Re: [PATCH] mempool: micro optimizations

[Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>]

On 3/27/25 20:15, Bruce Richardson wrote:
> 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.
> 
> In general not a big fan of using likely/unlikely, but if they give a perf
> benefit, we should probably take them.
> 
> Few more comments inline below.
> 
>> 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>
> 
> Acked-by: Bruce Richardson <bruce.richardson@intel.com>


Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>