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Re: Performance Question

by tachyon (Chancellor)
on May 08, 2002 at 16:06 UTC ( #165083=note: print w/replies, xml ) Need Help??

in reply to Performance Question

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;




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