Abstract
Using the Cray Gemini interconnect as our platform, we
present a study of an important class of communication
operations––the fan-in communication pattern. By its nature,
fan-in communications form ‘hot spots’ that present
significant challenges for any interconnect fabric and
communication software stack. Yet despite the inherent
challenges, these communication patterns are common in
both applications (which often perform reductions and other
collective operations that include fan-in communication such
as barriers) and system software (where they assume an
important role within parallel file systems and other
components requiring high-bandwidth or low-latency I/O).
Our study determines the effectiveness of differing clientserver
fan-in strategies. We describe fan-in performance in
terms of aggregate bandwidth in the presence of varying
degrees of congestion, as well as several other key attributes.
Comparison numbers are presented for the Cray Aries
interconnect. Finally, we provide recommended communication
strategies based on our findings.
