Re: [dpdk-dev] [memnic PATCH v2 6/7] pmd: add branch hint in recv/xmit

[Neil Horman <nhorman@tuxdriver.com>]

On Tue, Sep 30, 2014 at 11:14:40AM +0000, Hiroshi Shimamoto wrote:
> From: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> 
> To reduce instruction cache miss, add branch condition hints into
> recv/xmit functions. This improves a bit performance.
> 
> We can see performance improvements with memnic-tester.
> Using Xeon E5-2697 v2 @ 2.70GHz, 4 vCPU.
>  size |  before  |  after
>    64 | 5.54Mpps | 5.55Mpps
>   128 | 5.46Mpps | 5.44Mpps
>   256 | 5.21Mpps | 5.22Mpps
>   512 | 4.50Mpps | 4.52Mpps
>  1024 | 3.71Mpps | 3.73Mpps
>  1280 | 3.21Mpps | 3.22Mpps
>  1518 | 2.92Mpps | 2.93Mpps
> 
> Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
> Reviewed-by: Hayato Momma <h-momma@ce.jp.nec.com>
> ---
>  pmd/pmd_memnic.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/pmd/pmd_memnic.c b/pmd/pmd_memnic.c
> index 7fc3093..875d3ea 100644
> --- a/pmd/pmd_memnic.c
> +++ b/pmd/pmd_memnic.c
> @@ -289,26 +289,26 @@ static uint16_t memnic_recv_pkts(void *rx_queue,
>  	int idx, next;
>  	struct rte_eth_stats *st = &adapter->stats[rte_lcore_id()];
>  
> -	if (!adapter->nic->hdr.valid)
> +	if (unlikely(!adapter->nic->hdr.valid))
>  		return 0;
>  
>  	pkts = bytes = errs = 0;
>  	idx = adapter->up_idx;
>  	for (nr = 0; nr < nb_pkts; nr++) {
>  		p = &data->packets[idx];
> -		if (p->status != MEMNIC_PKT_ST_FILLED)
> +		if (unlikely(p->status != MEMNIC_PKT_ST_FILLED))
>  			break;
>  		/* prefetch the next area */
>  		next = idx;
> -		if (++next >= MEMNIC_NR_PACKET)
> +		if (unlikely(++next >= MEMNIC_NR_PACKET))
>  			next = 0;
>  		rte_prefetch0(&data->packets[next]);
> -		if (p->len > framesz) {
> +		if (unlikely(p->len > framesz)) {
>  			errs++;
>  			goto drop;
>  		}
>  		mb = rte_pktmbuf_alloc(adapter->mp);
> -		if (!mb)
> +		if (unlikely(!mb))
>  			break;
>  
>  		rte_memcpy(rte_pktmbuf_mtod(mb, void *), p->data, p->len);
> @@ -350,7 +350,7 @@ static uint16_t memnic_xmit_pkts(void *tx_queue,
>  	uint64_t pkts, bytes, errs;
>  	uint32_t framesz = adapter->framesz;
>  
> -	if (!adapter->nic->hdr.valid)
> +	if (unlikely(!adapter->nic->hdr.valid))
>  		return 0;
>  
>  	pkts = bytes = errs = 0;
> @@ -360,7 +360,7 @@ static uint16_t memnic_xmit_pkts(void *tx_queue,
>  		struct rte_mbuf *sg;
>  		void *ptr;
>  
> -		if (pkt_len > framesz) {
> +		if (unlikely(pkt_len > framesz)) {
>  			errs++;
>  			break;
>  		}
> @@ -379,7 +379,7 @@ retry:
>  			goto retry;
>  		}
>  
> -		if (idx != ACCESS_ONCE(adapter->down_idx)) {
> +		if (unlikely(idx != ACCESS_ONCE(adapter->down_idx))) {
Why are you using ACCESS_ONCE here?  Or for that matter, anywhere else in this
PMD?  The whole idea of the ACCESS_ONCE macro is to assign a value to a variable
once and prevent it from getting reloaded from memory at a later time, this is
exactly contrary to that, both in the sense that you're explicitly reloading the
same variable multiple times, and that you're using it as part of a comparison
operation, rather than an asignment operation

Neil