in reply to Re^4: Could we save the memory occupied by "undef" in an array?
in thread Could we save the memory occupied by "undef" in an array?

There are many ways to implement an array in C. You don't have to allocate pointers immediately for every bucket. It's irrelevant though, since Perl's implementation clearly does something that causes empty buckets to take up space.

40000 bytes is close enough to the 0.35 MB I saw that inaccuracies in measurement (I glanced at Apple's system monitor to check it) cover it. Looks like 5.10 has the same cost as 5.8.8.

  • Comment on Re^5: Could we save the memory occupied by "undef" in an array?

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Re^6: Could we save the memory occupied by "undef" in an array?
by JavaFan (Canon) on Nov 24, 2008 at 11:17 UTC
    There are many ways to implement an array in C.
    Really? And for decades, I assumed there was just one way: just line up the elements of the array in consecutive memory locations.

    And that's how Perl implements arrays as well. A sequence of pointers to SVs in consecutive memory locations. Each pointer taking 4 (32-bit platform) or 8 (64-bit platform) bytes.

    You don't have to allocate pointers immediately for every bucket.
    Not quite sure what you mean by 'allocate pointers'. But even if you don't initialize the array elements (which in the Perl cause would mean, not fill the slot with a pointer to an SV - but note that since Perl doesn't keep track of which pointers are valid and which aren't, it has to), you still have to allocate memory for it. To get to $a[10000] fast, Perl will use the pointer found 40000 (80000 on 64-bit platforms) from the start of the array.
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[d/l]
      Trading some speed and adding some complexity to an array implementation to save memory on sparse arrays is a pretty straightforward concept. It's obviously not how Perl does it, based on the report of ps. No big surprise, since Perl normally favors speed over memory.
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        As I said before, Perl gives the programmer the opportunity to trade speed for memory, and use a hash instead of an array.
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Re^6: Could we save the memory occupied by "undef" in an array?
by GrandFather (Saint) on Nov 24, 2008 at 01:57 UTC

    40,000 is about 1/10th of 0.35 MB (358,400)

    Perl reduces RSI - it saves typing
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      Oh, right, off by one decimal place. But ps also shows a difference of ~400KB, so it's consistent with Apple's tool (which probably uses the same APIs). This is RSS when the program is in memory, with a sleep(10) keeping it alive.
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