Re: [PATCH] mempool: micro optimizations

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

On Wed, Feb 26, 2025 at 03:59:22PM +0000, Morten Brørup wrote:
> The comparisons lcore_id < RTE_MAX_LCORE and lcore_id != LCORE_ID_ANY are
> equivalent, but the latter compiles to fewer bytes of code space.
> Similarly for lcore_id >= RTE_MAX_LCORE and lcore_id == LCORE_ID_ANY.
> 
> The rte_mempool_get_ops() function is also used in the fast path, so
> RTE_VERIFY() was replaced by RTE_ASSERT().
> 
> Compilers implicitly consider comparisons of variable == 0 likely, so
> unlikely() was added to the check for no mempool cache (mp->cache_size ==
> 0) in the rte_mempool_default_cache() function.
> 
> The rte_mempool_do_generic_put() function for adding objects to a mempool
> was refactored as follows:
> - The comparison for the request itself being too big, which is considered
>   unlikely, was moved down and out of the code path where the cache has
>   sufficient room for the added objects, which is considered the most
>   likely code path.
> - Added __rte_assume() about the cache length, size and threshold, for
>   compiler optimization when "n" is compile time constant.
> - Added __rte_assume() about "ret" being zero, so other functions using
>   the value returned by this function can be potentially optimized by the
>   compiler; especially when it merges multiple sequential code paths of
>   inlined code depending on the return value being either zero or
>   negative.
> - The refactored source code (with comments) made the separate comment
>   describing the cache flush/add algorithm superfluous, so it was removed.
> 
> A few more likely()/unlikely() were added.
> 
> A few comments were improved for readability.
> 
> Some assertions, RTE_ASSERT(), were added. Most importantly to assert that
> the return values of the mempool drivers' enqueue and dequeue operations
> are API compliant, i.e. 0 (for success) or negative (for failure), and
> never positive.
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---
>  lib/mempool/rte_mempool.h | 67 ++++++++++++++++++++++-----------------
>  1 file changed, 38 insertions(+), 29 deletions(-)
> 
Is there any measurable performance change with these modifications?

/Bruce