[dpdk-dev] [PATCH v3 6/7] examples/vhost: add an option to enable dequeue zero copy

[Yuanhan Liu]

Add an option, --dequeue-zero-copy, to enable dequeue zero copy.

One thing worth noting while using dequeue zero copy is the nb_tx_desc
has to be small enough so that the eth driver will hit the mbuf free
threshold easily and thus free mbuf more frequently.

The reason behind that is, when dequeue zero copy is enabled, guest Tx
used vring will be updated only when corresponding mbuf is freed. If mbuf
is not freed frequently, the guest Tx vring could be starved.

Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>
---

v3: - handle missing "Tx --> dequeue" renaming in usage
---
 examples/vhost/main.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/examples/vhost/main.c b/examples/vhost/main.c
index 195b1db..91000e8 100644
--- a/examples/vhost/main.c
+++ b/examples/vhost/main.c
@@ -127,6 +127,7 @@ static uint32_t enable_tx_csum;
 static uint32_t enable_tso;
 
 static int client_mode;
+static int dequeue_zero_copy;
 
 /* Specify timeout (in useconds) between retries on RX. */
 static uint32_t burst_rx_delay_time = BURST_RX_WAIT_US;
@@ -294,6 +295,17 @@ port_init(uint8_t port)
 
 	rx_ring_size = RTE_TEST_RX_DESC_DEFAULT;
 	tx_ring_size = RTE_TEST_TX_DESC_DEFAULT;
+
+	/*
+	 * When dequeue zero copy is enabled, guest Tx used vring will be
+	 * updated only when corresponding mbuf is freed. Thus, the nb_tx_desc
+	 * (tx_ring_size here) must be small enough so that the driver will
+	 * hit the free threshold easily and free mbufs timely. Otherwise,
+	 * guest Tx vring would be starved.
+	 */
+	if (dequeue_zero_copy)
+		tx_ring_size = 64;
+
 	tx_rings = (uint16_t)rte_lcore_count();
 
 	retval = validate_num_devices(MAX_DEVICES);
@@ -470,7 +482,8 @@ us_vhost_usage(const char *prgname)
 	"		--socket-file: The path of the socket file.\n"
 	"		--tx-csum [0|1] disable/enable TX checksum offload.\n"
 	"		--tso [0|1] disable/enable TCP segment offload.\n"
-	"		--client register a vhost-user socket as client mode.\n",
+	"		--client register a vhost-user socket as client mode.\n"
+	"		--dequeue-zero-copy enables dequeue zero copy\n",
 	       prgname);
 }
 
@@ -495,6 +508,7 @@ us_vhost_parse_args(int argc, char **argv)
 		{"tx-csum", required_argument, NULL, 0},
 		{"tso", required_argument, NULL, 0},
 		{"client", no_argument, &client_mode, 1},
+		{"dequeue-zero-copy", no_argument, &dequeue_zero_copy, 1},
 		{NULL, 0, 0, 0},
 	};
 
@@ -1501,6 +1515,9 @@ main(int argc, char *argv[])
 	if (client_mode)
 		flags |= RTE_VHOST_USER_CLIENT;
 
+	if (dequeue_zero_copy)
+		flags |= RTE_VHOST_USER_DEQUEUE_ZERO_COPY;
+
 	/* Register vhost user driver to handle vhost messages. */
 	for (i = 0; i < nb_sockets; i++) {
 		ret = rte_vhost_driver_register
-- 
1.9.0