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Postings from April 2014
On Sat, Mar 29, 2014 at 08:29:46PM -0700, Karl Williamson via RT wrote:
> Here are the 5.19.10+ blead numbers on a Linux box:
>
> Rate regex regex_opt eq
> regex 7650757/s -- -1% -84%
> regex_opt 7717895/s 1% -- -84%
> eq 48383994/s 532% 527% --
>
>
> And if we change $token to 'read', we get
>
> Rate regex regex_opt eq
> regex 7748530/s -- -2% -62%
> regex_opt 7899967/s 2% -- -61%
> eq 20146179/s 160% 155% --
I think the important thing is how much slower the trie optimisation is for
simple alternations that wouldn't benefit. Comparing a range of perl
versions on the same hardware, I get for the original bug report code
(where $token matches the first alternation):
5.008009 regex 10526316/s
5.010001 regex 3676471/s
5.012005 regex 3367003/s
5.014004 regex 5665722/s
5.016003 regex 6756757/s
5.018002 regex 5602241/s
5.019010 regex 6211180/s
And here's the same with TRIEs disabled (${^RE_TRIE_MAXBUF} = -1):
5.008009 regex 10309278/s
5.010001 regex 7017544/s
5.012005 regex 6993007/s
5.014004 regex 6920415/s
5.016003 regex 8130081/s
5.018002 regex 6451613/s
5.019010 regex 6644518/s
and with TRIEs enabled and $token as 'read', i.e. $token matches the third
alternation:
5.008009 regex 8438819/s
5.010001 regex 3809524/s
5.012005 regex 3344482/s
5.014004 regex 5917160/s
5.016003 regex 6711409/s
5.018002 regex 5524862/s
5.019010 regex 6097561/s
and with TRIEs enabled and with Yves modification, i.e. $token matches the
12th alternation:
5.008009 regex 4807692/s
5.010001 regex 3724395/s
5.012005 regex 3367003/s
5.014004 regex 5952381/s
5.016003 regex 6688963/s
5.018002 regex 5555556/s
5.019010 regex 6042296/s
What I conclude from the above is that:
* as with all such micro benchmarks, a certain amount of the variation is
just due to the luck of the compiler, e.g. instruction cache alignment;
* simple alternations slow down in proportion to the number of
alternatives that are tried (as you would expect);
* TRIEs match in roughly constant time regardless of which alternation
* matches (as you would expect);
* even non-TRIE alternations are slower in later perls, most likely due to
all the extra overhead we've accumulated, such as utf8 correctness,
not blowing the stack on recursion etc.
* TRIEs have got faster in 5.14;
* But TRIEs still have a bigger overhead than plain alts, so it may be
worth looking at
* profiling to see if the overhead can be reduced;
* seeing whether there's a cut-off point where it's not worth
building a TRIE, e.g. if there are less than N alternatives; and
there might be other factors such as lengths of the alt strings,
whether its utf8 etc.
--
The warp engines start playing up a bit, but seem to sort themselves out
after a while without any intervention from boy genius Wesley Crusher.
-- Things That Never Happen in "Star Trek" #17
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