[PATCH] examples/l3fwd: fix Tx performance deteriorate

[Konstantin Ananyev]

> The application send packets only when the buffer is full, or the
> buffer is empty and the number of packets to be sent extends half
> of the buffer.
> 
> The change of MAX_PKT_BURST increases TX buffer size, while the
> default size of local cache on each lcore is 256, which not greater
> than the limit of transmitting. That would make the mbuf not on the
> local cache be frequently used and the performance deteriorates.
> 
> This problem can be solved by making the TX threshold smaller than
> the local cache size. For example, use the '--mbcache' parameter to
> make the local cache greater. This patch optimizes the default
> performance by lowering TX threshold.

In  commit:
examples/l3fwd: add option to set Rx burst size
you introduced new global 
uint32_t nb_pkt_per_burst;
Why not to use it for both (rx and tx) paths?
Or if necessary introduce another one for tx, so we'll have:
uint32_t nb_rx_pkt_per_burst, nb_tx_pkt_per_burst,;
To me that is much better then create some hardcoded
and implicit thresholds.  

> Fixes: d5c4897ecfb2 ("examples/l3fwd: add option to set Rx burst size")
> 
> Signed-off-by: Jie Hai <haijie1 at huawei.com>
> ---
>  examples/l3fwd/l3fwd.h        | 8 +++++---
>  examples/l3fwd/l3fwd_common.h | 6 +++---
>  2 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/examples/l3fwd/l3fwd.h b/examples/l3fwd/l3fwd.h
> index 0cce3406ee7d..a01fecd51261 100644
> --- a/examples/l3fwd/l3fwd.h
> +++ b/examples/l3fwd/l3fwd.h
> @@ -35,7 +35,7 @@
>  /*
>   * Try to avoid TX buffering if we have at least MAX_TX_BURST packets to send.
>   */
> -#define	MAX_TX_BURST	  (MAX_PKT_BURST / 2)
> +#define	MAX_TX_BURST DEFAULT_PKT_BURST
> 
>  #define NB_SOCKETS        8
> 
> @@ -57,6 +57,8 @@
>  #define L3FWD_HASH_ENTRIES		(1024*1024*1)
>  #endif
> 
> +static_assert(MAX_TX_BURST <= MAX_PKT_BURST, "MAX_TX_BURST should be at most MAX_PKT_BURST");
> +
>  struct parm_cfg {
>  	const char *rule_ipv4_name;
>  	const char *rule_ipv6_name;
> @@ -152,8 +154,8 @@ send_single_packet(struct lcore_conf *qconf,
>  	len++;
> 
>  	/* enough pkts to be sent */
> -	if (unlikely(len == MAX_PKT_BURST)) {
> -		send_burst(qconf, MAX_PKT_BURST, port);
> +	if (unlikely(len == MAX_TX_BURST)) {
> +		send_burst(qconf, MAX_TX_BURST, port);
>  		len = 0;
>  	}
> 
> diff --git a/examples/l3fwd/l3fwd_common.h b/examples/l3fwd/l3fwd_common.h
> index d94e5f135791..3f504dc0a552 100644
> --- a/examples/l3fwd/l3fwd_common.h
> +++ b/examples/l3fwd/l3fwd_common.h
> @@ -71,7 +71,7 @@ send_packetsx4(struct lcore_conf *qconf, uint16_t port, struct rte_mbuf *m[],
>  	 * If TX buffer for that queue is empty, and we have enough packets,
>  	 * then send them straightway.
>  	 */
> -	if (num >= MAX_TX_BURST && len == 0) {
> +	if (num >= MAX_TX_BURST / 2 && len == 0) {
>  		n = rte_eth_tx_burst(port, qconf->tx_queue_id[port], m, num);
>  		if (unlikely(n < num)) {
>  			do {
> @@ -112,9 +112,9 @@ send_packetsx4(struct lcore_conf *qconf, uint16_t port, struct rte_mbuf *m[],
>  	len += n;
> 
>  	/* enough pkts to be sent */
> -	if (unlikely(len == MAX_PKT_BURST)) {
> +	if (unlikely(len > MAX_TX_BURST)) {
> 
> -		send_burst(qconf, MAX_PKT_BURST, port);
> +		send_burst(qconf, len, port);
> 
>  		/* copy rest of the packets into the TX buffer. */
>  		len = num - n;
> --
> 2.22.0