in reply to Re^6: selecting columns from a tab-separated-values file
in thread selecting columns from a tab-separated-values file

With 80GB of data and 384GB of ram, I's remove IO thrash from the picture entirely by slurping the entire dataset into ram first. Something like this: 

#! perl -slw
use strict;

open my $fh, '<:raw', $ARGV[ 0 ] or die $!;
sysread $fh, my $slurp, -s( $ARGV[0] );
close $fh;

local $, = "\t";
open RAM, '<', \$slurp;
while( <RAM> ) {
    my @f = split "\t";
    print @f[ 2,0,5 ], 
}

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Re^8: selecting columns from a tab-separated-values file
by ibm1620 (Hermit) on Jan 24, 2013 at 01:37 UTC

    Interesting procedure! However, this passed the 10M-record file in 83 seconds, as opposed to 60 (to my great surprise).

    UPDATE!!! Correction! I accidentally used perl 5.10 for the above test. I have been using 5.16 for everything else. Rerunning with 5.16 yielded a runtime of 60 seconds.

[reply]
      Rerunning with 5.16 yielded a runtime of 60 seconds.

      Conclusion: With 384GB of ram; your (relatively) tiny 10e6 lines test file is being read from system file cache, hence effectively disguising the disk IO costs.

      If your 80GB file fits in cache and will always be there when you need to do this; you can ignore the effects of disk.

      Otherwise ... you need to re-run all your testing using the real file and having flushed the cache before each test.

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
        And/or there's some pretty effective read-ahead action going on with the disk driver.
[reply]

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