So I would ask, as your memory climbed, did the system slowdown, or did things keep running normally.

I watched the RES column from ps's output, first via top, then also using a code snippet taken out from Process::MaxSize like

[...]
    my $size;

    open PIPE, "/bin/ps wwaxo 'pid,rss' |";
    while(<PIPE>) {
        next unless /^\s*$$\s/;
    s/^\s+//g;
        chomp;
        $size = (split(' ', $_))[1];
    }
    close PIPE;

    return $size;
}
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I didn't wait to let the system really slowdown, but stopped when it became obvious that swapping would have to start.