[dpdk-dev] [PATCH 02/13] mbuf: reorder fields by time of use

[Bruce Richardson]

*  Reorder the fields in the mbuf so that we have fields that are used$
together side-by-side in the structure. This means that we have a$
contiguous block of 8-bytes in the mbuf which are used to reset an mbuf$
of descriptor rearm, and a block of 16-bytes of data (excluding flags)
which are set on RX from the received packet descriptor.
* Use dummy fields as appropraite to ensure alignment or to reserve gaps
for later field additions.
* Place most items which are not used by fast-path RX separately at the end
of the structure so they can later be moved to a separate cache line.
[The l2/l3 length fields are not moved at this stage as doing so will
cause overflow to the next cache line].

Signed-off-by: Bruce Richardson <bruce.richardson at intel.com>
---
 lib/librte_mbuf/rte_mbuf.h | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 669e7f5..f136d37 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -115,16 +115,12 @@ extern "C" {
  * The generic rte_mbuf, containing a packet mbuf.
  */
 struct rte_mbuf {
-	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
 	void *buf_addr;           /**< Virtual address of segment buffer. */
 	phys_addr_t buf_physaddr; /**< Physical address of segment buffer. */
-	uint16_t buf_len;         /**< Length of segment buffer. */
 
-	/* valid for any segment */
-	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
+	/* next 8 bytes are initialised on RX descriptor rearm */
+	uint16_t buf_len;         /**< Length of segment buffer. */
 	uint16_t data_off;
-	uint16_t data_len;        /**< Amount of data in segment buffer. */
-	uint32_t pkt_len;         /**< Total pkt len: sum of all segments. */
 
 #ifdef RTE_MBUF_REFCNT
 	/**
@@ -142,14 +138,17 @@ struct rte_mbuf {
 #else
 	uint16_t refcnt_reserved;     /**< Do not use this field */
 #endif
-	uint16_t reserved;            /**< Unused field. Required for padding */
-	uint16_t ol_flags;            /**< Offload features. */
-
-	/* these fields are valid for first segment only */
 	uint8_t nb_segs;        /**< Number of segments. */
 	uint8_t port;           /**< Input port. */
 
-	/* offload features, valid for first segment only */
+	uint16_t ol_flags;      /**< Offload features. */
+	uint16_t reserved0;     /**< Unused field. Required for padding */
+	uint32_t reserved1;     /**< Unused field. Required for padding */
+
+	/* remaining bytes are set on RX when pulling packet from descriptor */
+	uint16_t reserved2;     /**< Unused field. Required for padding */
+	uint16_t data_len;      /**< Amount of data in segment buffer. */
+	uint32_t pkt_len;       /**< Total pkt len: sum of all segments. */
 	uint16_t l3_len:9;      /**< L3 (IP) Header Length. */
 	uint16_t l2_len:7;      /**< L2 (MAC) Header Length. */
 	uint16_t vlan_tci;      /**< VLAN Tag Control Identifier (CPU order). */
@@ -162,6 +161,10 @@ struct rte_mbuf {
 		uint32_t sched;     /**< Hierarchical scheduler */
 	} hash;                 /**< hash information */
 
+	/* fields only used in slow path or on TX */
+	struct rte_mempool *pool; /**< Pool from which mbuf was allocated. */
+	struct rte_mbuf *next;    /**< Next segment of scattered packet. */
+
 	union {
 		uint8_t metadata[0];
 		uint16_t metadata16[0];
-- 
1.9.3