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Subject: Re: [OMPI devel] Reviewing MPI_Dims_create
From: Andreas Schäfer (gentryx_at_[hidden])
Date: 2014-02-11 00:22:49


Christoph-

Jeff is right, the largest prime p we need to precompute is p <
sqrt(2^31), so in total we'd get away with a dozen kB of RAM. The only
prime in the factorization we'd not catch this way can be retrieved
later on during the factorization.

But I agree with you that's it's not really worth the effort.

Cheers
-Andreas

On 02:39 Tue 11 Feb , Christoph Niethammer wrote:
> sqrt(2^31)/log(sqrt(2^31))*(1+1.2762/log(sqrt(2^31)))/1024 * 4byte = 18,850133965051 kbyte should do it. ;)
> Amazing - I think our systems are still *too small* - lets go for MPI with int64 types. ^^
>
> ----- Ursprüngliche Mail -----
> Von: "Jeff Squyres (jsquyres)" <jsquyres_at_[hidden]>
> An: "Open MPI Developers" <devel_at_[hidden]>
> Gesendet: Dienstag, 11. Februar 2014 01:32:53
> Betreff: Re: [OMPI devel] Reviewing MPI_Dims_create
>
> On Feb 10, 2014, at 7:22 PM, Christoph Niethammer <niethammer_at_[hidden]> wrote:
>
> > 2.) Interesting idea: Using the approximation from the cited paper we should only need around 400 MB to store all primes in the int32 range. Potential for applying compression techniques still present. ^^
>
> Per Andreas' last mail, we only need primes up to sqrt(2B) + 1 more. That *has* to be less than 400MB... right?
>
> sqrt(2B) = 46340. So the upper limit on the size required to hold all the primes from 2...46340 is 46340*sizeof(int) = 185,360 bytes (plus one more, per Andreas, so 185,364).
>
> This is all SWAGing, but I'm assuming the actual number must be *far* less than that...
>
> --
> Jeff Squyres
> jsquyres_at_[hidden]
> For corporate legal information go to: http://www.cisco.com/web/about/doing_business/legal/cri/
>
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-- 
==========================================================
Andreas Schäfer
HPC and Grid Computing
Chair of Computer Science 3
Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
+49 9131 85-27910
PGP/GPG key via keyserver
http://www.libgeodecomp.org
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