[Octave-patch-tracker] [patch #9276] decide between array slice references and copies based on size

[Olaf Till]

URL:
  <http://savannah.gnu.org/patch/?9276>

                 Summary: decide between array slice references and copies
based on size
                 Project: GNU Octave
            Submitted by: i7tiol
            Submitted on: Thu 02 Mar 2017 08:12:57 PM UTC
                Category: Core : other
                Priority: 5 - Normal
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any

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Details:

Patched revision: default, 2bed6d6bb2e2, 2017-02-24 20:43+0100

Problem: with current array slice references, large arrays remain in memory
even if only a reference to a small part of them still exists.

Demonstration (in single user mode, swapping switched off):

octave:2> a = ones (500000000, 1);
octave:3> b = a(1:10);
octave:4> clear a
octave:5> a = ones (500000000, 1);
error: out of memory or dimension too large for Octave's index type
octave:5>


However, contrary to what I initially thought, no problem if array allocated
within function:

octave:1> function ret = mkarray ()
> a = ones (500000000, 1);
> ret = a(1:10);
> endfunction
octave:2> b = mkarray;
octave:3> c = mkarray;
octave:4> d = mkarray;
octave:5> ... and so on.

so if I had to decide now, I probably wouldn't apply the patch.

With patch, demonstrated problem is fixed:

octave:1> a = ones (500000000, 1);
octave:2> b = a(1:10);
octave:3> clear a
octave:4> a = ones (500000000, 1);
octave:5> c = a(1:10);
octave:6> clear a
octave:7> a = ones (500000000, 1);
octave:8> ... and so on.


Description of patch:

If slice reference leaves a certain proportion (configurable, default 50
percent) of array unreferenced, and this proportion corresponds to more than
getpagesize(), a copy is made instead of a reference. The same configure
option, set to 'no' or '0', disables slice references entirely. Set to '100'
for behaviour of unpatched Octave.

Make check gives:

Summary:

  PASS     15008
  FAIL        20
  XFAIL       38
  SKIPPED    204

with unpatched as well as with patched version, the latter both with slice
references enabled and disabled.

No test for patched behaviour can be supplied, since only feasible in single
user mode and without swapping.




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File Attachments:


-------------------------------------------------------
Date: Thu 02 Mar 2017 08:12:57 PM UTC  Name: array-slice-refs-or-copies.cset 
Size: 6kB   By: i7tiol

<http://savannah.gnu.org/patch/download.php?file_id=39882>

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