You can get data in whatever chunk size you want using read(). Here is an example that takes 24 seconds to process a 100MB file
on my PIII with slow disks. That gives a throughput of
4MB per second which will process your 81GB file in under 6 hours.
The optimal chunk size empirically is around 1MB with modest benefits increasing it to 2,4 and 8 MB. With smaller chunks you
can here the heads flipping from one file area to the other - bigger chunks allow the heads to chill. At 64kB the run time was
57 seconds and the disks screamed. At 4MB the runtime was 23 seconds.
If possible I would suggest reading from one disk and writing to a
completely separate one (I did the testing on a single partition of a single disk). You could
also roughly double the speed by forking a kid to do the disk write while the parent reads and processes
more info. This will only help if you are reading from one disk and
writing to another.
#!/usr/bin/perl -w
use strict;
my $chunk = 2**20; # try 1MB to start but it may be faster to go bigg
+er/smaller
my $infile = 'c:/test.txt';
my $outfile = 'c:/out.txt';
open IN, $infile or die "Can't open $infile $!\n";
open OUT, ">$outfile" or die "Can't open $outfile $!\n";
my $buffer;
my $partial_line = '';
my $start = time;
while (read(IN, $buffer, $chunk)) {
# we should only process full lines so we trim off the partial lin
+e
# that we inevitably get at the end of our read and save it into $
+2
$buffer =~ s/^(.*\n)([^\n]+)\z/$1/s;
# add last partial line to front of buffer
$buffer = $partial_line.$buffer;
# save the current partial line for next loop so we can add it bac
+k on
$partial_line = $2 || '';
# make changes
$buffer =~ s/this/that/g;
print OUT $buffer;
}
print "Took ", time - $start, " seconds\n";
close IN;
close OUT;
cheers
tachyon
s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print
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