comment on

> Is it possible that large allocations are also freed back to the OS on *nix, and you are measuring after it has been freed?

I'm not sure where this mem comes from. I tried with Memory::Usage, measuring the consumption with top running with 0.001s delay (was probably more in practice):

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use Memory::Usage;

my $file = shift;
open my $FH, '<', $file or die $!;

my $string;
my $mu = 'Memory::Usage'->new;

$mu->record('start');
{ local $/; $string = <$FH> };
$mu->record('after do');
undef $string;

seek $FH, 0, 0;
$string = do { local $/; <$FH> };
$mu->record('after no do');

$mu->dump;
[download]

Output:

  time    vsz (  diff)    rss (  diff) shared (  diff)   code (  diff)
+   data (  diff)
     0  17504 ( 17504)   3916 (  3916)   3328 (  3328)   1644 (  1644)
+   1060 (  1060) start
     0  1970632 ( 1953128)  1957112 ( 1953196)   3520 (   192)   1644 
+(     0)  1954188 ( 1953128) after do
     1  1970632 (     0)  1957248 (   136)   3520 (     0)   1644 (   
+  0)  1954188 (     0) after no do
[download]

Top output:

 8504 choroba   20   0 1970632   7992   3520 R 0.000 0.099   0:00.00 1
+.pl
 8504 choroba   20   0 1970632  26656   3520 R 0.000 0.329   0:00.00 1
+.pl
 8504 choroba   20   0 1970632  47136   3520 R 100.0 0.582   0:00.01 1
+.pl
 8504 choroba   20   0 1970632  65568   3520 R 0.000 0.809   0:00.01 1
+.pl
 8504 choroba   20   0 1970632  86048   3520 R 100.0 1.062   0:00.02 1
+.pl
 8504 choroba   20   0 1970632 104480   3520 R 100.0 1.289   0:00.03 1
+.pl
 8504 choroba   20   0 1970632 124960   3520 R 0.000 1.542   0:00.03 1
+.pl
 8504 choroba   20   0 1970632 143392   3520 R 100.0 1.770   0:00.04 1
+.pl
 8504 choroba   20   0 1970632 163872   3520 R 0.000 2.022   0:00.04 1
+.pl
 8504 choroba   20   0 1970632 184352   3520 R 100.0 2.275   0:00.05 1
+.pl
 8504 choroba   20   0 1970632 202784   3520 R 0.000 2.503   0:00.05 1
+.pl
 8504 choroba   20   0 1970632 223264   3520 R 100.0 2.755   0:00.06 1
+.pl
 8504 choroba   20   0 1970632 243744   3520 R 0.000 3.008   0:00.06 1
+.pl
 8504 choroba   20   0 1970632 262176   3520 R 100.0 3.236   0:00.07 1
+.pl
 8504 choroba   20   0 1970632 282656   3520 R 0.000 3.488   0:00.07 1
+.pl
 8504 choroba   20   0 1970632 303136   3520 R 100.0 3.741   0:00.08 1
+.pl
 8504 choroba   20   0 1970632 323616   3520 R 0.000 3.994   0:00.08 1
+.pl
 8504 choroba   20   0 1970632 344096   3520 R 0.000 4.247   0:00.08 1
+.pl
 8504 choroba   20   0 1970632 364576   3520 R 100.0 4.499   0:00.09 1
+.pl
 8504 choroba   20   0 1970632 385056   3520 R 100.0 4.752   0:00.10 1
+.pl
 8504 choroba   20   0 1970632 405536   3520 R 0.000 5.005   0:00.10 1
