Beefy Boxes and Bandwidth Generously Provided by pair Networks
P is for Practical

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
unexpectedly-good results can be obtained by using “an old COBOL trick,” namely, use an external disk-sort to sort the file first.

How many times do you need to be told. No, they cannot! It takes at least 20 times longer!


  1. Using a hash takes 38.54 seconds:
    [ 1:27:09.40] c:\test>wc -l rands.dat 100000000 rands.dat [ 1:27:48.30] c:\test>perl -nlE"++$h[ $_ ]" rands.dat [ 1:28:27.24] c:\test>
  2. Just sorting the same file takes almost 10 minutes!:
    [ 1:29:54.32] c:\test>sort -n rands.dat >rands.dat.sorted [ 1:39:03.08] c:\test>

    And that before you run another process to perform the actual counting!

To anyone with half a brain this is obvious.

  1. Using a hash requires:

    100e6 X the average time taken to read a record (IO).

    100e6 X the time taken to hash (H) the number and increment the value (i).

    ~Total time required: 100e6 * ( IO + H + I )

  2. Using a sort and then count (at least):

    100e6 X the average time taken to read a record (IOR).

    100e6 X log2( 100e6 ) = 2,657,542,476 X the time taken compare two lines (COMPL).

    100e6 X the average time taken to write a record (IOW).

    100e6 X the average time to read the sorted file (IOR)

    100e6 X the time taken to compare two lines (COMPL) + the time taken to increment a count (I) + the time taken to record that count (R).

    ~Total time required: 200e6*IOR + 100e6*IOW + 2,757e6*COMPL + 100e6*I + 100e6*R

    And that assumes that the whole dataset can be held and sorted in memory thus avoiding the additional, costly spill and merge stages. Which if it could there would be no point in using an external sort.

And please note: This isn't a personal attack. I will respond in a similar fashion to anyone pushing this outdated piece of bad information. It only seems personal, because you keep on doing it!

By now, I'm half tempted to believe you are only doing it to incur this response. But I dismiss that notion as it would require me to attribute you with some kind of Machiavellian intent, and I prefer to believe in Hanlon's Razor in these cases.

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.

In reply to Re^2: "Just use a hash": An overworked mantra? by BrowserUk
in thread "Just use a hash": An overworked mantra? by davido

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others romping around the Monastery: (7)
    As of 2021-04-21 12:22 GMT
    Find Nodes?
      Voting Booth?

      No recent polls found