Re: [dpdk-dev] [PATCH 2/5] ixgbe: add prefetch to improve slow-path tx perf

[Neil Horman <nhorman@tuxdriver.com>]

On Wed, Sep 17, 2014 at 11:01:39AM +0100, Bruce Richardson wrote:
> Make a small improvement to slow path TX performance by adding in a
> prefetch for the second mbuf cache line.
> Also move assignment of l2/l3 length values only when needed.
> 
> Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> ---
>  lib/librte_pmd_ixgbe/ixgbe_rxtx.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> index 6f702b3..c0bb49f 100644
> --- a/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> +++ b/lib/librte_pmd_ixgbe/ixgbe_rxtx.c
> @@ -565,25 +565,26 @@ ixgbe_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts,
>  		ixgbe_xmit_cleanup(txq);
>  	}
>  
> +	rte_prefetch0(&txe->mbuf->pool);
> +

Can you explain what all of these prefetches are doing?  It looks to me like
they're just fetching the first caheline of the mempool structure, which it
appears amounts to the pools name.  I don't see that having any use here.

>  	/* TX loop */
>  	for (nb_tx = 0; nb_tx < nb_pkts; nb_tx++) {
>  		new_ctx = 0;
>  		tx_pkt = *tx_pkts++;
>  		pkt_len = tx_pkt->pkt_len;
>  
> -		RTE_MBUF_PREFETCH_TO_FREE(txe->mbuf);
> -
>  		/*
>  		 * Determine how many (if any) context descriptors
>  		 * are needed for offload functionality.
>  		 */
>  		ol_flags = tx_pkt->ol_flags;
> -		vlan_macip_lens.f.vlan_tci = tx_pkt->vlan_tci;
> -		vlan_macip_lens.f.l2_l3_len = tx_pkt->l2_l3_len;
>  
>  		/* If hardware offload required */
>  		tx_ol_req = ol_flags & PKT_TX_OFFLOAD_MASK;
>  		if (tx_ol_req) {
> +			vlan_macip_lens.f.vlan_tci = tx_pkt->vlan_tci;
> +			vlan_macip_lens.f.l2_l3_len = tx_pkt->l2_l3_len;
> +
>  			/* If new context need be built or reuse the exist ctx. */
>  			ctx = what_advctx_update(txq, tx_ol_req,
>  				vlan_macip_lens.data);
> @@ -720,7 +721,7 @@ ixgbe_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts,
>  				    &txr[tx_id];
>  
>  				txn = &sw_ring[txe->next_id];
> -				RTE_MBUF_PREFETCH_TO_FREE(txn->mbuf);
> +				rte_prefetch0(&txn->mbuf->pool);
>  
>  				if (txe->mbuf != NULL) {
>  					rte_pktmbuf_free_seg(txe->mbuf);
> @@ -749,6 +750,7 @@ ixgbe_xmit_pkts(void *tx_queue, struct rte_mbuf **tx_pkts,
>  		do {
>  			txd = &txr[tx_id];
>  			txn = &sw_ring[txe->next_id];
> +			rte_prefetch0(&txn->mbuf->pool);
>  
>  			if (txe->mbuf != NULL)
>  				rte_pktmbuf_free_seg(txe->mbuf);
> -- 
> 1.9.3
> 
>