19
Networks
•Full
cross-bar is not practical
•Direct
and Indirect Networks are alternatives
–Indirect,
e.g. W-network
•Has
only “long” paths of O(log P), no nearest neighbor
•Multiple references to a location can collide, so try combining at the
switches
•In
fact, exploit combining with Fetch&Add -- it’s better for shared memory
than Test&Set because it “schedules”
•Both
Fetch&Adds and Load/Stores can be combined
•Combining
requires “smart” switches that slow net
•Analysis
shows combining opportunities are rare; hot spots due to colliding references to
different locations is the problem
F&A + combine is smart but
flawed