Some PMDs, when using RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE, don't call rte_mempool_put_bulk() to free the mbufs, but instead manipulate the mempool and cache structures directly. This means that mempool instrumentation is bypassed, and such mbuf bulk frees are neither traced nor counted in the mempool statistics.
*** Bug 923 has been marked as a duplicate of this bug. ***