6. IXGBE Driver
6.1. Vector PMD for IXGBE
Vector PMD uses Intel® SIMD instructions to optimize packet I/O. It improves load/store bandwidth efficiency of L1 data cache by using a wider SSE/AVX register 1 (1). The wider register gives space to hold multiple packet buffers so as to save instruction number when processing bulk of packets.
There is no change to PMD API. The RX/TX handler are the only two entries for vPMD packet I/O. They are transparently registered at runtime RX/TX execution if all condition checks pass.
- To date, only an SSE version of IX GBE vPMD is available. To ensure that vPMD is in the binary code, ensure that the option CONFIG_RTE_IXGBE_INC_VECTOR=y is in the configure file.
Some constraints apply as pre-conditions for specific optimizations on bulk packet transfers. The following sections explain RX and TX constraints in the vPMD.
6.1.1. RX Constraints
126.96.36.199. Prerequisites and Pre-conditions
The following prerequisites apply:
- To enable vPMD to work for RX, bulk allocation for Rx must be allowed.
Ensure that the following pre-conditions are satisfied:
- rxq->rx_free_thresh >= RTE_PMD_IXGBE_RX_MAX_BURST
- rxq->rx_free_thresh < rxq->nb_rx_desc
- (rxq->nb_rx_desc % rxq->rx_free_thresh) == 0
- rxq->nb_rx_desc < (IXGBE_MAX_RING_DESC - RTE_PMD_IXGBE_RX_MAX_BURST)
These conditions are checked in the code.
Scattered packets are not supported in this mode. If an incoming packet is greater than the maximum acceptable length of one “mbuf” data size (by default, the size is 2 KB), vPMD for RX would be disabled.
By default, IXGBE_MAX_RING_DESC is set to 4096 and RTE_PMD_IXGBE_RX_MAX_BURST is set to 32.
188.8.131.52. Feature not Supported by RX Vector PMD
Some features are not supported when trying to increase the throughput in vPMD. They are:
- Header split
- RX checksum off load
Other features are supported using optional MACRO configuration. They include:
- HW VLAN strip
- HW extend dual VLAN
- Enabled by RX_OLFLAGS (RTE_IXGBE_RX_OLFLAGS_ENABLE=y)
To guarantee the constraint, configuration flags in dev_conf.rxmode will be checked:
fdir_conf->mode will also be checked.
184.108.40.206. RX Burst Size
As vPMD is focused on high throughput, it assumes that the RX burst size is equal to or greater than 32 per burst. It returns zero if using nb_pkt < 32 as the expected packet number in the receive handler.
6.1.2. TX Constraint
The only prerequisite is related to tx_rs_thresh. The tx_rs_thresh value must be greater than or equal to RTE_PMD_IXGBE_TX_MAX_BURST, but less or equal to RTE_IXGBE_TX_MAX_FREE_BUF_SZ. Consequently, by default the tx_rs_thresh value is in the range 32 to 64.
220.127.116.11. Feature not Supported by RX Vector PMD
TX vPMD only works when txq_flags is set to IXGBE_SIMPLE_FLAGS.
This means that it does not support TX multi-segment, VLAN offload and TX csum offload. The following MACROs are used for these three features:
6.1.3. Sample Application Notes
By default, using CONFIG_RTE_IXGBE_RX_OLFLAGS_ENABLE=y:
./x86_64-native-linuxapp-gcc/app/testpmd -c 300 -n 4 -- -i --burst=32 --rxfreet=32 --mbcache=250 --txpt=32 --rxht=8 --rxwt=0 --txfreet=32 --txrst=32 --txqflags=0xf01
When CONFIG_RTE_IXGBE_RX_OLFLAGS_ENABLE=n, better performance can be achieved:
./x86_64-native-linuxapp-gcc/app/testpmd -c 300 -n 4 -- -i --burst=32 --rxfreet=32 --mbcache=250 --txpt=32 --rxht=8 --rxwt=0 --txfreet=32 --txrst=32 --txqflags=0xf01 --disable-hw-vlan
When running l3fwd with vPMD, there is one thing to note. In the configuration, ensure that port_conf.rxmode.hw_ip_checksum=0. Otherwise, by default, RX vPMD is disabled.
As in the case of l3fwd, set configure port_conf.rxmode.hw_ip_checksum=0 to enable vPMD. In addition, for improved performance, use -bsz “(32,32),(64,64),(32,32)” in load_balancer to avoid using the default burst size of 144.