+.pl
 8504 choroba   20   0 1970632 426016   3520 R 100.0 5.257   0:00.11 1
+.pl
 8504 choroba   20   0 1970632 446496   3520 R 0.000 5.510   0:00.11 1
+.pl
 8504 choroba   20   0 1970632 466976   3520 R 100.0 5.763   0:00.12 1
+.pl
 8504 choroba   20   0 1970632 489504   3520 R 100.0 6.041   0:00.13 1
+.pl
 8504 choroba   20   0 1970632 507936   3520 R 0.000 6.268   0:00.13 1
+.pl
 8504 choroba   20   0 1970632 530464   3520 R 100.0 6.546   0:00.14 1
+.pl
 8504 choroba   20   0 1970632 550944   3520 R 0.000 6.799   0:00.14 1
+.pl
 8504 choroba   20   0 1970632 571424   3520 R 100.0 7.052   0:00.15 1
+.pl
 8504 choroba   20   0 1970632 591904   3520 R 0.000 7.305   0:00.15 1
+.pl
 8504 choroba   20   0 1970632 612384   3520 R 0.000 7.557   0:00.15 1
+.pl
 8504 choroba   20   0 1970632 632864   3520 R 100.0 7.810   0:00.16 1
+.pl
 8504 choroba   20   0 1970632 655392   3520 R 0.000 8.088   0:00.16 1
+.pl
 8504 choroba   20   0 1970632 675872   3520 R 100.0 8.341   0:00.17 1
+.pl
 8504 choroba   20   0 1970632 696352   3520 R 100.0 8.594   0:00.18 1
+.pl
 8504 choroba   20   0 1970632 716832   3520 R 0.000 8.846   0:00.18 1
+.pl
 8504 choroba   20   0 1970632 739360   3520 R 100.0 9.124   0:00.19 1
+.pl
 8504 choroba   20   0 1970632 759840   3520 R 0.000 9.377   0:00.19 1
+.pl
 8504 choroba   20   0 1970632 780320   3520 R 0.000 9.630   0:00.19 1
+.pl
 8504 choroba   20   0 1970632 800800   3520 R 100.0 9.883   0:00.20 1
+.pl
 8504 choroba   20   0 1970632 823328   3520 R 0.000 10.16   0:00.20 1
+.pl
 8504 choroba   20   0 1970632 843808   3520 R 100.0 10.41   0:00.21 1
+.pl
 8504 choroba   20   0 1970632 864288   3520 R 100.0 10.67   0:00.22 1
+.pl
 8504 choroba   20   0 1970632 884768   3520 R 0.000 10.92   0:00.22 1
+.pl
 8504 choroba   20   0 1970632 905248   3520 R 0.000 11.17   0:00.22 1
+.pl
 8504 choroba   20   0 1970632 927776   3520 R 100.0 11.45   0:00.23 1
+.pl
 8504 choroba   20   0 1970632 948256   3520 R 0.000 11.70   0:00.23 1
+.pl
 8504 choroba   20   0 1970632 968736   3520 R 100.0 11.96   0:00.24 1
+.pl
 8504 choroba   20   0 1970632 989216   3520 R 0.000 12.21   0:00.24 1
+.pl
 8504 choroba   20   0 1970632 988.0m   3520 R 100.0 12.49   0:00.25 1
+.pl
 8504 choroba   20   0 1970632 0.984g   3520 R 0.000 12.74   0:00.25 1
+.pl
 8504 choroba   20   0 1970632 1.004g   3520 R 100.0 12.99   0:00.26 1
+.pl
 8504 choroba   20   0 1970632 1.023g   3520 R 0.000 13.24   0:00.26 1
+.pl
 8504 choroba   20   0 1970632 1.043g   3520 R 100.0 13.50   0:00.27 1
+.pl
 8504 choroba   20   0 1970632 1.064g   3520 R 0.000 13.77   0:00.27 1
+.pl
 8504 choroba   20   0 1970632 1.084g   3520 R 100.0 14.03   0:00.28 1
+.pl
