Dear Wise Monks,

I am testing changes to my

limit.conf

on my RHEL4 system. I want to see what happens if I set my virtual memory very low ... like ...

$ ulimit -a
...
virtual memory        (kbytes, -v) 8192
[download]

... and then I run Perl script that fork's off some memory draining processes ... like so ...

$ cat swaphogger.pl
#!/usr/bin/perl -w

use strict;

sub start_hogging_memory  {
        my @blackhole;
        my $count = 0;
        print __FILE__ . " [" . __LINE__ . "] child [$$]\n";
        open ( FH, "> ${0}_${$}.output" ) or die " could not open file
+:$!\n";
        my $swapon = `swapon -s | tail -1`;
        while ( 1 ) { 
                chomp( $swapon );
                print FH "[$swapon $count]\n"; 
                push @blackhole, "$count -----------------------------
+------------------------------------------\n"; 
                $count++;
                $swapon = `swapon -s | tail -1` # strangeness
        }
        close (FH);
}

my $maxChildren = shift;
my $numChildren = 0;

while ( $numChildren < $maxChildren ) {
        $numChildren++;
        my $pid = fork();
        if ( $pid == 0 ) {
                start_hogging_memory ();
        } elsif ( $pid ) {
                print __FILE__ . " [" . __LINE__ . "] parent [$$]\n";
        } else {
                die __FILE__ . " [" . __LINE__ . "] couldn't fork() :$
+!\n";
        }
}
[download]

The funny thing is that above code does NOT use up any swap! But if I comment out the line :

                $swapon = `swapon -s | tail -1` # strangeness
[download]

... things work the way I would expect ....

$ ./swaphogger.pl  10
./swaphogger.pl [8] child [2583]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2584]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2585]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2588]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2589]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2590]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2591]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2592]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2593]
./swaphogger.pl [30] parent [2582]
./swaphogger.pl [8] child [2594]
./swaphogger.pl [30] parent [2582]
rcecala@eman-sjc1-001: Priority=4 scripts$ Out of memory!
Out of memory!
Out of memory!
Out of memory!
Out of memory!
Out of memory!
Out of memory!
Out of memory!
Out of memory!
Out of memory!
[download]

Why does the line ...

                $swapon = `swapon -s | tail -1` # strangeness
[download]

... seem to fix my memory hole?

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