[dpdk-dev] [PATCH] mbuf: replace c memcpy() code semantics with optimized rte_memcpy()

[Sarosh Arif]

Hello,
The following things made me think that rte_memcpy() is more optimized
than memcpy():
1. dpdk documentation recommends to use rte_memcpy() instead of memcpy():
    https://doc.dpdk.org/guides/prog_guide/writing_efficient_code.html
2. Here some benchmarks are available:
    https://software.intel.com/content/www/us/en/develop/articles/performance-optimization-of-memcpy-in-dpdk.html
3. rte_memcpy() has __attribute__((always_inline)) associated with it,
so compiler also tries to inline it.

Using rte_memcpy() everywhere ensures consistency in code-base.
Here are the results of the performance number measurement using "perf":

rte_memcpy()

 Performance counter stats
          1.573864      task-clock (msec)         #    0.898 CPUs
utilized
                 0      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
               342      page-faults               #    0.217 M/sec
         5,483,016      cycles                    #    3.484 GHz
         5,554,017      instructions              #    1.01  insn per
cycle
         1,114,593      branches                  #  708.189 M/sec
            33,796      branch-misses             #    3.03% of all
branches
         1,369,247      L1-dcache-loads           #  869.991 M/sec
     <not counted>      L1-dcache-load-misses
               (0.00%)
     <not counted>      LLC-loads
               (0.00%)
     <not counted>      LLC-load-misses
               (0.00%)

       0.001753373 seconds time elapsed



memcpy()

 Performance counter stats
          1.631135      task-clock (msec)         #    0.902 CPUs
utilized
                 0      context-switches          #    0.000 K/sec
                 0      cpu-migrations            #    0.000 K/sec
               342      page-faults               #    0.210 M/sec
         5,676,549      cycles                    #    3.480 GHz
               (73.99%)
         5,739,593      instructions              #    1.01  insn per
cycle
         1,141,121      branches                  #  699.587 M/sec
            34,553      branch-misses             #    3.03% of all
branches
         1,417,494      L1-dcache-loads           #  869.023 M/sec
            67,312      L1-dcache-load-misses     #    4.75% of all
L1-dcache hits    (26.01%)
     <not counted>      LLC-loads
               (0.00%)
     <not counted>      LLC-load-misses
               (0.00%)

      0.001808500 seconds time elapsed



On Thu, Jul 23, 2020 at 8:47 PM Stephen Hemminger
<stephen at networkplumber.org> wrote:
>
> On Thu, 23 Jul 2020 12:02:40 +0500
> Sarosh Arif <sarosh.arif at emumba.com> wrote:
>
> > Since rte_memcpy is more optimized it should be used instead of memcpy
> >
> > Signed-off-by: Sarosh Arif <sarosh.arif at emumba.com>
>
> Really did you measure this.
> For fixed size structures, compiler can inline memcpy small set of instructions.