Polling vs. Interrupts

Onload User Guide (UG1586)

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1.2 English

Interrupts are useful because they allow the CPU to do other useful work while simultaneously waiting for asynchronous events (such as the reception of packets from the network). The historical alternative to interrupts was for the CPU to periodically poll for asynchronous events and on single processor systems this could result in greater latency than would be observed with interrupts. Historically it was accepted that interrupts were “good for latency”.

On modern, multicore systems the tradeoffs are different. It is often possible to dedicate an entire CPU core to the processing of a single source of asynchronous events (such as network traffic). The CPU dedicated to processing network traffic can be spinning (also known as busy waiting), continuously polling for the arrival of packets. When a packet arrives, the CPU can begin processing it almost immediately.

Contrast the polling model to an interrupt-driven model. Here the CPU is likely in its “idle loop” when an interrupt occurs. The idle loop is interrupted, the interrupt handler executes, typically marking a worker task as runnable. The OS scheduler will then run and switches to the kernel thread that will process the incoming packet. There is typically a subsequent task switch to a user-mode thread where the real work of processing the event (for example acting on the packet payload) is performed. Depending on the system, it can take on the order of a microsecond to respond to an interrupt and switch to the appropriate thread context before beginning the real work of processing the event. A dedicated CPU spinning in a polling loop can begin processing the asynchronous event in a matter of nanoseconds.

It follows that spinning only becomes an option if a CPU core can be dedicated to the asynchronous event. If there are more threads awaiting events than CPU cores (that is if all CPU cores are oversubscribed to application worker threads), then spinning is not a viable option, (at least, not for all events). One thread will be spinning, polling for the event while another could be doing useful work. Spinning in such a scenario can lead to (dramatically) increased latencies. But if a CPU core can be dedicated to each thread that blocks waiting for network I/O, then spinning is the best method to achieve the lowest possible latency.