[dpdk-dev] [PATCH] net/i40e: update tx_free_threshold to improve zero copy performance

[Junjie Chen]

From: "Chen, Junjie" <junjie.j.chen at intel.com>

When vhost backend works in dequeue zero copy mode, nic locks virtio's
buffer until there is less or equal than tx_free_threshold buffer remain
and then free number of tx burst buffer. This causes packets drop in
virtio side and impacts zero copy performance. So we need to increase
the tx_free_threshold to let nic free virtio's buffer as soon as possible.
Also we keep the upper limit to tx max burst size to ensure least
performance impact on non zero copy.

Signed-off-by: Chen, Junjie <junjie.j.chen at intel.com>
---
 drivers/net/i40e/i40e_rxtx.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c
index 56a854cec..d9569bdc9 100644
--- a/drivers/net/i40e/i40e_rxtx.c
+++ b/drivers/net/i40e/i40e_rxtx.c
@@ -2039,6 +2039,8 @@ i40e_dev_tx_queue_setup(struct rte_eth_dev *dev,
 		tx_conf->tx_rs_thresh : DEFAULT_TX_RS_THRESH);
 	tx_free_thresh = (uint16_t)((tx_conf->tx_free_thresh) ?
 		tx_conf->tx_free_thresh : DEFAULT_TX_FREE_THRESH);
+	if (tx_free_thresh < nb_desc - I40E_TX_MAX_BURST)
+		tx_free_thresh = nb_desc - I40E_TX_MAX_BURST;
 	if (tx_rs_thresh >= (nb_desc - 2)) {
 		PMD_INIT_LOG(ERR, "tx_rs_thresh must be less than the "
 			     "number of TX descriptors minus 2. "
-- 
2.16.0