RE: [PATCH] eal/x86: optimize memcpy of small 64-byte blocks

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> > > > > > > > > > > +	/* Common way for small copy size of 64-byte
> > blocks.
> > > > > > > > Unlikely, so
> > > > > > > > > > constant size only */
> > > > > > > > > > > +	if (__rte_constant(n) && (n & 63) == 0 && n <=
> > > > > > > > > > RTE_MEMCPY_BLOCK_64_MAX) {
> > > > > > > > > > > +		void *ret = dst;
> > > > > > > > > > > +
> > > > > > > > > >
> > > > > > > > > > Maybe just let compiler decide, it will generate vector
> > > > > > > > instructions in
> > > > > > > > > > most cases.
> > > > > > > > > >
> > > > > > > > > > 	if (__rte_constant(n))
> > > > > > > > > > 		return mempcpy(dst, src, n);
> > > > > > > > >
> > > > > > > > > Maybe in most, but not in all:
> > > > > > > > > https://godbolt.org/z/KvdKqT5rY
> > > > > > > >
> > > > > > > > With '-mavx' or '-mavx512f' it looks like it does for your
> > > > sample
> > > > > > code.
> > > > > > >
> > > > > > > It also does with -msse4.2 when SZ is reduced to 256 bytes.
> > > > > > > Clang switches to inline when SZ is reduced to 128 bytes.
> > > > > > >
> > > > > > > It seems the compiler has a threshold for when to inline and
> > when
> > > > to
> > > > > > call the C
> > > > > > > library's memcpy subroutine.
> > > > > > > The threshold depends on both copy size and vector register
> > size.
> > > > > > > And it is compiler dependent.
> > > > > >
> > > > > > I think there are compiler options to specify desired threshold
> > > > values.
> > > > > > Let say for gcc there is  ' -mmemcpy-strategy=strategy'.
> > > > > > For that example in that particular case
> > > > > > -mmemcpy-strategy=vector_loop:512:align,loop:-1:align
> > > > > > generates sse loads/stores.
> > > > > > Might be we can exploit it somehow?
> > > > >
> > > > > That could give us higher granularity/control over memcpy for
> > > > individual
> > > > > memcpy instances; might be useful for hot code paths where we
> > have
> > > > more
> > > > > knowledge about the copy operation than the compiler can infer.
> > > > > However, pragmas are discouraged in DPDK, and this looks like a
> > very
> > > > similar
> > > > > path.
> > > >
> > > > Well, right now  rte_memcpy.h is 700+ lines and keeps growing.
> > > > Considering that probably pragmas are not that bad.
> > > > Of course, pragmas have their own issues and it is hard to ensure
> > that
> > > > they will produce same code between different compilers/versions,
> > etc.
> > > >
> > > > > > I am not really happy that our home-brewed memcpy code-block
> > keeps
> > > > > > growing,
> > > > > > while we keep talking that it would be good to eliminate it
> > > > completely.
> > > > >
> > > > > I agree in principle.
> > > > > However, this rte_memcpy() optimization is for the pile/mempool
> > > > optimizations
> > > > > I'm working on, so there is a specific use case motivating the
> > added
> > > > code.
> > > >
> > > > I understand that you probably have some specific use-case in mind.
> > > > BTW for this optimization you mentioned above: what is the gain
> > with
> > > > these changes?
> > >
> > > IMO, the primary benefit is the much simpler (and smaller) assembly
> > output due
> > > to avoiding the address alignment check (and the resulting duplicated
> > code).
> >
> > I think that should be measurable too: whole binary  and/or hot path
> > function size reduction, etc.
> 
> The size of the generated code for the copy operation is reduced to slightly less
> than half (only one instance of the copy operation instead of two, and the
> address alignment comparison is omitted).
> 
> >
> > > I haven't measured the performance gain.
> > > Based on the perf gain in a previous mempool optimization patch [1],
> > it seems
> > > avoiding the address alignment check shaves ~2 cycles off the copy
> > operation (for
> > > cache-to-cache copy).
> > > I expect that the same gain (from avoiding the address alignment
> > check) applies
> > > here.
> >
> > Ok, then I suggest we do some measurements first, before going forward
> > with it.
> 
> I tried memcpy_perf_autotest, but the branch predictor kicks in and eliminates
> the cost of the address alignment comparison. So the results are very similar.

Indeed, they are the same.

> BTW, that's a general issue with our perf tests: Repeated testing doesn't show
> the cost of branches, because they are always eliminated by the branch
> predictor.

Might be...
But we do need some measurable evidence that such change improve things,
otherwise - why to bother?
If that perf test is not goof enough - let's try to extend it or use different one.
BTW, in your mempool pile RFC, I see you also use rte_memcpy over fixed sized buffers.
Do you see any improvement there (wit/without rte_memcpy optimixation)?  

> ======= ================= ================= =================
> =================
>    Size   Cache to cache     Cache to mem      Mem to cache        Mem to mem
> (bytes)          (ticks)          (ticks)           (ticks)           (ticks)
> ------- ----------------- ----------------- ----------------- -----------------
> ================================= 32B aligned
> =================================
> Before
> C    64  2 -  2(  9.60%)  22 - 21(  4.48%)  33 - 31(  5.15%)  57 - 56(  0.28%)
> C   128  4 -  4(  5.56%)  41 - 40(  3.59%)  53 - 52(  1.49%) 101 -100(  0.71%)
> C   192  6 -  6(  4.06%)  64 - 60(  5.61%)  68 - 69( -1.68%) 136 -138( -1.49%)
> C   256 10 -  9(  5.21%)  82 - 81(  1.20%)  93 - 87(  7.47%) 172 -175( -1.46%)
> After
> C    64  2 -  2( -0.71%)  22 - 21(  7.62%)  31 - 30(  4.32%)  57 - 59( -3.03%)
> C   128  4 -  4( -0.34%)  40 - 40(  0.60%)  50 - 50(  0.41%)  97 - 99( -1.96%)
> C   192  6 -  6(  5.27%)  59 - 59( -0.27%)  69 - 72( -4.42%) 139 -138(  0.90%)
> C   256  9 -  9(  9.14%)  77 - 77(  0.25%)  89 - 90( -0.88%) 180 -177(  1.57%)
> ================================== Unaligned
> ==================================
> Before
> C    64  5 -  5(  0.98%)  32 - 32(  0.72%)  42 - 42(  0.44%)  82 - 82(  0.51%)
> C   128  8 -  9(-11.26%)  51 - 50(  0.53%)  60 - 59(  0.89%) 117 -115(  1.37%)
> C   192 13 - 12(  1.62%)  66 - 67( -1.12%)  83 - 80(  3.26%) 157 -165( -5.25%)
> C   256 21 - 22( -5.78%)  94 - 93(  1.65%) 100 - 99(  1.52%) 202 -204( -1.34%)
> After
> C    64  5 -  5(  0.05%)  31 - 31(  0.16%)  40 - 39(  1.37%)  77 - 77( -0.93%)
> C   128  9 -  9( -0.69%)  50 - 50( -0.02%)  61 - 60(  2.71%) 118 -122( -3.15%)
> C   192 12 - 11(  7.68%)  68 - 66(  3.64%)  79 - 81( -2.85%) 164 -164( -0.12%)
> C   256 21 - 20(  1.80%)  90 - 87(  2.70%)  96 - 97( -0.99%) 202 -201(  0.09%)
> 
> >
> > > [1]:
> > https://patchwork.dpdk.org/project/dpdk/patch/20260521185631.116046-1-
> > > mb@smartsharesystems.com/