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
open PIPE, "/bin/ps wwaxo 'pid,rss' |";
next unless /^\s*$$\s/;
$size = (split(' ', $_));
I didn't wait to let the system really slowdown, but stopped when it became obvious that swapping would have to start.