8.8. Receive Flow Steering (RFS)
Receive Flow Steering (RFS) extends RPS behavior to increase the CPU cache hit rate and thereby reduce network latency. Where RPS forwards packets based solely on queue length, RFS uses the RPS backend to calculate the most appropriate CPU, then forwards packets based on the location of the application consuming the packet. This increases CPU cache efficiency.
RFS is disabled by default. To enable RFS, you must edit two files:
/proc/sys/net/core/rps_sock_flow_entries- Set the value of this file to the maximum expected number of concurrently active connections. We recommend a value of
32768for moderate server loads. All values entered are rounded up to the nearest power of 2 in practice. /sys/class/net/device/queues/rx-queue/rps_flow_cnt- Replace device with the name of the network device you wish to configure (for example,
eth0), and rx-queue with the receive queue you wish to configure (for example,rx-0).Set the value of this file to the value ofrps_sock_flow_entriesdivided byN, whereNis the number of receive queues on a device. For example, ifrps_flow_entriesis set to32768and there are 16 configured receive queues,rps_flow_cntshould be set to2048. For single-queue devices, the value ofrps_flow_cntis the same as the value ofrps_sock_flow_entries.
Data received from a single sender is not sent to more than one CPU. If the amount of data received from a single sender is greater than a single CPU can handle, configure a larger frame size to reduce the number of interrupts and therefore the amount of processing work for the CPU. Alternatively, consider NIC offload options
or faster CPUs.
Consider using
numactl or taskset in conjunction with RFS to pin applications to specific cores, sockets, or NUMA nodes. This can help prevent packets from being processed out of order.
