Intel admits its crossbar interconnect is power hungry

CHIPMAKER Intel has admitted that its crossbar core interconnect has the worst energy effiency of all the topologies it has evaluated.

Intel's crossbar interconnect topology is what the firm uses to connect cores in multi-core processors, however Intel senior fellow Steve Pawlowski noted that in internal experiments "crossbar [interconnect] had the worst energy efficiency". He continued by saying Intel would eventually move away from crossbar interconnect due to both power considerations and bandwidth.

Pawlowski said that bandwidth between cores is becoming a serious performance issue, with bandwidth not scaling with the number of cores being put on a single die. In some cases this means that the cores are starved of data due to the bandwidth limitations of the crossbar, which works against the advantage of having many cores.

Intel has tested mesh and ring topologies between processor cores, with its internal tests showing a ring topology providing the best power efficiency at a particular bandwidth. However Pawlowski said, "Intel is moving towards a ring but going forward it will implement mesh."

Pawlowski also talked up Intel's photonics research, though he wouldn't go into any details. The INQUIRER asked Pawlowski whether Intel's interest in photonics was to create fibre-optic - not copper - interconnects between cores, to which he agreed. He added that currently Intel is interested in "off board interconnects" using photonics and he expects it will use copper in core interconnects for some time yet.

Earlier Justin Rattner, Intel's CTO, said "moving photons as well as electrons will become increasingly important". While Rattner didn't go into more detail, by combining his comments with Pawlowski's it is fair to say that moving data around between processor cores and other system components is quickly becoming a performance bottleneck, or perhaps rather a set of performance bottlenecks.

Pawlowski didn't give a timeline for the interconnect change from crossbar to ring to mesh, but he did say it will be "a while before photonics can be manufactured", meaning that whatever core interconnect topology Intel does implement, copper will still be used for a while yet. µ