
> > Opteron),
> > but I don't understand why.
>
> First touch (or data placement in general) is not something
> typically handled by a compiler. It is controlled by the Operating
> System. Solaris has cc-NUMA support for example, but I believe
> Linux supports it too these days.
>
> The general rule is that the thread first touching (a chunk of)
> the data gets it in it's local memory. Typically such first touch
> happens when initializing or reading the data for the first time.
>
> If for example, you use "malloc" to allocate a chunk of memory,
> nothing has happened yet. All the OS does is to reserve that
> chunk for you.
>
> The minute a thread then _accesses_ a portion (or all) of it,
> first touch causes it to be owned by that thread.
>
> This is why one can speed up an OpenMP program running on a
> cc-NUMA system by parallelizing the data initialization phase.
> Even adding a redudant initialization upfront could work out
> well (in case the first touch is through sequential I/O for
> example).
>
> How you want to initialize the data in parallel depends on
> how you access the data later on.
>
> Kind regards,
> Ruud
>
> PS I did mention "malloc" for a good reason. With "calloc" the
>     data gets pre-initialized to zero and may therefore end up on
>     the wrong node.
> ----------------------------------------------------------------
> Senior Staff Engineer             Email: ruud.vanderpas at sun.com
> Scalable Systems Group            Phone: +31-33-4515000 (x15920)
> Sun Microsystems                  Fax  : +31-33-4515001
> ----------------------------------------------------------------
>
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