Interrupt Moderation

To reduce the number of interrupts, many NICs use interrupt moderation. With interrupt moderation, the NIC hardware will not generate an interrupt immediately after it receives a packet. Instead, the hardware waits for more packets to arrive, or for a time-out to expire, before generating an interrupt. The hardware vendor specifies the maximum number of packets, time-out interval, or other interrupt moderation algorithm.

The measured round-trip time for a packet is one of the most commonly used techniques to determine the network bandwidth between two endpoints. However, when interrupt moderation is enabled, receiving a packet does not generate an immediate interrupt and therefore the perceived round-trip time for a particular packet becomes larger than the average time. To allow accurate measurement of round trip time for a packet, NDIS provides the ability to disable and enable interrupt moderation on demand.

All NDIS 6.0 and later miniport drivers must support the OID_GEN_INTERRUPT_MODERATION OID. If a miniport driver does not support interrupt moderation, the driver must specify NdisInterruptModerationNotSupported in the InterruptModeration member of the NDIS_INTERRUPT_MODERATION_PARAMETERS structure.

NDIS 6.0 and later miniport drivers must support both the OID_GEN_INTERRUPT_MODERATION OID set and query requests. The set request directs the miniport driver to enable or disable interrupt moderation and the query request reports the current state of interrupt moderation.

A miniport driver that supports interrupt moderation should turn this capability on by default unless the InterruptModeration standard keyword in the registry disables it. For more information about the standard keywords, see Standardized INF Keywords for Network Devices.