RCU fanout leaf balancing

[Daniel J Blueman]

Hi Paul,

Finding earlier reference to increasing RCU fanout leaf for the purpose 
of "decrease[ing] cache-miss overhead for large systems", would your 
suggestion be to increase the value to the next hierarchy core-count 
above 16?

If we have say 32 interconnected 48-core servers; 3 sockets of dual-node 
8-core Opteron 6300s, so 1536 cores in all. Latency across the coherent 
interconnect is O(100x) higher than the internal Hypertransport 
interconnect, so if we set RCU_FANOUT_LEAF to 48 to keep leaf-checking 
local to one Hypertransport fabric, what wisdom would one use for 
RCU_FANOUT? 4x leaf?

Or, would it be more cache-friendly to set RCU_FANOUT_LEAF to 8 and 
RCU_FANOUT to 48?

Many thanks,
  Daniel
--
Daniel J Blueman
Principal Software Engineer, Numascale
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