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Postings from August 2010
Jan Dubois schrieb:
> On Fri, 30 Jul 2010, Wolfram Humann wrote:
>
> [...]
>
>> I would like to propose that perl requests an additional amount of
>> memory from realloc when strings grow by a small amount. After
>> discussion on c.l.p.m. and with the help of ideas from Ilya Zakharevich
>> and Peter J. Holzer my proposal reads like this:
>> (I'm not a regular user of diff and using it wrong might do more harm
>> than good)
>>
>> In sv.c the following section in Perl_sv_grow():
>>
>> if (newlen> SvLEN(sv)) { /* need more room? */
>> #ifndef Perl_safesysmalloc_size
>> newlen = PERL_STRLEN_ROUNDUP(newlen);
>> #endif
>>
>> should be expanded to:
>>
>> #ifndef PERL_STRLEN_EXPAND_SHIFT
>> # define PERL_STRLEN_EXPAND_SHIFT 2
>> #endif
>>
>> if (newlen> SvLEN(sv)) { /* need more room? */
>> size_t minlen = SvCUR(sv);
>> minlen += (minlen>> PERL_STRLEN_EXPAND_SHIFT) + 10;
>> if (newlen< minlen) newlen = minlen;
>> /* newlen += (newlen>> 1) + 10; */
>> #ifndef Perl_safesysmalloc_size
>> newlen = PERL_STRLEN_ROUNDUP(newlen);
>> #endif
>
> Thank you for your patch! I've now committed a slightly cleaned up version
> to the Perl repository, and added your email address to the AUTHORS file:
>
> http://perl5.git.perl.org/perl.git/commitdiff/f12005599f648e675af22dfef1047191e260bc48
> http://perl5.git.perl.org/perl.git/commitdiff/1fd8fa480e535be8885c2c1b2e7e95c3aabbdf7f
>
> Beyond formatting changes I did:
>
> * change size_t to STRLEN for internal consistency
> * removed the commented out code
> * moved the PERL_STRLEN_EXPAND_SHIFT definition to perl.h
>
> The discussion of this change seemed to have stalled, but I see
> +1 votes from David Golden and Marvin Humphrey, with additional
> information from Ben Morrow that the patch also helps on FreeBSD
> (when building without -Dusemymalloc), with nobody voicing
> any disagreement about applying the patch.
This particular slowdown was only recognized for WIN32 native malloc,
but not for other platforms with better malloc libs.
Those platforms are now hurt by using overallocation,
i.e. need more memory, e.g. with piping.
Why was this patch not applied with the appropriate
#if defined(_WIN32) or what is used for MSVC and mingw?
> a) the algorithm in sv_grow() for expanding SVs with SvOOK
>
> if (newlen> SvLEN(sv))
> newlen += 10 * (newlen - SvCUR(sv)); /* avoid copy each time */
>
> This doesn't make much sense, and is catastrophic when
> you append a very long string to a short one. I have not looked
> into the heritage of this algorithm yet.
--
Reini Urban
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