From help-request at octave dot org Wed Oct  6 22:54:58 2004
Subject: Re: poisson_rnd() excruciatingly slow
From: Paul Kienzle <pkienzle at users dot sourceforge dot net>
To: mddejong at xs4all dot nl
Cc: help at octave dot org
Date: Wed, 6 Oct 2004 23:53:58 -0400

I've posted randp() to octave-forge/FIXES.   The code for
the 5 (?!) generators  is in randpoisson.c.  There are
different generators for large (>10) and small lambda (<10),
and for repeated and varying lambda.  Some algorithms have
considerable initialization overhead when changing
lambda.   Also, for huge lambda (>1e8) a normal approximation
is used.

Be careful to test that the generated numbers look
reasonable for all these cases before putting your
trust in this code.  Test showing that the resulting
numbers are indeed Poisson would be appreciated.

Here are some times:

uniform generator
octave> tic; rand(400); toc
ans = 0.042940

repeated lambda, small and large
octave> tic; randp(5,400); toc
ans = 0.055537
octave> tic; randp(155,400); toc
ans = 0.11648

varying lambda, small and large
octave> tic; randp(5*ones(400),400); toc
ans = 0.20203
octave> tic; randp(155*ones(400),400); toc
ans = 0.58247

I've just noticed that randn is very slow on OS X:

octave> tic; randn(400); toc
ans = 12.868

Some concerned mac users may want to pursue this.
It should be just a little slower than rand(), not 300x
slower.

	- Paul


On Oct 6, 2004, at 8:24 PM, mddejong at xs4all dot nl wrote:

> Is there a reason why poisson_rnd() is so *glacially slow* for high 
> values
> of lambda? On a 2.0 GHz Athlon system, I measured poisson_rnd(200000) 
> to
> take approximately 18 seconds, making this function next to useless for
> practical use. Compare that to the 0.16 seconds for 
> binomial_rnd(400000,
> 0.5) which effectively computes the same thing, which I still consider 
> to
> be a sizable amount, by the way.
>
> If my questions have already been answered on various lists, then I
> apologize for asking them, but as the search-function of said lists 
> seems
> to be down, I cannot check myself.
>
>
> Kind regards,
> Maarten
>
>
>
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