On 4/19/26 12:55 PM, Morten Brørup wrote:
This patch refactors the mempool cache to eliminate some unexpected behaviour and reduce the mempool cache miss rate.1. The actual cache size was 1.5 times the cache size specified at run-time mempool creation. This was obviously not expected by application developers. 2. In get operations, the check for when to use the cache as bounce buffer did not respect the run-time configured cache size, but compared to the build time maximum possible cache size (RTE_MEMPOOL_CACHE_MAX_SIZE, default 512). E.g. with a configured cache size of 32 objects, getting 256 objects would first fetch 32 + 256 = 288 objects into the cache, and then move the 256 objects from the cache to the destination memory, instead of fetching the 256 objects directly to the destination memory. This had a performance cost. However, this is unlikely to occur in real applications, so it is not important in itself. 3. When putting objects into a mempool, and the mempool cache did not have free space for so many objects, the cache was flushed completely, and the new objects were then put into the cache. I.e. the cache drain level was zero. This (complete cache flush) meant that a subsequent get operation (with the same number of objects) completely emptied the cache, so another subsequent get operation required replenishing the cache. Similarly, When getting objects from a mempool, and the mempool cache did not hold so many objects, the cache was replenished to cache->size + remaining objects, and then (the remaining part of) the requested objects were fetched via the cache, which left the cache filled (to cache->size) at completion. I.e. the cache refill level was cache->size (plus some, depending on request size). (1) was improved by generally comparing to cache->size instead of cache->flushthresh, when considering the capacity of the cache. The cache->flushthresh field is kept for API/ABI compatibility purposes, and initialized to cache->size instead of cache->size * 1.5. (2) was improved by generally comparing to cache->size / 2 instead of RTE_MEMPOOL_CACHE_MAX_SIZE, when checking the bounce buffer limit. (3) was improved by flushing and replenishing the cache by half its size, so a flush/refill can be followed randomly by get or put requests. This also reduced the number of objects in each flush/refill operation. As a consequence of these changes, the size of the array holding the objects in the cache (cache->objs[]) no longer needs to be 2 * RTE_MEMPOOL_CACHE_MAX_SIZE, and can be reduced to RTE_MEMPOOL_CACHE_MAX_SIZE at an API/ABI breaking release. Performance data: With a real WAN Optimization application, where the number of allocated packets varies (as they are held in e.g. shaper queues), the mempool cache miss rate dropped from ca. 1/20 objects to ca. 1/48 objects. This was deployed in production at an ISP, and using an effective cache size of 384 objects. As a consequence of the improved mempool cache algorithm, some drivers were updated accordingly: - The Intel idpf PMD was updated regarding how much to backfill the mempool cache in the AVX512 code. - The NXP dpaa and dpaa2 mempool drivers were updated to not set the mempool cache flush threshold; doing this no longer has any effect, and thus became superfluous. Bugzilla ID: 1027 Fixes: ea5dd2744b90 ("mempool: cache optimisations") Signed-off-by: Morten Brørup <[email protected]> --- Depends-on: patch-163181 ("net/intel: do not bypass mbuf lib for mbuf fast-free")
Acked-by: Andrew Rybchenko <[email protected]>
