NonStop NS14000 Planning Guide (H06.10+)

In a TMR or triplex processor, each LSU has inputs from three PEs. As with the duplex processor,
the LSU keeps the three PEs in loose lockstep. The triplex processor provides fault tolerance
upon failure of a PE by comparing the output from the three NonStop Blade Elements to determine
which one is errant. This method eliminates the question of which output is valid when two
outputs disagree in a duplex system.
Processor Synchronization and Rendezvous
Synchronization and rendezvous at the LSUs perform two main functions:
• Keep the individual PEs in a logical processor in loose lock-step through a technique called
rendezvous. Rendezvous occurs to:
— Periodically synchronize the PEs so they execute the same instruction at the same time.
Synchronization accommodates the slightly different clock speed within each PE.
— Allow each PE to individually and deterministically respond to asynchronous incoming
interrupts and then to respond collectively as a single logical processor.
— Exchange software state information when performing operations that are distributed
across PEs; for example, memory reintegration, error handling, and memory scrubbing.
• Compare output from each PE. If identical, the output is transmitted over the ServerNet
fabrics. If the PE outputs are not the same, appropriate actions occur to identify the errant
one and to recover from the failure. Under some failure conditions, it can be necessary to
stop normal operations of the erring PE.
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