 8504 choroba   20   0 1970632 1.104g   3520 R 0.000 14.28   0:00.28 1
+.pl
 8504 choroba   20   0 1970632 1.123g   3520 R 100.0 14.53   0:00.29 1
+.pl
 8504 choroba   20   0 1970632 1.143g   3520 R 100.0 14.79   0:00.30 1
+.pl
 8504 choroba   20   0 1970632 1.162g   3520 R 0.000 15.04   0:00.30 1
+.pl
 8504 choroba   20   0 1970632 1.182g   3520 R 0.000 15.29   0:00.30 1
+.pl
 8504 choroba   20   0 1970632 1.203g   3520 R 100.0 15.57   0:00.31 1
+.pl
 8504 choroba   20   0 1970632 1.223g   3520 R 100.0 15.82   0:00.32 1
+.pl
 8504 choroba   20   0 1970632 1.242g   3520 R 0.000 16.07   0:00.32 1
+.pl
 8504 choroba   20   0 1970632 1.262g   3520 R 0.000 16.33   0:00.32 1
+.pl
 8504 choroba   20   0 1970632 1.281g   3520 R 100.0 16.58   0:00.33 1
+.pl
 8504 choroba   20   0 1970632 1.301g   3520 R 0.000 16.83   0:00.33 1
+.pl
 8504 choroba   20   0 1970632 1.320g   3520 R 100.0 17.09   0:00.34 1
+.pl
 8504 choroba   20   0 1970632 1.340g   3520 R 0.000 17.34   0:00.34 1
+.pl
 8504 choroba   20   0 1970632 1.359g   3520 R 100.0 17.59   0:00.36 1
+.pl
 8504 choroba   20   0 1970632 1.379g   3520 R 0.000 17.84   0:00.36 1
+.pl
 8504 choroba   20   0 1970632 1.398g   3520 R 0.000 18.10   0:00.36 1
+.pl
 8504 choroba   20   0 1970632 1.418g   3520 R 0.000 18.35   0:00.36 1
+.pl
 8504 choroba   20   0 1970632 1.438g   3520 R 100.0 18.60   0:00.37 1
+.pl
 8504 choroba   20   0 1970632 1.457g   3520 R 0.000 18.86   0:00.37 1
+.pl
 8504 choroba   20   0 1970632 1.475g   3520 R 100.0 19.08   0:00.39 1
+.pl
 8504 choroba   20   0 1970632 1.494g   3520 R 0.000 19.34   0:00.39 1
+.pl
 8504 choroba   20   0 1970632 1.512g   3520 R 0.000 19.56   0:00.39 1
+.pl
 8504 choroba   20   0 1970632 1.531g   3520 R 100.0 19.82   0:00.40 1
+.pl
 8504 choroba   20   0 1970632 1.551g   3520 R 0.000 20.07   0:00.40 1
+.pl
 8504 choroba   20   0 1970632 1.572g   3520 R 100.0 20.35   0:00.41 1
+.pl
 8504 choroba   20   0 1970632 1.592g   3520 R 0.000 20.60   0:00.41 1
+.pl
 8504 choroba   20   0 1970632 1.611g   3520 R 100.0 20.85   0:00.42 1
+.pl
 8504 choroba   20   0 1970632 1.631g   3520 R 0.000 21.10   0:00.42 1
+.pl
 8504 choroba   20   0 1970632 1.650g   3520 R 100.0 21.36   0:00.43 1
+.pl
 8504 choroba   20   0 1970632 1.670g   3520 R 0.000 21.61   0:00.43 1
+.pl
 8504 choroba   20   0 1970632 1.691g   3520 R 100.0 21.89   0:00.44 1
+.pl
 8504 choroba   20   0 1970632 1.711g   3520 R 0.000 22.14   0:00.44 1
+.pl
 8504 choroba   20   0 1970632 1.730g   3520 R 0.000 22.39   0:00.44 1
+.pl
 8504 choroba   20   0 1970632 1.750g   3520 R 100.0 22.65   0:00.45 1
+.pl
 8504 choroba   20   0 1970632 1.770g   3520 R 0.000 22.90   0:00.45 1
+.pl
