Re: [dpdk-dev] [PATCH] mempool: fix mempool obj alignment for non x86

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> From: Jerin Jacob <jerinj@marvell.com>
> 
> The exiting optimize_object_size() function address the memory object
> alignment constraint on x86 for better performance.
> 
> Different (Mirco) architecture may have different memory alignment
> constraint for better performance and it not same as the existing
> optimize_object_size() function. Some use, XOR(kind of CRC) scheme to
> enable DRAM channel distribution based on the address and some may have
> a different formula.
If I understand correctly, address interleaving is the characteristic of the memory controller and not the CPU.
For ex: different SoCs using the same Arm architecture might have different memory controllers. So, the solution should not be architecture specific, but SoC specific.

> 
> Introducing arch_mem_object_align() function to abstract the differences in
> different (mirco) architectures and avoid wasting memory for mempool
> object alignment for the architecture the existing optimize_object_size() is
> not valid.
> 
> Additional details:
> https://www.mail-archive.com/dev@dpdk.org/msg149157.html
> 
> Fixes: af75078fece3 ("first public release")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Jerin Jacob <jerinj@marvell.com>
> ---
>  doc/guides/prog_guide/mempool_lib.rst |  6 +++---
>  lib/librte_mempool/rte_mempool.c      | 17 +++++++++++++----
>  2 files changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/doc/guides/prog_guide/mempool_lib.rst
> b/doc/guides/prog_guide/mempool_lib.rst
> index 3bb84b0a6..eea7a2906 100644
> --- a/doc/guides/prog_guide/mempool_lib.rst
> +++ b/doc/guides/prog_guide/mempool_lib.rst
> @@ -27,10 +27,10 @@ In debug mode
> (CONFIG_RTE_LIBRTE_MEMPOOL_DEBUG is enabled),  statistics about get
> from/put in the pool are stored in the mempool structure.
>  Statistics are per-lcore to avoid concurrent access to statistics counters.
> 
> -Memory Alignment Constraints
> -----------------------------
> +Memory Alignment Constraints on X86 architecture
> +------------------------------------------------
> 
> -Depending on hardware memory configuration, performance can be greatly
> improved by adding a specific padding between objects.
> +Depending on hardware memory configuration on X86 architecture,
> performance can be greatly improved by adding a specific padding between
> objects.
>  The objective is to ensure that the beginning of each object starts on a
> different channel and rank in memory so that all channels are equally loaded.
> 
>  This is particularly true for packet buffers when doing L3 forwarding or flow
> classification.
> diff --git a/lib/librte_mempool/rte_mempool.c
> b/lib/librte_mempool/rte_mempool.c
> index 78d8eb941..871894525 100644
> --- a/lib/librte_mempool/rte_mempool.c
> +++ b/lib/librte_mempool/rte_mempool.c
> @@ -45,6 +45,7 @@ EAL_REGISTER_TAILQ(rte_mempool_tailq)
>  #define CALC_CACHE_FLUSHTHRESH(c)	\
>  	((typeof(c))((c) * CACHE_FLUSHTHRESH_MULTIPLIER))
> 
> +#if defined(RTE_ARCH_X86)
>  /*
>   * return the greatest common divisor between a and b (fast algorithm)
>   *
> @@ -74,12 +75,13 @@ static unsigned get_gcd(unsigned a, unsigned b)  }
> 
>  /*
> - * Depending on memory configuration, objects addresses are spread
> + * Depending on memory configuration on x86 arch, objects addresses are
> + spread
>   * between channels and ranks in RAM: the pool allocator will add
>   * padding between objects. This function return the new size of the
>   * object.
>   */
> -static unsigned optimize_object_size(unsigned obj_size)
> +static unsigned
> +arch_mem_object_align(unsigned obj_size)
>  {
>  	unsigned nrank, nchan;
>  	unsigned new_obj_size;
> @@ -99,6 +101,13 @@ static unsigned optimize_object_size(unsigned
> obj_size)
>  		new_obj_size++;
>  	return new_obj_size * RTE_MEMPOOL_ALIGN;  }
> +#else
This applies to add Arm (PPC as well) SoCs which might have different schemes depending on the memory controller. IMO, this should not be architecture specific.

> +static unsigned
> +arch_mem_object_align(unsigned obj_size) {
> +	return obj_size;
> +}
> +#endif
> 
>  struct pagesz_walk_arg {
>  	int socket_id;
> @@ -234,8 +243,8 @@ rte_mempool_calc_obj_size(uint32_t elt_size,
> uint32_t flags,
>  	 */
>  	if ((flags & MEMPOOL_F_NO_SPREAD) == 0) {
>  		unsigned new_size;
> -		new_size = optimize_object_size(sz->header_size + sz-
> >elt_size +
> -			sz->trailer_size);
> +		new_size = arch_mem_object_align
> +			    (sz->header_size + sz->elt_size + sz->trailer_size);
>  		sz->trailer_size = new_size - sz->header_size - sz->elt_size;
>  	}
> 
> --
> 2.24.1