[dpdk-dev] [PATCH v5 15/15] mbuf: move pool pointer in hotter first half

[Thomas Monjalon <thomas@monjalon.net>]

The mempool pointer in the mbuf struct is moved
from the second to the first half.
It should increase performance on most systems having 64-byte cache line,
i.e. mbuf is split in two cache lines.
On such system, the first half (also called first cache line) is hotter
than the second one where the pool pointer was.

Moving this field gives more space to dynfield1.

This is how the mbuf layout looks like (pahole-style):

word  type                              name                byte  size
 0    void *                            buf_addr;         /*   0 +  8 */
 1    rte_iova_t                        buf_iova          /*   8 +  8 */
      /* --- RTE_MARKER64               rearm_data;                   */
 2    uint16_t                          data_off;         /*  16 +  2 */
      uint16_t                          refcnt;           /*  18 +  2 */
      uint16_t                          nb_segs;          /*  20 +  2 */
      uint16_t                          port;             /*  22 +  2 */
 3    uint64_t                          ol_flags;         /*  24 +  8 */
      /* --- RTE_MARKER                 rx_descriptor_fields1;        */
 4    uint32_t             union        packet_type;      /*  32 +  4 */
      uint32_t                          pkt_len;          /*  36 +  4 */
 5    uint16_t                          data_len;         /*  40 +  2 */
      uint16_t                          vlan_tci;         /*  42 +  2 */
 5.5  uint64_t             union        hash;             /*  44 +  8 */
 6.5  uint16_t                          vlan_tci_outer;   /*  52 +  2 */
      uint16_t                          buf_len;          /*  54 +  2 */
 7    struct rte_mempool *              pool;             /*  56 +  8 */
      /* --- RTE_MARKER                 cacheline1;                   */
 8    struct rte_mbuf *                 next;             /*  64 +  8 */
 9    uint64_t             union        tx_offload;       /*  72 +  8 */
10    uint16_t                          priv_size;        /*  80 +  2 */
      uint16_t                          timesync;         /*  82 +  2 */
      uint32_t                          seqn;             /*  84 +  4 */
11    struct rte_mbuf_ext_shared_info * shinfo;           /*  88 +  8 */
12    uint64_t                          dynfield1[4];     /*  96 + 32 */
16    /* --- END                                             128      */

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
---
 doc/guides/rel_notes/deprecation.rst | 5 -----
 lib/librte_kni/rte_kni_common.h      | 3 ++-
 lib/librte_mbuf/rte_mbuf_core.h      | 5 ++---
 3 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
index 72dbb25b83..07ca1dcbb2 100644
--- a/doc/guides/rel_notes/deprecation.rst
+++ b/doc/guides/rel_notes/deprecation.rst
@@ -88,11 +88,6 @@ Deprecation Notices
 
   - ``seqn``
 
-  As a consequence, the layout of the ``struct rte_mbuf`` will be re-arranged,
-  avoiding impact on vectorized implementation of the driver datapaths,
-  while evaluating performance gains of a better use of the first cache line.
-
-
 * ethdev: the legacy filter API, including
   ``rte_eth_dev_filter_supported()``, ``rte_eth_dev_filter_ctrl()`` as well
   as filter types MACVLAN, ETHERTYPE, FLEXIBLE, SYN, NTUPLE, TUNNEL, FDIR,
diff --git a/lib/librte_kni/rte_kni_common.h b/lib/librte_kni/rte_kni_common.h
index 36d66e2ffa..ffb3182731 100644
--- a/lib/librte_kni/rte_kni_common.h
+++ b/lib/librte_kni/rte_kni_common.h
@@ -84,10 +84,11 @@ struct rte_kni_mbuf {
 	char pad2[4];
 	uint32_t pkt_len;       /**< Total pkt len: sum of all segment data_len. */
 	uint16_t data_len;      /**< Amount of data in segment buffer. */
+	char pad3[14];
+	void *pool;
 
 	/* fields on second cache line */
 	__attribute__((__aligned__(RTE_CACHE_LINE_MIN_SIZE)))
-	void *pool;
 	void *next;             /**< Physical address of next mbuf in kernel. */
 };
 
diff --git a/lib/librte_mbuf/rte_mbuf_core.h b/lib/librte_mbuf/rte_mbuf_core.h
index 52ca1c842f..ee185fa32b 100644
--- a/lib/librte_mbuf/rte_mbuf_core.h
+++ b/lib/librte_mbuf/rte_mbuf_core.h
@@ -584,12 +584,11 @@ struct rte_mbuf {
 
 	uint16_t buf_len;         /**< Length of segment buffer. */
 
-	uint64_t unused;
+	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
 
 	/* second cache line - fields only used in slow path or on TX */
 	RTE_MARKER cacheline1 __rte_cache_min_aligned;
 
-	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
 	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
 
 	/* fields to support TX offloads */
@@ -646,7 +645,7 @@ struct rte_mbuf {
 	 */
 	struct rte_mbuf_ext_shared_info *shinfo;
 
-	uint64_t dynfield1[3]; /**< Reserved for dynamic fields. */
+	uint64_t dynfield1[4]; /**< Reserved for dynamic fields. */
 } __rte_cache_aligned;
 
 /**
-- 
2.28.0