On 08/23/2016 10:10 AM, Yuanhan Liu wrote: > Add an option, --tx-zero-copy, to enable Tx zero copy. > > One thing worth noting while using Tx 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 Tx 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> > --- > 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 9974f0b..e3437ad 100644 > --- a/examples/vhost/main.c > +++ b/examples/vhost/main.c > @@ -130,6 +130,7 @@ static uint32_t enable_tx_csum; > static uint32_t enable_tso; > > static int client_mode; > +static int tx_zero_copy; > > /* Specify timeout (in useconds) between retries on RX. */ > static uint32_t burst_rx_delay_time = BURST_RX_WAIT_US; > @@ -297,6 +298,17 @@ port_init(uint8_t port) > > rx_ring_size = RTE_TEST_RX_DESC_DEFAULT; > tx_ring_size = RTE_TEST_TX_DESC_DEFAULT; > + > + /* > + * When Tx 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 (tx_zero_copy) > + tx_ring_size = 64;
I have a concern about more complex applications, where the mbufs might not be consumed sequentially. If one mbuf gets stuck for a while, whereas all others are consumed, we would face starvation. For example, the packet is to be routed to a VM, which is paused, and the routing thread keeps retrying to enqueue the packet for a while. Anyway, this feature is optional and off by default, so having the feature applied is not a blocker. Thanks! Maxime
