[R] slowness of plot(x, type='l')

[apjaworski@mmm.com]

Thanks for the quick response and apologies for my sloppy post (nor
mentioning the device) and sloppy example (the use of date()).

Indeed, I was using the windows device.  There is no timing problem with
the postscript device.

I just installed the R-devel May-9 and the problem went away.  Here is an
example:

> n <- 15000
> system.time(plot(rnorm(n), type='l'))
[1] 0.07 0.41 0.48   NA   NA

An interesting footnote is the fact that the slowdown of the windows device
did not happen for plot(*, type='c').  The 'c' option plots line segments
without points, so it probably used the "bunches of lines" anyway.

Andy

__________________________________
Andy Jaworski
518-1-01
Process Laboratory
3M Corporate Research Laboratory
-----
E-mail: apjaworski at mmm.com
Tel:  (651) 733-6092
Fax:  (651) 736-3122


                                                                           
             Prof Brian Ripley                                             
             <ripley at stats.ox.                                             
             ac.uk>                                                     To 
             Sent by:                  apjaworski at mmm.com                  
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             at.math.ethz.ch           r-help at stat.math.ethz.ch            
                                                                   Subject 
                                       Re: [R] slowness of plot(x,         
             05/07/2005 12:36          type='l')                           
             AM                                                            
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




Plotting times depend on the graphics device.  That is nowhere mentioned
here, which is unhelpful, and we have already seen a post saying it does
not happen on another unmentioned device (presumably X11).

Let us assume the unmentioned device was windows(), as that is the only
one I see any slowdown for.  (Others like win.metafile are windows() under
the skin.)

On Fri, 6 May 2005 apjaworski at mmm.com wrote:

> A couple of days ago a few messages indicated that something changed in
the
> basic plot routine that made plot(*, type='l') slow for large data sets.
> Some people even reported crashes for very large data sets.  As far as I
> remember, this was not reported as a formal bug.

Well, _is_ there a bug in R (as distinct from in Windows graphics
internals)?  I am almost certain there is not in R and this is a bug in
Windows.

> I am still not sure if this is a bug, so I report my findings here.
First
> of all, I think I see a slowdown of the plot function, although I do not
> have older versions of R installed, so I cannot do side-by-side
> comparisons.  Secondly, I noticed that the behavior of plot(*, type='l')
> differs.  Before R-2.1, the plotted lines would appear on the plot
> gradually.  Now, after the wait, the whole plot appears at once.
>
> Here are my timing results.  I am on Windows2000, IBM Intellistation with
> Xenon 2.8MHz with 1Gb of memory.  I checked May-06 versions of R-patched
> and R-devel built from sources.  I ran the following simple test:
>
> x <- rnorm(n)
> date(); plot(x, type='l'); date()

Oh, PLEASE, use system.time() to time things.  Had you done so you might
have seen things like

> windows()
> n <- 10000
> system.time(plot(rnorm(n), type="l"))
[1]  0.03 13.11 13.21    NA    NA
> postscript()
> system.time(plot(rnorm(n), type="l"))
[1] 0.07 0.00 0.08   NA   NA
> dev.off()
> system.time(plot(rnorm(n), type="p"))
[1] 0.07 0.93 1.00   NA   NA

so the time is not being taken by R but by Windows.

I can tell you the reason: it is the support for mitred etc line ends
introduced in R 2.0.0 and only supported in windows() from 2.1.0.  This
has slowed solid lines down to the sort of times taken for dashed lines
previously.

Now, the best we can do to work around this is to follow what we did for
dashed lines, and not attempt to be accurate for very large numbers of
line segments.  By plotting in bunches of 1000 lines I get

> system.time(plot(rnorm(n), type="l"))
[1] 0.03 0.36 0.42   NA   NA
> system.time(plot(rnorm(n), type="l", lty=3))
[1] 0.22 2.89 3.11   NA   NA

We have been here before, and as I recall this slowdown happens only in
NT-based versions of Windows which seem _de facto_ restricted to about
1000 line elements in a path: what we were not aware of was that it
happened for solid lines as well as dashed ones.

I've put the bunching into R-patched.

It is very regretable that this sort of thing was not tested for during
beta-testing.

--
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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