Re: fork(): where does copy happen?

One thing you'll need to look for is places where a scalar's internal type might change. For example:

   my $number = 10;
   print "$number\n";
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That second line is going to actually write to $number by changing it from a pure integer (IV) into a string/integer (PVIV). You can prove this by using Devel::Peek:

   use Devel::Peek;
   my $number = 10;
   Dump($number);
   print "Number is $number\n";
   Dump($number);
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Which outputs:

SV = IV(0x9a5cfe0) at 0x9a41774
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 10
Number is 10
SV = PVIV(0x9a42b10) at 0x9a41774
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,POK,pIOK,pPOK)
  IV = 10
  PV = 0x9a57758 "10"\0
  CUR = 2
  LEN = 4
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If that variable was part of your shared pre-fork data then that data page just got unshared, all 4k of it! It's possible to avoid this in some cases, for example by using printf() instead of print() above, but it's quite difficult to find all the possible cases.

To learn more about how Perl manages memory check out perlguts.

-sam

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Re^2: fork(): where does copy happen? by almut (Canon) on Apr 16, 2008 at 18:06 UTC
It might be worth pointing out that even the module Readonly will - contrary to what one might expect - not prevent the internal structures from being modified by implicit type conversions. (In addition, the structures are more complex to start with.) E.g. `use Devel::Peek; use Readonly; Readonly::Scalar my $number => 10; Dump($number); printf "Number is %d\n", $number; Dump($number); print "Number is $number\n"; Dump($number);` [download] Output: SV = PVMG(0x68ba80) at 0x65eb40 REFCNT = 1 FLAGS = (PADBUSY,PADMY,GMG,SMG,RMG) IV = 0 NV = 0 PV = 0 MAGIC = 0x662c90 MG_VIRTUAL = &PL_vtbl_packelem MG_TYPE = PERL_MAGIC_tiedscalar(q) MG_FLAGS = 0x02 REFCOUNTED MG_OBJ = 0x63c410 SV = RV(0x6ba0d0) at 0x63c410 REFCNT = 1 FLAGS = (ROK) RV = 0x6dc2e0 SV = PVMG(0x68ba48) at 0x6dc2e0 REFCNT = 1 FLAGS = (PADBUSY,PADMY,OBJECT,IOK,pIOK) IV = 10 NV = 0 PV = 0 STASH = 0x6dc120 "Readonly::Scalar" Number is 10 SV = PVMG(0x68ba80) at 0x65eb40 REFCNT = 1 FLAGS = (PADBUSY,PADMY,GMG,SMG,RMG,pIOK) IV = 10 NV = 0 PV = 0 MAGIC = 0x662c90 MG_VIRTUAL = &PL_vtbl_packelem MG_TYPE = PERL_MAGIC_tiedscalar(q) MG_FLAGS = 0x02 REFCOUNTED MG_OBJ = 0x63c410 SV = RV(0x6ba0d0) at 0x63c410 REFCNT = 1 FLAGS = (ROK) RV = 0x6dc2e0 SV = PVMG(0x68ba48) at 0x6dc2e0 REFCNT = 1 FLAGS = (PADBUSY,PADMY,OBJECT,IOK,pIOK) IV = 10 NV = 0 PV = 0 STASH = 0x6dc120 "Readonly::Scalar" Number is 10 SV = PVMG(0x68ba80) at 0x65eb40 REFCNT = 1 FLAGS = (PADBUSY,PADMY,GMG,SMG,RMG,pIOK,pPOK) IV = 10 NV = 0 PV = 0x6519b0 "10"\0 CUR = 2 LEN = 8 MAGIC = 0x662c90 MG_VIRTUAL = &PL_vtbl_packelem MG_TYPE = PERL_MAGIC_tiedscalar(q) MG_FLAGS = 0x02 REFCOUNTED MG_OBJ = 0x63c410 SV = RV(0x6ba0d0) at 0x63c410 REFCNT = 1 FLAGS = (ROK) RV = 0x6dc2e0 SV = PVMG(0x68ba48) at 0x6dc2e0 REFCNT = 1 FLAGS = (PADBUSY,PADMY,OBJECT,IOK,pIOK) IV = 10 NV = 0 PV = 0 STASH = 0x6dc120 "Readonly::Scalar" _____ Update: ...similarly with Scalar::Readonly, btw (though here the internal structures are as lean as without using the module): `use Devel::Peek; use Scalar::Readonly ':all'; my $number = 10; readonly_on($number); Dump($number); print "Number is $number\n"; Dump($number);` [download] SV = IV(0x661dc8) at 0x65eb50 REFCNT = 1 FLAGS = (PADBUSY,PADMY,IOK,READONLY,pIOK) IV = 10 Number is 10 SV = PVIV(0x63e130) at 0x65eb50 REFCNT = 1 FLAGS = (PADBUSY,PADMY,IOK,POK,READONLY,pIOK,pPOK) IV = 10 PV = 0x654fb0 "10"\0 CUR = 2 LEN = 8	[reply] [d/l] [select]
Re^2: fork(): where does copy happen? by Anonymous Monk on Apr 16, 2008 at 16:47 UTC
ouch, if even your example causes a copy, then there will be alot to be cleaned up ;)	[reply]