Re: What is PERL_COPY_ON_WRITE Compile Option About

[BrowserUk]

PERL_COPY_ON_WRITE has nothing to do with threads or process as far as I can see.

It is used by code within the pp_hot.c, regcomp.h and other places to avoid copying data until (and unless) it is written to.

An example with informative comment comes from pp_hot.c:

#ifdef PERL_COPY_ON_WRITE
/* The match may make the string COW. If so, brilliant, because that's
   just saved us one malloc, copy and free - the regexp has donated
   the old buffer, and we malloc an entirely new one, rather than the
   regexp malloc()ing a buffer and copying our original, only for
   us to throw it away here during the substitution.  */
if (SvIsCOW(TARG)) {
    sv_force_normal_flags(TARG, SV_COW_DROP_PV);
} else
#endif

[DeadPoet]

To All:

Here is what I just found at
http://yapc.mongueurs.net/yapc/talk/?tid=85

"Currently in perl when you write $a = $b, the contents of $b are immediately copied into $a. In theory perl could just pretend that it did the copy, and only actually do it if one of $a or $b is modified. If $b is a large string, and $a is never modified this could potentially be a big efficiency gain. This talk consists of simple pictures - how I didn't do it, and how I did."



DeadPoet

[BrowserUk]

It appears that currently, COW is only used for certain copy operations--some of those in the regex engine, shared hash keys (or values?), and a few other places--but not currently simple scalars.

It looks like the proposal is to extend it's reach.

[nobull]

That was an excelent talk. Highly informative and imaginatively illustrated with several people holding up cards in a carefully choreographed sequence.