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Re: Perl's poor disk IO performance

by snoopy (Curate)
on Apr 29, 2010 at 23:09 UTC ( #837663=note: print w/replies, xml ) Need Help??

in reply to Perl's poor disk IO performance

Are you really wanting to skip over records as per your example above?

Memory mapping can be a better choice, if I/O has been identified as a bottleneck and you want 'semi-random' access to you data. I.e. if you can be a bit selective, skipping records, based on the headers and thus skipping significant blocks of data.

For example, the following uses Sys::Mmap:

#!/usr/bin/perl use common::sense; use Sys::Mmap; my $path = '/tmp/stuff'; my $file_size = -s $path; die "empty or missing file: $path" unless $file_size; open (my $fh, '+<', $path) or die "unable to open $path for read: $!"; mmap(my $data, 0, PROT_READ, MAP_SHARED, $fh) or die "mmap: $!"; my $pos = 0; while ($pos < $file_size) { my ($size, $code, $ftype) = unpack ("nCC", substr($data, $pos, 4)); $pos += 4; # advance past header $size = $size - 4; if ($size > 0) { $pos += $size; # advance past record } }
If you've identified I/O as a bottleneck, it's worthwhile benchmarking this against your above solution anyway, even if you are reading sequentially. It'll help to determine if read really is imposing a performance penalty!

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