DMA (direct memory access) allows the network device to move packet data directly to the system's memory, reducing CPU utilization. However, the frequency and random intervals at which packets arrive do not allow the system to enter a lower power state. DMA coalescing allows the NIC to collect packets before it initiates a DMA event. This may increase network latency but also increases the chances that the system will consume less energy. Adapters and network devices based on the Intel® Ethernet Controller I350 (and later controllers) support DMA coalescing.
Higher DMA coalescing values result in more energy saved but may increase your system's network latency. If you enable DMA coalescing, you should also set the interrupt moderation rate to 'Minimal'. This minimizes the latency impact imposed by DMA coalescing and results in better peak network throughput performance. You must enable DMA coalescing on all active ports in the system. You may not gain any energy savings if it is enabled only on some of the ports in your system. There are also several BIOS, platform, and application settings that will affect your potential energy savings.
Intel® I350 Ethernet Controller and DMA Coalescing White Paper (PDF)
A white paper containing additional information and tuning recommendations for DMA Coalescing.
Size: 232 KB
Date: August 2011
Note: PDF files require Adobe Acrobat Reader*
Technology Brief: DMA Coalescing: Reducing Data Center Power Consumption with DMA Coalescing