[R] Antwort: Re: Antwort: Buying more computer for GLM

g.russell at eos-finance.com g.russell at eos-finance.com
Fri Sep 1 15:33:58 CEST 2006

Prof Brian Ripley wrote:
> I would not have expected glm to be more than say 5x slower than lm if 
> cycles and not memory were the limiting factor.  In that case more RAM 
> might be all you need.

The ratio between glm and lm might well be about 5x, but that's still a 
big difference for us.   I am pretty sure that RAM is not the main 
problem; according to the Windows Task Manager the computer is at close to 
100% CPU usage, and swapping is not going on.   Of course L1/L2 caches may 
still be
something one can work on, but I'm not sure whether glm has enough 
repeated access to the same data for that to help.   (I don't know how glm 
but I guess it does a lot of scans through the whole data set, and that 
the amount of working memory it needs during these scans is basically a 
function of the number of parameters, not the number of observations, is 
that right?)

Many thanks for your observations about subset selection by the way, they 
are a lot of help.   Would a good approach be, say, to use some stricter 
criteria like BIC for choosing a model, and then use non-statistical 
methods to improve the plausibility of the chosen parameters?

best wishes,

George Russell

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