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Re^2: Strings and numbers: losing memory and mind. (SV sizes)

by almut (Canon)
on Sep 28, 2007 at 13:28 UTC ( #641526=note: print w/replies, xml ) Need Help??


in reply to Re: Strings and numbers: losing memory and mind. (SV sizes)
in thread Strings and numbers: losing memory and mind.

Using an integer as a string usually makes for a considerably larger (in memory) scalar than using a string as a number.

How does this fit in with the size that Devel::Size reports? If you can trust it, a stringified number, and a numified string result in the same size (32 byte for the value 123, on a 32-bit Perl).

use Devel::Size qw(size); use Devel::Peek; sub info { print Dump($_[0]); print "size = ",size($_[0])," ($_[1])\n\n"; } $num = 123; # or int("123") info($num, "integer"); $num .= ""; info($num, "integer stringified"); $str = "123"; info($str, "string"); $str += 0; info($str, "string numified"); $str += 45678900; info($str, "... with bigger integer"); $str .= ""; info($str, "... re-stringified");

outputs something like

SV = IV(0x816983c) at 0x8192124 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 123 size = 16 (integer) SV = PVIV(0x8150b10) at 0x8192124 REFCNT = 1 FLAGS = (POK,pPOK) IV = 123 PV = 0x81c9f18 "123"\0 CUR = 3 LEN = 4 size = 32 (integer stringified) SV = PV(0x814fb90) at 0x81ca934 REFCNT = 1 FLAGS = (POK,pPOK) PV = 0x81950f0 "123"\0 CUR = 3 LEN = 4 size = 28 (string) SV = PVIV(0x8150b20) at 0x81ca934 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 123 PV = 0x81950f0 "123"\0 CUR = 3 LEN = 4 size = 32 (string numified) SV = PVIV(0x8150b20) at 0x81ca934 REFCNT = 1 FLAGS = (IOK,pIOK) IV = 45679023 PV = 0x81950f0 "123"\0 CUR = 3 LEN = 4 size = 32 (... with bigger integer) SV = PVIV(0x8150b20) at 0x81ca934 REFCNT = 1 FLAGS = (POK,pPOK) IV = 45679023 PV = 0x81950f0 "45679023"\0 CUR = 8 LEN = 12 size = 40 (... re-stringified)

Devel::Peek shows a comparable resulting structure for "number stringified" and "string numified" (with respect to IV and PV usage). Also, one can observe that the overall size gets larger if you make the number bigger, and then re-stringify the variable...

Anyhow, does your comment mean that Devel::Size is not reporting the size related to the entire PV buffer allocated for the cached stringified form, but rather its currently used part only (up to and including the \0)? — which would make it a less useful tool for determining real memory usage. Actually, the size that Devel::Size reports seems to be related to the LEN in the Devel::Peek dump (which itself you can observe to increment in steps of 4, if you play around a bit). Just wondering...

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Re^3: Strings and numbers: losing memory and mind. (SV sizes)
by tye (Sage) on Sep 28, 2007 at 15:58 UTC
    Anyhow, does your comment mean that Devel::Siz­e is not reporting the size related to the entire PV buffer allocated

    Wow, you are actually considering believing some second-hand hear-say over numbers output by a module in black-and-white? (:

    I just restated what ysth said. It made sense to me and I trust ysth but I didn't do any experiments to validate the claims. Perhaps ysth will provide some details. It certainly could be a "problem" only on a different version of Perl than what you tested on, for example. Or it may have been a misinterpretation of some data on ysth's part; after all, it was a rather casual comment and so I may have erred to elevate it to the level of a node or just misinterpretted it. We'll see what others contribute.

    Thanks for testing it.

    Looking at some source code, using an NV instead of an IV likely makes the difference (which testing shows is true on my version of Perl, allocating 36 bytes for the string "1.1", roughly doubling the size of ($x=1.1).='' over ($y='1.1')+=0; not a huge difference in most situations). The code appears to pre-construct the string then allocate/copy just the required size for an IV or UV but to allocate the buffer in the SV first when converting an NV. And based on ysth's comment, I wouldn't be surprised if the NV case has changed in some development version of Perl.

    - tye        

      Wow, you are actually considering believing some second-hand hear-say over numbers output by a module in black-and-white? (:

      Greatly do we respect our monks and greatly do we suspect our tools.

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