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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: dwz@sourceware.org
Subject: [Bug default/27507] New: [dwz] Low-mem mode slowness
Date: Wed, 03 Mar 2021 15:04:38 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27507

            Bug ID: 27507
           Summary: [dwz] Low-mem mode slowness
           Product: dwz
           Version: unspecified
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: default
          Assignee: nobody at sourceware dot org
          Reporter: vries at gcc dot gnu.org
                CC: dwz at sourceware dot org
  Target Milestone: ---

Low-mem was originally advertised with:
...
commit d3b509689e17cd4885e0dfb9ad777e6337297282
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 29 19:00:58 2012 +0200

    Implement special low_mem mode, which roughly halves amount of
    needed RAM at the expense of small slowdown on very large
    inputs (more than -l COUNT DIEs).  Give up if input has more than
    -L COUNT DIEs.
...

Now consider the test-case Checker.so.debug, ~48 million DIEs, .debug_info
~0.5GB:
...
$ ./src/contrib/bytes-per-die.sh $f
11.15   size: 542547271 nr_dies: 48634711
...

We compare normal and low-mem mode:
...
+ ../measure/time.sh ./dwz debug/Checker.so.debug -o debug/Checker.so.debug=
.z
-lnone -Lnone --no-odr
maxmem: 5912860
real: 69.10
user: 65.82
system: 3.27
+ ../measure/time.sh ./dwz debug/Checker.so.debug -o debug/Checker.so.debug=
.z
-l0 -Lnone --no-odr
maxmem: 3048072
real: 135.65
user: 133.19
system: 2.40
...

The memory savings is correct, roughly 50%. But the increase in computation
time is almost a factor two.

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