what is the difference between RPS and XPS technology
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XPS selects a transmit queue during packet transmission based
on configuration. This is done by mapping the CPU transmitting the
packet to a queue. This is the transmit side analogue to RPS-- where
RPS is selecting a CPU based on receive queue, XPS selects a queue
based on the CPU (previously there was an XPS patch from Eric
Dumazet, but that might more appropriately be called transmit completion
steering).
Each transmit queue can be associated with a number of CPUs which will
use the queue to send packets.
RPS
distributes the load of received packet processing across multiple CPUs.
Problem statement: Protocol processing done in the NAPI context for received
packets is serialized per device queue and becomes a bottleneck under high
packet load. This substantially limits pps that can be achieved on a single
queue NIC and provides no scaling with multiple cores.
This solution queues packets early on in the receive path on the backlog queues
of other CPUs. This allows protocol processing (e.g. IP and TCP) to be
performed on packets in parallel. For each device (or NAPI instance for
a multi-queue device) a mask of CPUs is set to indicate the CPUs that can
process packets for the device. A CPU is selected on a per packet basis by
hashing contents of the packet header (the TCP or UDP 4-tuple) and using the
result to index into the CPU mask. The IPI mechanism is used to raise
networking receive softirqs between CPUs. This effectively emulates in
software what a multi-queue NIC can provide, but is generic requiring no device
support.
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