Re: [dpdk-dev] [dpdk-stable] [PATCH v3] mempool: fix mempool obj alignment for non x86

[David Marchand <david.marchand@redhat.com>]

On Mon, Jan 13, 2020 at 7:49 AM <jerinj@marvell.com> wrote:
>
> From: Jerin Jacob <jerinj@marvell.com>
>
> The existing optimize_object_size() function address the memory object
> alignment constraint on x86 for better performance.
>
> Different (micro) architecture may have different memory alignment
> constraint for better performance and it not the same as the existing
> optimize_object_size().
>
> Some use, XOR(kind of CRC) scheme to enable DRAM channel distribution
> based on the address and some may have a different formula.
>
> Introducing arch_mem_object_align() function to abstract
> the difference between different (micro) architectures to avoid
> wasting memory for mempool object alignment for the architecture
> that it is not required to do so.
>
> Details on the amount of memory saving:
>
> Currently, arm64 based architectures use the default (nchan=4,
> nrank=1). The worst case is for an object whose size (including mempool
> header) is 2 cache lines, where it is optimized to 3 cache lines (+50%).
>
> Examples for cache lines size = 64:
>   orig     optimized
>   64    -> 64           +0%
>   128   -> 192          +50%
>   192   -> 192          +0%
>   256   -> 320          +25%
>   320   -> 320          +0%
>   384   -> 448          +16%
>   ...
>   2304  -> 2368         +2.7%  (~mbuf size)
>
> Additional details:
> https://www.mail-archive.com/dev@dpdk.org/msg149157.html
>
> Fixes: af75078fece3 ("first public release")

Weird to flag this as a problem in this sha1.
x86 was the only architecture supported at the time.
Either we mark the introduction of new architectures as the point of
backport, or we remove this tag and just let Cc: stable@dpdk.org


> Cc: stable@dpdk.org

It seems more an optimisation than a fix to me, but in any case, the
stable maintainers will be the judges.


>
> Signed-off-by: Jerin Jacob <jerinj@marvell.com>
> Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> ---
> v3:
> - Change comment for MEMPOOL_F_NO_SPREAD flag as
> " Spreading among memory channels not required." (Stephen Hemminger)
>
> v2:
> - Changed the return type of arch_mem_object_align() to "unsigned int" from
>   "unsigned" to fix the checkpatch issues (Olivier Matz)
> - Updated the comments for MEMPOOL_F_NO_SPREAD (Olivier Matz)
> - Update the git comments to share the memory saving details.
>
>  doc/guides/prog_guide/mempool_lib.rst |  6 +++---
>  lib/librte_mempool/rte_mempool.c      | 17 +++++++++++++----
>  lib/librte_mempool/rte_mempool.h      |  3 ++-
>  3 files changed, 18 insertions(+), 8 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
> +------------------------------------------------

Nit: afaics in the docs, x86 is preferred to X86.


>
> -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.


-- 
David Marchand