Re: [PATCH v2 5/7] net/mlx4: remove unnecessary variables in Tx burst

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

Hi Ophir/Matan,

On Mon, Oct 23, 2017 at 02:21:58PM +0000, Ophir Munk wrote:
> From: Matan Azrad <matan@mellanox.com>
> 
> Remove usage of variables which doesn't add new information for
> performance improvement.
> 
> Signed-off-by: Matan Azrad <matan@mellanox.com>

I'm almost 100% sure this commit wasn't validated for performance on its
own. Don't mention "performance improvement" in that case.

If you're removing a couple of local variables for readability, just say
so.

More below.

> ---
>  drivers/net/mlx4/mlx4_rxtx.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/mlx4/mlx4_rxtx.c b/drivers/net/mlx4/mlx4_rxtx.c
> index 014a6d3..e8d9a35 100644
> --- a/drivers/net/mlx4/mlx4_rxtx.c
> +++ b/drivers/net/mlx4/mlx4_rxtx.c
> @@ -285,8 +285,6 @@ mlx4_tx_burst(void *dpdk_txq, struct rte_mbuf **pkts, uint16_t pkts_n)
>  	struct txq *txq = (struct txq *)dpdk_txq;
>  	unsigned int elts_head = txq->elts_head;
>  	const unsigned int elts_n = txq->elts_n;
> -	unsigned int elts_comp = 0;
> -	unsigned int bytes_sent = 0;
>  	unsigned int i;
>  	unsigned int max;
>  	struct mlx4_sq *sq = &txq->msq;
> @@ -498,8 +496,7 @@ mlx4_tx_burst(void *dpdk_txq, struct rte_mbuf **pkts, uint16_t pkts_n)
>  						       MLX4_BIT_WQE_OWN : 0));
>  		sq->head += nr_txbbs;
>  		elt->buf = buf;
> -		bytes_sent += buf->pkt_len;
> -		++elts_comp;
> +		txq->stats.obytes += buf->pkt_len;
>  		elts_head = elts_head_next;
>  	}
>  	/* Take a shortcut if nothing must be sent. */
> @@ -507,13 +504,12 @@ mlx4_tx_burst(void *dpdk_txq, struct rte_mbuf **pkts, uint16_t pkts_n)
>  		return 0;
>  	/* Increment send statistics counters. */
>  	txq->stats.opackets += i;
> -	txq->stats.obytes += bytes_sent;
>  	/* Make sure that descriptors are written before doorbell record. */
>  	rte_wmb();
>  	/* Ring QP doorbell. */
>  	rte_write32(txq->msq.doorbell_qpn, txq->msq.db);
>  	txq->elts_head = elts_head;
> -	txq->elts_comp += elts_comp;
> +	txq->elts_comp += i;
>  	return i;
>  }

For bytes_sent, reading these changes and assuming -O0 with the compiler
attempting to convert the code without reordering/improving things, this
replaces register variables used in a loop with memory operations on a large
structure through a pointer (txq->stats update for every iteration instead
of once at the end).

Are you really sure this is more optimized that way? Although the compiler
likely does it already with -O3, helping it avoid unnecessary memory writes
is good in my opinion.

OK for the removal of redundant elts_comp though. Although I'm pretty sure
once again the compiler didn't wait for this patch to optimize it away.

-- 
Adrien Mazarguil
6WIND