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Cc: OCaml List <caml-list@yquem.inria.fr>
Message-Id: <59BD1B3A-B449-4963-9910-ED5E755D00E6@cs.berkeley.edu>
From: Brighten Godfrey <pbg@cs.berkeley.edu>
To: Markus Mottl <markus.mottl@gmail.com>
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Subject: Re: [Caml-list] Strange performance bug
Date: Tue, 28 Apr 2009 21:31:21 -0700
References: <324B24CA-9671-42C0-B722-C7710C0C45C7@cs.berkeley.edu> <f8560b80904282037t61b27ce7lbe532b0f2dd54ee9@mail.gmail.com>
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X-Spam: no; 0.00; bug:01 markus:01 mottl:01 bug:01 compiler:01 pcre:01 pcre:01 regexp:01 regexp:01 28,:98 28,:98 2009:98 wrote:01 wrote:01 parsing:01 

On Apr 28, 2009, at 8:37 PM, Markus Mottl wrote:
> On Tue, Apr 28, 2009 at 22:43, Brighten Godfrey  
> <pbg@cs.berkeley.edu> wrote:
>> I've encountered a very odd performance problem which I suspect is  
>> not a bug
>> in my code.  Could it be the compiler, or maybe PCRE?
>
> I'm not sure it solves your problem (haven't tried the example), but
> just looking at the code there is clearly a performance bug: the
> pattern is passed to Pcre.pmatch "on the fly" using label "~pat".
> This is ok and convenient if it is used only once, but is bad if it
> happens in a loop.  Precompile the regular expression outside of the
> loop (let rex = Pcre.regexp "...") and pass it in with label "~rex" to
> solve this problem.


I can't see how this explains the problem.  Why should the parsing get  
dramatically slower when starting to parse the file a *second* time?

(Changing to the precompiled regexp does make this bug go away -- but  
so do many other small changes, like commenting out the last line of  
the code, *after* the parsing is complete.)

Thanks very much,
~Brighten