[R] strange fluctuations in system.time with kernapply

From: Alexander Senger <senger_at_physik.hu-berlin.de>
Date: Fri, 29 Apr 2011 23:38:03 +0200

Hello expeRts,

here is something which strikes me as kind of odd and I would like to ask for some enlightenment:

First let's do this:

tkern <- kernel("modified.daniell", c(5,5)) test <- rep(1,1000000)
system.time(kernapply(test,tkern))

        User      System verstrichen
       1.100       0.040       1.136

That was easy. Now this:

test <- rep(1,1100000)
system.time(kernapply(test,tkern))

        User      System verstrichen
        1.40        0.02        1.43

Still fine. Now this:

test <- rep(1,1110000)
system.time(kernapply(test,tkern))

        User      System verstrichen
       1.390       0.020       1.409

Ok, by now it seems boring. But wait:

test <- rep(1,1110300)
system.time(kernapply(test,tkern))

        User      System verstrichen
      12.270       0.030      12.319

There is a sudden - and repeatable! - jump in the time needed to execute kernapply. At least from a naive point of view there should not be much difference between applying a kernel to a vector 1110000 or 1110300 entries long. But maybe there is some limit here?

So I tried this:

test <- rep(1,1110400)
system.time(kernapply(test,tkern))

        User      System verstrichen
        1.96        0.01        1.97

which doesn't fit into the pattern. But the best thing is still to come. When I try this

test <- rep(1,1110308)
system.time(kernapply(test,tkern))

then the computer starts to run and does so for longer than 15 minutes until when I normally kill the process. As noted above this behaviour is repeatable and occurs every time I issue these commands.

I really would like to know if there is some magic to the number 1110308 I'm not aware of.

Last but not least, here is my

sessionInfo()
R version 2.10.1 (2009-12-14)
x86_64-pc-linux-gnu

locale:
[1] LC_CTYPE=de_DE.utf8 LC_NUMERIC=C
[3] LC_TIME=de_DE.utf8 LC_COLLATE=de_DE.utf8
[5] LC_MONETARY=C LC_MESSAGES=de_DE.utf8
[7] LC_PAPER=de_DE.utf8 LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C
[11] LC_MEASUREMENT=de_DE.utf8 LC_IDENTIFICATION=C

attached base packages:
[1] stats graphics grDevices utils datasets methods base

loaded via a namespace (and not attached): [1] tools_2.10.1

Thank you,

Alex

-- 
Dipl.-Phys. Alexander Senger        Tel   : +49 30 2093 4941
Humboldt-Universitaet zu Berlin     Fax   : +49 30 2093 4718
AG Quantenoptik und Metrologie
Hausvogteiplatz 5-7                 Email :
10117 Berlin, Germany               senger_at_physik.hu-berlin.de

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Received on Fri 29 Apr 2011 - 21:40:55 GMT

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