ldurandadomia
commented
4 years ago Yes, DMA is a memory transfer method.
both interrupt and polling modes are using DMA to copy packets between NIC cards and host memory. The difference is :
- interrupt: is not using continuously CPU resources to check if a packet is available or not. It save CPU resources but is less efficient for high PPS traffic (lots of interrupts are generated ==> lots of time is waste in save/restore context)
- polling: is using continuously CPU resources to incoming process packets. If each time the CPU is checking a queue, a packet is present this is very efficient. But at low speed, this mechanism is wasting lots of CPU resources to check NIC queues where there are not packet to process.
To sum up:
- interrupt mode is well suited for low speed (more accurately low PPS) VNF
- polling mode is well suited for high speed (more accurately high PPS) VNF
as explained well in page ch2/page 7, these 2 were different things. interrupt mode can also have DMA. it is the polling mode that avoids the dependence on interrupt.