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Additional network configuration restrictions apply when physical channels are used:
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| • | If two tasks are to be connected by a physical channel, there must be at least one wire running directly between the processors on which the tasks are placed. Intermediate nodes cannot forward messages sent on physical channels. |
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| • | An FPGA can provide links to its attached processors without violating the previous restriction. |
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| • | Each link (wire) can carry only two physical channels, one in each direction. If more physical channels are required than there are links available, the configurer reports an error. Note that any virtual channels using a wire consumes both of the two physical channels available on that wire. |
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| • | Any wire that is allocated to a physical channel is not available for use by UPR (see the previous restriction). This may result in not enough spare wires being available to connect all the UPR nodes. If that happens, the configurer reports a 'UPR connectivity failure'. For example, consider a network of only two processors, in which a physical channel connects one task on the root processor to another task on the second processor. This can be done with just one wire. However, if any virtual channel connections are also required between the two processors, or any task on the second processor is linked with the full run-time library and requires GFS, then two wires are needed: one for the physical channel and one for UPR. If a virtual channel were used instead of a physical one then a single wire would be enough. |
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