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Mon, 13 Nov 2023 18:14:53 +0000 From: Aditya Ambadipudi To: "dev@dpdk.org" Subject: Question about loop unrolling in rte_ring datastructure. Thread-Topic: Question about loop unrolling in rte_ring datastructure. 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charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hello all. My name is Aditya Ambadipudi. I am not the sharpest tool in the shed. I was reading through the rte_ring datastructure. And I have two questions = about the optimizations that are being made there. 1. Loop unrolling: https://github.com/DPDK/dpdk/blob/main/lib/ring/rte_ring_elem_pvt.h#L28-L35 Why are we unrolling these loops manually. GCC will generate SIMD instructi= ons for these loops automatically. Irrespective of wheither or not we unrol= l the loops Unrolled loop: https://godbolt.org/z/n97noqYn7 Regular loop:https://godbolt.org/z/h6G9o9773 This is true of both x86 and ARM. 2. Normalizing to few fixed types: It looks like we separate out enqueue/dequeue operations into 3 functions. = One for each element size 32, 64, 128. Again I am not clear on why we are doing this. Both 128 & 64 are multiples = of 32. Why can't we just normalize everything to 32? I feel like this is in some shape or form related to loop unrolling. But I = am not able to figure it out on my own. I am working on a patch that is closely related to this. And I would greatl= y appreciate any assistance anyone can provide on this. Thank you, Aditya Ambadipudi IMPORTANT NOTICE: The contents of this email and any attachments are confid= ential and may also be privileged. If you are not the intended recipient, p= lease notify the sender immediately and do not disclose the contents to any= other person, use it for any purpose, or store or copy the information in = any medium. Thank you. --_000_PAVPR08MB9185BCDC0DDF8CB6FB4AAD88EFB3APAVPR08MB9185eurp_ Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hello all.

My name is Aditya Ambadipudi. I am not the sharpest tool in the shed.

I was reading through the rte_ring datastructure. And I have two questions = about the optimizations that are being made there.

  1. = Loop unrolling:
    https://github.c= om/DPDK/dpdk/blob/main/lib/ring/rte_ring_elem_pvt.h#L28-L35 Why= are we unrolling these loops manually. GCC will generate SIMD instructions= for these loops automatically. Irrespective of wheither or not we unroll the loops
    Unrolled loop: htt= ps://godbolt.org/z/n97noqYn7
    Regular loop:https://godbolt.org/z/h6G9o9773<= /span>

    This is true of both x86 and ARM.
  2. = Normalizing to few fixed types:

    It looks like we separate out enqueue/dequeue operations into 3 functions. = One for each element size 32, 64, 128.

    Again I am not clear on why we are doing this. Both 128 & 64 are multip= les of 32. Why can't we just normalize everything to 32?

    I feel like this is in some shape or form related to loop unrolling. But I = am not able to figure it out on my own.

I am working on a patch that is closely related = to this. And I would greatly appreciate any assistance anyone can provide on this.

Thank you,
Aditya Ambadipudi
IMPORTANT NOTICE: The contents of this email and any attachments are confid= ential and may also be privileged. If you are not the intended recipient, p= lease notify the sender immediately and do not disclose the contents to any= other person, use it for any purpose, or store or copy the information in any medium. Thank you. --_000_PAVPR08MB9185BCDC0DDF8CB6FB4AAD88EFB3APAVPR08MB9185eurp_--