cache miss increases when change rx descriptor from 512 to 2048

["Xiaoping Yan (NSB)" <xiaoping.yan@nokia-sbell.com>]

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Hi experts,

I had a traffic throughput test for my dpdk application, with same software and test case, only difference is the number of rx/tx descriptor:
Rx/tx descriptor 512, test result 3.2mpps
Rx/tx descriptor 2048, test result 3mpp
From perf data, rx descriptor 2048 case has more cache miss, and lower instruction per cycle
Perf for 512 rx descriptor
      114289237792      cpu-cycles
      365408402395      instructions              #    3.20  insn per cycle
       74186289932      branches
          36020793      branch-misses             #    0.05% of all branches
        1298741388      bus-cycles
           3413460      cache-misses              #    0.723 % of all cache refs
         472363654      cache-references
Perf for 2048 rx descriptor:
       57038451185      cpu-cycles
      173805485573      instructions              #    3.05  insn per cycle
       35289607389      branches
          15418885      branch-misses             #    0.04% of all branches
         648164239      bus-cycles
          13170596      cache-misses              #    1.702 % of all cache refs
         773765263      cache-references

I understand it means more rx descriptor somehow causes more cache miss and then less instruction per cycle, so lower performance.

Any one observe similar results?
Any idea to mitigate (or investigate further) the impact? (we want to use 2048 to better tolerate some jitter/burst)
Any comment?

Thank you.

Br, Xiaoping


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