Re: [PATCH] doc: remove obsolete mempool creation advice

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Mar 19, 2026 at 09:13:00AM +0000, Morten Brørup wrote:
> The descriptions for the mempool creation functions contained advice for
> choosing the optimum (in terms of memory usage) number of elements and
> cache size.
> The advice was based on implementation details, which was changed long
> ago, making the advice completely irrelevant.
> 

The comment is still correct in most cases, since the default backing
storage remains an rte_ring. If passing a power-of-2 size to mempool create
one will get a backing rte_ring store which is twice as large as requested,
leading to lots of ring slots being wasted. For example, for a pool with
16k elements, the actual ring size allocated will be 32k, leading to
wasting 128k of RAM, and also potentially cache too. The latter will occur
because of the nature of the ring to iterate through all mempool/ring
entries, meaning that even if only 16k of the 32k slots will ever be used,
all 32k slots will be passed through the cpu cache if it works on the
mempool directly and not just from the per-core cache.

On the other hand, I'd be in favour of removing this text if we switched
the default mempool in DPDK to being stack-based. While the stack may not
be lock-free like the ring, with per-lcore caches the number of accesses to
the stack should be small, and it gives much better cache utilization
overall - especially in cases where buffers are allocated on one core and
freed on a different one! Even in cases where we are not transferring
between cores, in a single-core case we still will get better reuse of
"hot" buffers than in an rte_ring-backed case.

/Bruce