What are the advantages NAPI before the IRQ Coalesce?

Let's begin with no napi and interupt coalescing.

The first case: livelock. That means when many interrupts post from servel process continuely, the CPU only processes interrupts and never allows a user-level process to run and actually service the requests. For it, we create napi, which handle it with hybrid mode (interupt + polling). When an interrupt happens, handle it and polling for a while to solve subsequence requests.

The second case: optimization. Before raise an interupt, a device first waits for a bit before delivering the interrupt to the CPU. While waiting, other requests may soon complete, and thus multiple interrupts can be merged into a single interrupt delivery, thus lowering the overhead of interrupt processing.

To conclude, there're no conflict between them. And they're for different cases though napi can also optimize the CPU overhead.

Ref: Principles of Computer System Design.

I view NAPI as a form of interrupt coalescing. I think your question may stem from a misunderstanding about NAPI. First of all, interrupts are involved with NAPI. Also, NAPI's polling is actually not "in vain". Remember, for NAPI, the idea is that high throughput traffic is bursty. NAPI only "starts" after an "packet received interrupt" happens.

Here's a quick overview of how NAPI is supposed to be used:

The kernel kicks off the "packet received" interrupt, which a network device driver using NAPI detects. The network device driver then disables interrupts related to receiving packets and using NAPI, tells the Linux networking subsystem to poll the device driver. The poll function is implemented by the device driver, and is passed to the networking subsystem, and contains the device driver's packet handler. After enough packets are received or a timeout is reached, packet-receiving interrupts are re-enabled, and everything starts over again.

So NAPI is basically just a centralized API in the Linux networking subsystem for supporting interrupt coalescing to reduce receive livelock situations. NAPI gives device driver developers a clean framework for interrupt coalescing. NAPI does not run all the time, but only happens when traffic is actually received, making it essentially an interrupt coalescing scheme... At least in my book.

Note: This was all in the context of a network device driver using NAPI, but in fact NAPI can be used for any kind of interrupt. This is one of the benefits of NAPI, as well.

If there are any errors in my understanding, please feel free to point them out!