Re^5: Can you improve upon my algorithm.

[salva]

Oh, well, I meant if you do it in C, Perl has too much overhead for that.

For instance, removing everything not in the hot path for your script:

while( @fhs ) {
    printf "\r%d\t", scalar( @fhs );
    my $r = int rand @fhs;
    57 unless( length( $bufs[ $r ] ) );
    my $nextRec = substr( $bufs[ $r ], -16, 16, '' );
}

Perl compiles it into 40 OPs, 35 inside the loop:

$ perl -MO=Concise,-exec buk.pl
1  <0> enter 
2  <;> nextstate(main 5 buk.pl:1) v:{
3  <{> enterloop(next->z last->13 redo->4) v
10 <#> gv[*fhs] s
11 <1> rv2av[t2] sK/1
12 <|> and(other->4) vK/1
4      <;> nextstate(main 1 buk.pl:2) v
5      <0> pushmark sM
6      <$> const[PV "\r%d\t"] sM
7      <#> gv[*fhs] s
8      <1> rv2av[t5] sK/1
9      <@> prtf vK
a      <;> nextstate(main 1 buk.pl:3) v
b      <#> gv[*fhs] s
c      <1> rv2av[t8] sK/1
d      <1> rand[t9] sK/1
e      <1> int[t10] sK/1
f      <0> padsv[$r:1,3] sRM*/LVINTRO
g      <2> sassign vKS/2
h      <;> nextstate(main 2 buk.pl:4) v:{
i      <#> gv[*bufs] s
j      <1> rv2av sKR/1
k      <0> padsv[$r:1,3] s
l      <2> aelem sK/2
m      <1> length[t12] sKP/1
n      <|> or(other->o) vK/1
o      <;> nextstate(main 2 buk.pl:5) v:{
p      <#> gv[*bufs] s
q      <1> rv2av sKR/1
r      <0> padsv[$r:1,3] s
s      <2> aelem sKM/2
t      <$> const[IV -16] s/FOLD
u      <$> const[IV 16] s
v      <$> const[PV ""] s
w      <@> substr[t16] sK/4
x      <0> padsv[$nextRec:2,3] sRM*/LVINTRO
y      <2> sassign vKS/2
z      <0> unstack v
           goto 10
13 <2> leaveloop vKP/2
14 <@> leave[1 ref] vKP/REFC

My estimation is that every OP performs between 50 and 100 machine instructions with a good proportion of performance-unfriendly conditional branches. So, roughly, every run of the loop above would require at least 2000 instructions and may be reaching the capacity of the L1 code cache.

On the other hand, an N-way merge in C, on the hot path of the inner loop, requires just to perform a couple of operations plus the key comparisons log₂N times. That may sum up to 50 instructions for every loop cycle (=sorted record)!

update: Oops, I forgot to say, in theory!

1 hour to sort 100GB, actually, seems too fast to me. But I think the analysis is sound, and unless I am overlooking some hidden bottleneck, the order or magnitude should be right!

[BrowserUk]

The code has been running for 8:20 mins and processed 2.8GB; I'll let you do the math.

On the other hand, an N-way merge in C, on the hot path of the inner loop, requires just to perform a couple of operations plus the key comparisons log2N times. That may sum up to 50 instructions for every loop cycle (=sorted record)!

Show me the code!

[salva]

Show me the code!

Be patient!

I am still playing with your cookies problem!

[BrowserUk]

Be patient!

I didn't say when :)

FWIW: After killing the process I left running; I made one change and re-ran it:

my $nextRec = substr( $bufs[ $r ], -1024, 1024, '' );  // 1/64th itera
+tions/buffer.

And it completed in under 10 minutes:

E:\junk>c:t-salvaTheory.pl
0Took 492 seconds.