[Melbourne-pm] An old one but an important one

[Scott Penrose]

I am playing with B::Lint to find them - doesn't help heaps because of  
dynamic compiled code.

Scott

On 21/01/2008, at 9:43 AM, leif.eriksen at hpa.com.au wrote:

> I wonder  if there's a way to hook into $& and family, to see stack
> traces where they are used ? Being global, can you tie them to  
> something
> useful ?
>
> L
>
>> -----Original Message-----
>> From: melbourne-pm-bounces+leif.eriksen=hpa.com.au at pm.org
>> [mailto:melbourne-pm-bounces+leif.eriksen=hpa.com.au at pm.org]
>> On Behalf Of Scott Penrose
>> Sent: Saturday, 19 January 2008 10:51 AM
>> To: Melbourne Perlmongers
>> Subject: [Melbourne-pm] An old one but an important one
>>
>> Hey Dudes,
>>
>> I was fixing a performance problem with our portal. It was taking 2  
>> or
>> 3 minutes to render an HTML form. I narrowed it down to the
>> fact that this particular form had a 500K XML file to parse,
>> and it did it 10 times. If the duplicate parsing and size
>> wasn't bad enough, it was the hidden problem.
>>
>> Making command line code that did EXACTLY the same thing (so I
>> thought) it ran in sub 1 second. So what was going on.
>>
>> After a full day of debugging I narrowed it down to one
>> module Filter::Simple, only it wasn't, it was Text::Balanced
>>
>> And after even more work I found the real problem - "$&"
>> after a regular expression.
>>
>> Yep - it is a known killer of regular expression performance,
>> but here is the actual differences:
>>
>> (env53) vmwmi1:~/simple# time perl test_direct.pl
>> 	Fake loop start - size 488604
>> 	Fake loop end - for 21581
>> 	real	0m26.586s
>> 	user	0m9.305s
>> 	sys	0m17.269s
>>
>> (env53) vmwmi1:~/simple# time perl test_direct.pl
>> 	Fake loop start - size 488604
>> 	Fake loop end - for 21581
>> 	real	0m0.131s
>> 	user	0m0.112s
>> 	sys	0m0.012s
>>
>> Yep, even including compile time, and reading the file from
>> disk - the time goes from 0.012s to 26.58 second - that is
>> 2000 times slower !
>>
>> So... take heed !
>>
>> 	Don't use $&, $` or $'
>>
>> But more importantly - check the modules you use on CPAN.
>>
>> Scooter
>> P.S. I have removed all uses in our code, and still it has a
>> problem, so I suspect there is yet another CPAN module using one.
>>
>> _______________________________________________
>> Melbourne-pm mailing list
>> Melbourne-pm at pm.org
>> http://mail.pm.org/mailman/listinfo/melbourne-pm
>>
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