STLinux

NAPI

All the stmmac driver versions implement NAPI (New API) for interrupt receive mitigation. See Documentation/networking/NAPI_HOWTO.txt in the kernel source tree for the details.

Zero-copy

All the stmmac driver versions implement zero-copy mechanism. If the packet length is under a fixed threshold, the memory copy of the socket buffers won't be performed. Each DMA buffer, in the ring, directly points to a preallocate socket buffer. During the reception process, after passing the frame to the kernel, the "dirty" socket buffers will be re-filled.

Timer-Driver Interrupt

Instead of having the device asynchronously notifies the frame receptions, the driver configures a timer to generate an interrupt at regular intervals. Based on the granularity of the timer the frame that are received by the device will be experience different levels of latency. Some NICs have dedicated timer device to perform this task. STMMAC can use either the SH4-RTC device or the TMU channel 2. The timers frequency can be passed to the stmmac driver as parameter. In any case, pay attention to play with it and take care of both hardware capability and network stability/performance impact.

Interrupt aggregation (or coalescence or mitigation)

This is to reduce the number of the interrupts by using an interrupt aggregation schema. It can help us to save the CPU especially when Timer optimization is enabled. To tune this kind of parameter, see the relative entry within the following directory: /sys/module/stmmac/parameters/.