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Re^5: Perl is dying (better sorting than the ST)

by demerphq (Chancellor)
on Feb 10, 2007 at 17:46 UTC ( #599374=note: print w/replies, xml ) Need Help??

in reply to Better sorting than the ST (was: Perl is dying)
in thread Perl is dying

I'm wondering if being concerned about the 100 byte overhead is a form of premature optimisation. Hypothetically perl should be able to determine if the arrays are going to have their size changed, or the arrays will be preserved or whatever, and if possible use a something more efficient.

Actually here is where Perl5 could steal a trick from Python and add a Tuple data type, that is an array like data type that is of constant size and contains constant values. Having such a data type would make it possible to write ST's and be MUCH more efficient, since the constantness of the structure would allow the structure to be represented in a much more efficient way. Much of the size issues of perls data structures are due to its dynamic nature, and the fact that we optimise for speed and not space.


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Re^6: Perl is dying (better sorting than the ST)
by diotalevi (Canon) on Feb 11, 2007 at 06:04 UTC

    How can perl know the extent of of the arrays? I can see from looking at a sample optree that the size of the input array can be known at compile time. I can also see sometimes sort's function can be known to be "simple" and won't increase the extent of the array. Lastly, I can also see that the expression used by the outer, unpacking map is simple. If I thought this were an interesting thing to chase down I'd probably go write some prolog to match stuff in our optree and if we were lisp instead of perl I'd write a macro to automatically optimize the code. We're not and we won't get that goodness til Perl 6 so it'd be a waste to do this to Perl 5.

    perl -MO=Concise -e 'print map $_->[1],sort{$a->[3]<=>$b->[3]}map[lc,$ +_],1,2,3' -e syntax OK r <@> leave[1 ref] vKP/REFC ->(end) 1 <0> enter ->2 2 <;> nextstate(main 2 -e:1) v ->3 q <@> print vK ->r 3 <0> pushmark s ->4 k <|> mapwhile(other->l)[t8] lK/1 ->q j <@> mapstart lK/2 ->k 4 <0> pushmark s ->5 - <1> null lK/1 ->5 ;; You can see the ->[1] access and you already know there's only two +elements. p <2> aelem sK/2 ->k n <1> rv2av[t1] sKR/1 ->o m <1> rv2sv sKM/DREFAV,1 ->n l <$> gv(*_) s ->m o <$> const(IV 1) s ->p ;; You can see the entire sort block got optimized into something nati +ve (I guess. It's certainly not in perl anymore) i <@> sort lKMS* ->j 5 <0> pushmark s ->6 - <1> null sK/1 ->6 - <@> scope sK ->(end) - <0> ex-nextstate v ->- - <2> ncmp[t4] sK/2 ->- - <2> aelem sK/2 ->- - <1> rv2av[t2] sKR/1 ->- - <1> rv2sv sKM/DREFAV,1 ->- - <$> gv(*a) s ->- - <$> const(IV 3) s ->- - <2> aelem sK/2 ->- - <1> rv2av[t3] sKR/1 ->- - <1> rv2sv sKM/DREFAV,1 ->- - <$> gv(*b) s ->- - <$> const(IV 3) s ->- b <|> mapwhile(other->c)[t7] lK/1 ->i a <@> mapstart lK/2 ->b 6 <0> pushmark s ->7 - <1> null lK/1 ->7 ;; You can see the array get allocated and can see it has only two ele +ments. h <1> srefgen sK/1 ->b - <1> ex-list lKRM ->h g <@> anonlist sKRM/1 ->h c <0> pushmark s ->d e <1> lc[t6] sK/1 ->f - <1> ex-rv2sv sK/1 ->e d <$> gvsv(*_) s ->e - <1> ex-rv2sv sK/1 ->g - <1> ex-rv2sv sK/1 ->g f <$> gvsv(*_) s ->g 7 <$> const(IV 1) sM ->8 8 <$> const(IV 2) sM ->9 9 <$> const(IV 3) sM ->a

    ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊

      If perl knew the array was going to be thrown away at the end of the statement, and that all indexed look ups were within the bounds of the original.

      Its a big if, hence the reason I mentioned Python's tuples, which could be seen from a perl perspective to be a form of compiler hinting, telling the compiler that an AV need not be constructed. You could prototype the syntax using a RO AV for instance.


        I'd make you say map tuple( ... ) and then you'd get your promised thing which I guess is smaller and has no mutators.

        ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊

Re^2: Better sorting than the ST
by Aristotle (Chancellor) on Feb 11, 2007 at 05:10 UTC

    Optimisation, yes; premature, how? Please show me how maintenance or verifiability suffers in any way from picking this algorithm over the ST. Tuples would help the ST, but even if its key container overhead were much smaller, itíd still be O(n), whereas itís O(1) for this algorithm.

    Makeshifts last the longest.

      Er, maybe im missing something here, how is an ST any different from what you have done in terms of big O. Leaving the sort aside since presumably it is as efficient either way, it appears to me that with an ST you have N steps to create the arrays, and N steps to unpack them, with your code you have N steps to create the key array, and N steps to map the sorted keys back to real values.

      It strikes me that your routine might be more efficient because the unit cost of each step will be cheaper, but that seems to me to be a form of optimisation that could be argued should be left to the compiler.

      So what am I missing?

      Also, if you are going down this route you might as well go as far you can. According to benchmark the true GRT I whipped up for this is about 50% faster than your routine, and uses less memory (by two whole AV's :-).

      use strict; use warnings; use Benchmark qw(cmpthese); my @a_of_a = map { [($_) x 5] } qw( a list of words to sort but the list needs to be fairl +y long so we will blab on a bit just because we have to which sorta sucks because it would be more interesting to write code or eat ice-cream ); my @grt; my @ari; cmpthese -2,{ grt => sub { my @grt = map { $a_of_a[unpack"x4N",$_] } sort map { pack "A4N", lc $a_of_a[$_][ 4 ], $_ } 0..$#a_of_a; }, ari => sub { my @ari = do { my @key = map { lc substr $_->[ 4 ], 0, 3 } @a_of_a; my @idx = sort { $key[ $a ] cmp $key[ $b ] } 0 .. $#key; @a_of_a[ @idx ]; }; } }; if (@grt) { for (0..$#a_of_a) { if ( $ari[$_] != $grt[$_] ) { die "$_ is bad\n"; } } } __END__ Rate ari grt ari 3946/s -- -37% grt 6260/s 59% -- Rate ari grt ari 4111/s -- -33% grt 6179/s 50% -- Rate ari grt ari 3989/s -- -38% grt 6393/s 60% -- Rate ari grt ari 4049/s -- -36% grt 6300/s 56% -- Rate ari grt ari 3982/s -- -38% grt 6467/s 62% --

      Because the GRT avoids the substr, needs no indirection on the key which in turn means you dont need provide a custom sort function which in turn means the per cost of each comparison is quite a bit lower. AND you get the added bonus that the sort is stable, and a lower memory profile.

      But it also arguable that this too is a form of premature optimisation.

      Update: when i compare your approach to a ST like the following, i see no performance difference between it and your approach.

      st => sub { my @st = map { pop @$_; $_ } sort { $a->[-1] cmp $b->[-1] } map { my $k = lc substr $_->[ 4 ], 0, 3; push @$_,$k; +$_ } @a_of_a; }

        Interestingly, when I looked at the optree above you'll notice that the custom sort sub { $a->[0] cmp $b->[0] } got entirely elided. Perhaps someone snuck in an interesting sort of optimization to the sort function where even more complicated things like that are actually performed in native C and not actually done in perl at all.

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        it appears to me that with an ST you have N steps to create the arrays, and N steps to unpack them, with your code you have N steps to create the key array, and N steps to map the sorted keys back to real values.

        I was talking about memory, not efficiency. Even if Perl had tuples, ST still requires extra memory for container structures proportional to the size of the list being sorted, whereas index sorting only requires a single container. Although in terms of big-O, thatís irrelevant, since if you consider the keys as well, then the ST requires extra memory on the order of n · (s + a) (where s and a are constants and stand for the overhead of a scalar and array, respectively) whereas index sorting only requires n · s + 1 · a; but these are both O(n).

        My bad.

        Of course, in Perl, thereís a huge difference between the two because n · a easily becomes a considerable quantity.

        However, I donít see how index sorting is a premature optimisation. Itís not just parsimonious with memory, itís also easier to follow (in my opinion) than the ST for someone who has never seen either idiom.

        The real way to go about making sorting faster would of course be to decouple key extraction from the comparator function, which would allow almost all sorts to be written declaratively. There was discussion about this on perl6-language back when I was subscribed, but I donít know if anything came of it.

        Makeshifts last the longest.

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