On 15/01/2015 12:45, Stewart Morris wrote:
Hi,
I am dealing with very large datasets and it takes a long time to save a
workspace image.
Sounds like bad practice on your part ... saving images is not
recommended for careful work.
The options to save compressed data are: "gzip", "bzip2" or "xz", the
default being gzip. I wonder if it's possible to include the pbzip2
(http://compression.ca/pbzip2/) algorithm as an option when saving.
It is not an 'algorithm', it is a command-line utility widely available
for Linux at least.
"PBZIP2 is a parallel implementation of the bzip2 block-sorting file
compressor that uses pthreads and achieves near-linear speedup on SMP
machines. The output of this version is fully compatible with bzip2
v1.0.2 or newer"
I tested this as follows with one of my smaller datasets, having only
read in the raw data:
============
# Dumped an ascii image
save.image(file='test', ascii=TRUE)
Why do that if you are at all interested in speed? A pointless (and
inaccurate) binary to decimal conversion is needed.
# At the shell prompt:
ls -l test
-rw-rw-r--. 1 swmorris swmorris 1794473126 Jan 14 17:33 test
time bzip2 -9 test
364.702u 3.148s 6:14.01 98.3% 0+0k 48+1273976io 1pf+0w
time pbzip2 -9 test
422.080u 18.708s 0:11.49 3836.2% 0+0k 0+1274176io 0pf+0w
============
As you can see, bzip2 on its own took over 6 minutes whereas pbzip2 took
11 seconds, admittedly on a 64 core machine (running at 50% load). Most
modern machines are multicore so everyone would get some speedup.
But R does not by default save bzip2-ed ASCII images ... and gzip is the
default because its speed/compression tradeoffs (see ?save) are best for
the typical R user.
And your last point is a common misunderstanding, that people typically
have lots of spare cores which are zero-price. Even on my 8 (virtual)
core desktop when I typically do have spare cores, using them has a
price in throttling turbo mode and cache contention. Quite a large
price: an R session may run 1.5-2x slower if 7 other tasks are run in
parallel.
Is this feasible/practical? I am not a developer so I'm afraid this
would be down to someone else...
Not in base R. For example one would need a linkable library, which the
site you quote is not obviously providing.
Nothing is stopping you writing a sensible uncompressed image and
optionally compressing it externally, but note that for some file
systems compressed saves are faster because of reduced I/O.
Thoughts?
Cheers,
Stewart
--
Brian D. Ripley, rip...@stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford
1 South Parks Road, Oxford OX1 3TG, UK
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