 8504 choroba   20   0 1970632 1.789g   3520 R 100.0 23.15   0:00.46 1
+.pl
 8504 choroba   20   0 1970632 1.809g   3520 R 100.0 23.40   0:00.47 1
+.pl
 8504 choroba   20   0 1970632 1.828g   3520 R 100.0 23.66   0:00.48 1
+.pl
 8504 choroba   20   0 1970632 1.848g   3520 R 0.000 23.91   0:00.48 1
+.pl
 8504 choroba   20   0 1970632 1.866g   3520 R 0.000 24.15   0:00.48 1
+.pl
 8504 choroba   20   0 1970632   8128   3520 R 100.0 0.100   0:00.49 1
+.pl
 8504 choroba   20   0 1970632  26560   3520 R 0.000 0.328   0:00.49 1
+.pl
 8504 choroba   20   0 1970632  47040   3520 R 100.0 0.581   0:00.50 1
+.pl
 8504 choroba   20   0 1970632  65472   3520 R 0.000 0.808   0:00.50 1
+.pl
 8504 choroba   20   0 1970632  85952   3520 R 100.0 1.061   0:00.51 1
+.pl
 8504 choroba   20   0 1970632 104480   3520 R 0.000 1.289   0:00.51 1
+.pl
 8504 choroba   20   0 1970632 124960   3520 R 0.000 1.542   0:00.51 1
+.pl
 8504 choroba   20   0 1970632 143392   3520 R 100.0 1.770   0:00.53 1
+.pl
 8504 choroba   20   0 1970632 163872   3520 R 0.000 2.022   0:00.53 1
+.pl
 8504 choroba   20   0 1970632 184352   3520 R 100.0 2.275   0:00.54 1
+.pl
 8504 choroba   20   0 1970632 202784   3520 R 0.000 2.503   0:00.54 1
+.pl
 8504 choroba   20   0 1970632 223264   3520 R 0.000 2.755   0:00.54 1
+.pl
 8504 choroba   20   0 1970632 243744   3520 R 100.0 3.008   0:00.55 1
+.pl
 8504 choroba   20   0 1970632 262176   3520 R 0.000 3.236   0:00.55 1
+.pl
 8504 choroba   20   0 1970632 280608   3520 R 100.0 3.463   0:00.56 1
+.pl
 8504 choroba   20   0 1970632 301088   3520 R 100.0 3.716   0:00.57 1
+.pl
 8504 choroba   20   0 1970632 321568   3520 R 0.000 3.968   0:00.57 1
+.pl
 8504 choroba   20   0 1970632 342048   3520 R 100.0 4.221   0:00.58 1
+.pl
 8504 choroba   20   0 1970632 362528   3520 R 0.000 4.474   0:00.58 1
+.pl
 8504 choroba   20   0 1970632 383008   3520 R 0.000 4.727   0:00.58 1
+.pl
 8504 choroba   20   0 1970632 401440   3520 R 100.0 4.954   0:00.59 1
+.pl
 8504 choroba   20   0 1970632 421920   3520 R 0.000 5.207   0:00.59 1
+.pl
 8504 choroba   20   0 1970632 444448   3520 R 100.0 5.485   0:00.60 1
+.pl
 8504 choroba   20   0 1970632 464928   3520 R 0.000 5.738   0:00.60 1
+.pl
 8504 choroba   20   0 1970632 485408   3520 R 100.0 5.990   0:00.61 1
+.pl
 8504 choroba   20   0 1970632 503840   3520 R 0.000 6.218   0:00.61 1
+.pl
 8504 choroba   20   0 1970632 526368   3520 R 100.0 6.496   0:00.62 1
+.pl
 8504 choroba   20   0 1970632 544800   3520 R 0.000 6.723   0:00.62 1
+.pl
 8504 choroba   20   0 1970632 565280   3520 R 100.0 6.976   0:00.63 1
+.pl
 8504 choroba   20   0 1970632 587808   3520 R 0.000 7.254   0:00.63 1
+.pl
 8504 choroba   20   0 1970632 608288   3520 R 100.0 7.507   0:00.65 1
