  The ATLAS Virtual Organization is grid's largest Virtual Organization which
is currently in full production stage. Hereby a case is being made that a user
working within that VO is going to face a wide spectrum of different systems,
whose heterogeneity is enough to count as "orders of magnitude" according to a
number of metrics; including integer/float operations, memory throughput
(STREAM) and communication latencies. Furthermore, the spread of performance
does not appear to follow any known distribution pattern, which is demonstrated
in graphs produced during May 2007 measurements. It is implied that the current
practice where either "all-WNs-are-equal" or, the alternative of SPEC-based
rating used by LCG/EGEE is an oversimplification which is inappropriate and
expensive from an operational point of view, therefore new techniques are
needed for optimal grid resources allocation.
