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Hi all,
I have a design question regarding rte_ring that I didn= =A1=A6t find a historical rationale for in the archives.
Most modern high-perf ring buffers (e.g. some NIC drivers, some DB queue im= plementations, etc.) eliminate wrap-around branches by taking the ring=A1= =A6s element array and mapping two consecutive VA ranges to the same physical backing pages.
i.e. you allocate N elements, commit enough pages to cover N, then call mmap (or equivalent) again immediately following it, pointin= g to the same physical pages. So from the CPU=A1=A6s POV the element array = is logically [0 .. N*2) but physically it=A1=A6s the same backing. The= refore a batch read/write can always be done as a single contiguous memcpy/= CLD/STOS without conditionals, even if (head+bulk) exceeds N.
Pseudo illustration:
[phys buffer of N =
slots]=0AVA: [0 .. N) -> phys=0AVA: [N .. 2N) -> same phys=0A<=
/div>
For multi-element enqueue/dequeue it eliminates the =A1=A7if wrap =A1=F7 sp= lit=A1=A8 case entirely =A1X you can always memcpy in one contiguous op.
Question:
Is there an explicit reason DPDK doesn=A1=A6t use this technique for = rte_ring?
e.g.
  • portability? (hugepages / VFIO?)
  • i= nability to rely on mmap trickery for hugepage backed memzones?
  • N= UMA locality considerations?
  • h= istorical reason: first gen ring didn=A1=A6t bulk enqueue so the branch did= n=A1=A6t matter?
  • r= eluctance to add VA aliasing because of security / introspection tooling / = ASan issues?
I just want to understand the architectural trade that was made.
Because on 64-bit Linux, double-mapping a 1=A1V2 MB region is pretty trivia= l, and bulk ops in DPDK are extremely common =A1X it feels like an obvious = microarchitectural win for the =A1=A7hot ring=A1=A8 case.
So: is there a concrete blocker? or simply =A1=A7no one pushed a patch beca= use current perf was =A1=A5good enough=A1=A6=A1=A8?
Pointers to prior mailing list discussion / patches would be appreciated.
Thanks,
Rami Neiman

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