+.pl
 8504 choroba   20   0 1970632 626720   3520 R 0.000 7.734   0:00.65 1
+.pl
 8504 choroba   20   0 1970632 649248   3520 R 0.000 8.012   0:00.65 1
+.pl
 8504 choroba   20   0 1970632 667680   3520 R 100.0 8.240   0:00.66 1
+.pl
 8504 choroba   20   0 1970632 688160   3520 R 0.000 8.493   0:00.66 1
+.pl
 8504 choroba   20   0 1970632 710688   3520 R 100.0 8.771   0:00.67 1
+.pl
 8504 choroba   20   0 1970632 731168   3520 R 0.000 9.023   0:00.67 1
+.pl
 8504 choroba   20   0 1970632 749600   3520 R 100.0 9.251   0:00.68 1
+.pl
 8504 choroba   20   0 1970632 770080   3520 R 0.000 9.504   0:00.68 1
+.pl
 8504 choroba   20   0 1970632 790560   3520 R 0.000 9.756   0:00.68 1
+.pl
 8504 choroba   20   0 1970632 811040   3520 R 100.0 10.01   0:00.69 1
+.pl
 8504 choroba   20   0 1970632 831520   3520 R 100.0 10.26   0:00.70 1
+.pl
 8504 choroba   20   0 1970632 852000   3520 R 0.000 10.51   0:00.70 1
+.pl
 8504 choroba   20   0 1970632 872480   3520 R 100.0 10.77   0:00.71 1
+.pl
 8504 choroba   20   0 1970632 892960   3520 R 0.000 11.02   0:00.71 1
+.pl
 8504 choroba   20   0 1970632 913440   3520 R 100.0 11.27   0:00.72 1
+.pl
 8504 choroba   20   0 1970632 931872   3520 R 0.000 11.50   0:00.72 1
+.pl
 8504 choroba   20   0 1970632 954400   3520 R 0.000 11.78   0:00.72 1
+.pl
 8504 choroba   20   0 1970632 974880   3520 R 100.0 12.03   0:00.73 1
+.pl
 8504 choroba   20   0 1970632 993312   3520 R 100.0 12.26   0:00.74 1
+.pl
 8504 choroba   20   0 1970632 990.0m   3520 R 100.0 12.51   0:00.75 1
+.pl
 8504 choroba   20   0 1970632 0.988g   3520 R 0.000 12.79   0:00.75 1
+.pl
 8504 choroba   20   0 1970632 1.006g   3520 R 100.0 13.02   0:00.76 1
+.pl
 8504 choroba   20   0 1970632 1.025g   3520 R 0.000 13.27   0:00.76 1
+.pl
 8504 choroba   20   0 1970632 1.047g   3520 R 0.000 13.55   0:00.76 1
+.pl
 8504 choroba   20   0 1970632 1.064g   3520 R 0.000 13.77   0:00.76 1
+.pl
 8504 choroba   20   0 1970632 1.084g   3520 R 100.0 14.03   0:00.78 1
+.pl
 8504 choroba   20   0 1970632 1.105g   3520 R 0.000 14.31   0:00.78 1
+.pl
 8504 choroba   20   0 1970632 1.123g   3520 R 0.000 14.53   0:00.78 1
+.pl
 8504 choroba   20   0 1970632 1.143g   3520 R 100.0 14.79   0:00.79 1
+.pl
 8504 choroba   20   0 1970632 1.164g   3520 R 0.000 15.06   0:00.79 1
+.pl
 8504 choroba   20   0 1970632 1.182g   3520 R 100.0 15.29   0:00.80 1
+.pl
 8504 choroba   20   0 1970632 1.201g   3520 R 0.000 15.54   0:00.80 1
+.pl
 8504 choroba   20   0 1970632 1.223g   3520 R 100.0 15.82   0:00.81 1
+.pl
 8504 choroba   20   0 1970632 1.240g   3520 R 0.000 16.05   0:00.81 1
+.pl
 8504 choroba   20   0 1970632 1.260g   3520 R 100.0 16.30   0:00.82 1
