in reply to Re: code optimization
in thread code optimization

Except.. it's pretty hard to compete with the cost of a division(which is rather low).

Agreed. Almost no matter how this is coded, the runtime is going to be dominated by the time taken to read the data from the file. It's doubtful that you could achieve a meaningful saving even by moving to C.

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Re^3: code optimization
by spx2 (Deacon) on Nov 04, 2011 at 12:53 UTC
    Yes, I think the C version would however be faster by some factor not depending on the input size. That's because Perl has some overhead because of the data structures it uses, because of the garbage collection and so forth. There's also the silly version of comparing a/b < c/d by multiplying everything with bd then you reach ad<cb. Now, are two multiplications faster than a division, even if the multiplication is carried out with Karatsuba's algorithm (the article says that "Karatsuba is usually faster when the multiplicands are longer than 320–640 bits" and also gives complexity) or linear time multiplication ? I'm just wondering what the cost of a normal division is in relation to the cost of two multiplications..
[reply]

      I'm not 100% certain, and I've failed to find confirmation with a quick search, but I am pretty sure that on Intel's recent (last 5 or so years) processors, 32&64-bit, that integer division and integer multiplication take the same number of clock cycles. I'll knock up a quick test to verify that though.

      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]
        I tried the following test for division. 
        perl -MTime::HiRes=gettimeofday,tv_interval -e '$istart=1000;$iend=500
+0;  $t0=[gettimeofday]; for $x($istart..$iend){for $y($istart..$iend)
+{ $x/$y }}; $t1=[gettimeofday]; print "lasted->".tv_interval($t0,$t1)
+."\n"'

        [download]
        I did the same thing for multiplication. On a couple of runs for division I got the times:
        • 1.032998
        • 1.029043
        • 1.043523
        • 1.059354
        • 1.034561
        • 1.072301
        • 1.10864
        • 1.034843
        On a couple of runs for multiplication I got the times:
        • 1.075402
        • 1.093403
        • 1.089273
        • 1.077661
        • 1.074203
        • 1.091646
        • 1.080421

        These numbers are seconds.

        I couldn't draw any conclusions from this...

[reply]
[d/l]

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