[Rd] Varying as.Date performance

[Prof Brian Ripley]

One other possibly difference would be locale, but this is slow on FC3 
(2.3.4 now) in the C locale.  Almost all the time is in strptime:
R profiling shows

> summaryRprof()
$by.self
                     self.time self.pct total.time total.pct
"strptime"              29.58     99.7      29.58      99.7
"as.Date.character"      0.10      0.3      29.68     100.0
"as.Date"                0.00      0.0      29.68     100.0
"eval"                   0.00      0.0      29.68     100.0
"system.time"            0.00      0.0      29.68     100.0

Now on a glibc 2.3.x system R's internal replacement for strptime will be 
used (to work around bugs) so it must be some other part of the POSIX 
time-handling that has changed.

The next step would be to do C-level profiling and then retrofit the 
crucial code from glibc 2.3.2.

It does seem a pretty unusual application of R for 10^5 date conversions 
to be needed and for 30 secs to be an appreciable part of the analysis 
time on such a data set.

On Wed, 4 May 2005, Jeff Enos wrote:

> R-devel,
>
> The performance of as.Date differs by a large degree between one of my
> machines with glibc 2.3.2:
>
>> system.time(x <- as.Date(rep("01-01-2005", 100000), format = "%m-%d-%Y"))
> [1] 1.17 0.00 1.18 0.00 0.00
>
> and a comparable machine with glibc 2.3.3:
>
>> system.time(x <- as.Date(rep("01-01-2005", 100000), format = "%m-%d-%Y"))
> [1] 31.20 46.89 81.01  0.00  0.00
>
> both with the same R version:
>
>> R.version
>         _
> platform i686-pc-linux-gnu
> arch     i686
> os       linux-gnu
> system   i686, linux-gnu
> status
> major    2
> minor    1.0
> year     2005
> month    04
> day      18
> language R
>
> I'm focusing on differences in glibc versions because of as.Date's use
> of strptime.
>
> Does it seem likely that the cause of this discrepancy is in fact
> glibc?  If so, can anyone tell me how to make the performance of the
> second machine more like the first?
>
> I have verified that using the chron package, which I don't believe
> uses strptime, for the above character conversion performs equally
> well on both machines.

-- 
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