+.pl
 8504 choroba   20   0 1970632 1.281g   3520 R 100.0 16.58   0:00.83 1
+.pl
 8504 choroba   20   0 1970632 1.299g   3520 R 0.000 16.81   0:00.83 1
+.pl
 8504 choroba   20   0 1970632 1.318g   3520 R 100.0 17.06   0:00.84 1
+.pl
 8504 choroba   20   0 1970632 1.340g   3520 R 0.000 17.34   0:00.84 1
+.pl
 8504 choroba   20   0 1970632 1.357g   3520 R 0.000 17.57   0:00.84 1
+.pl
 8504 choroba   20   0 1970632 1.377g   3520 R 100.0 17.82   0:00.85 1
+.pl
 8504 choroba   20   0 1970632 1.397g   3520 R 100.0 18.07   0:00.86 1
+.pl
 8504 choroba   20   0 1970632 1.416g   3520 R 0.000 18.32   0:00.86 1
+.pl
 8504 choroba   20   0 1970632 1.436g   3520 R 100.0 18.58   0:00.87 1
+.pl
 8504 choroba   20   0 1970632 1.453g   3520 R 0.000 18.80   0:00.87 1
+.pl
 8504 choroba   20   0 1970632 1.471g   3520 R 0.000 19.03   0:00.87 1
+.pl
 8504 choroba   20   0 1970632 1.490g   3520 R 100.0 19.28   0:00.88 1
+.pl
 8504 choroba   20   0 1970632 1.508g   3520 R 0.000 19.51   0:00.88 1
+.pl
 8504 choroba   20   0 1970632 1.527g   3520 R 100.0 19.77   0:00.89 1
+.pl
 8504 choroba   20   0 1970632 1.547g   3520 R 100.0 20.02   0:00.90 1
+.pl
 8504 choroba   20   0 1970632 1.564g   3520 R 0.000 20.25   0:00.90 1
+.pl
 8504 choroba   20   0 1970632 1.586g   3520 R 100.0 20.52   0:00.91 1
+.pl
 8504 choroba   20   0 1970632 1.605g   3520 R 0.000 20.78   0:00.91 1
+.pl
 8504 choroba   20   0 1970632 1.625g   3520 R 100.0 21.03   0:00.92 1
+.pl
 8504 choroba   20   0 1970632 1.643g   3520 R 0.000 21.26   0:00.92 1
+.pl
 8504 choroba   20   0 1970632 1.664g   3520 R 0.000 21.53   0:00.92 1
+.pl
 8504 choroba   20   0 1970632 1.682g   3520 R 100.0 21.76   0:00.93 1
+.pl
 8504 choroba   20   0 1970632 1.701g   3520 R 0.000 22.01   0:00.93 1
+.pl
 8504 choroba   20   0 1970632 1.721g   3520 R 100.0 22.27   0:00.94 1
+.pl
 8504 choroba   20   0 1970632 1.742g   3520 R 0.000 22.55   0:00.94 1
+.pl
 8504 choroba   20   0 1970632 1.760g   3520 R 100.0 22.77   0:00.95 1
+.pl
 8504 choroba   20   0 1970632 1.779g   3520 R 0.000 23.03   0:00.95 1
+.pl
 8504 choroba   20   0 1970632 1.799g   3520 R 100.0 23.28   0:00.97 1
+.pl
 8504 choroba   20   0 1970632 1.818g   3520 R 0.000 23.53   0:00.97 1
+.pl
 8504 choroba   20   0 1970632 1.838g   3520 R 100.0 23.78   0:00.98 1
+.pl
 8504 choroba   20   0 1970632 1.857g   3520 R 0.000 24.04   0:00.98 1
+.pl
 8504 choroba   20   0 1970632   4244   3520 R 100.0 0.052   0:00.99 1
+.pl
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

In reply to Re^6: memory use array vs ref to array by choroba
in thread memory use array vs ref to array by dkhosla1

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