User Manual

Rev 4.70

Mellanox Technologies

8.13.2 Drop Mode

In this mode, a packet arriving to a descriptor ring with no free descriptors is dropped, after veri-

fying that there are really no free descriptors. This allows isolation of the host driver execution

delays from the network, as well as isolation between different SW entities sharing the adapter

(e.g. SR-IOV VMs).

8.13.3 Poll Mode

In this mode, a packet arriving to a descriptor ring with no free descriptors will patiently wait

until a free descriptor is posted. All processing for this packet and the following packets is halted,

while free descriptor status is polled. This behavior will propagate the backpressure into the Rx

buffer which will accumulate incoming packets. When XOFF threshold is crossed, Flow Control

mechanisms mentioned earlier will stop the remote transmitters, thus avoiding packets from

being dropped.

Since this mode breaks the aforementioned isolation, the adapter offers a mitigation mechanism

that limits the amount of time a packet may wait for a free descriptor, while halting all packet

processing. When the allowed time expires the adapter reverts to the 'Drop Mode' behavior.

8.13.4 Default behavior

By default the adapter works in 'Drop Mode'. The adapter reverts to this mode upon initialization/

restart.

8.13.5 Known Limitations

• The feature is not available for SR-IOV Virtual Functions

• It is recommended that the feature be used only when the port is configured to maintain

flow control.

• It is recommended not to exceed typical timeout values of management protocols, usu-

ally in the order of several seconds.

• In order for the feature to effectively prevent packet drops, the DPC load duration needs

to be lower than the TCP retransmission timeout.

• The feature is only activated if neither of the ports is IB.

8.13.6 System Requirements

• Operating System: Windows 2012 or Windows 2012 R2

• Firmware: 2.31.5050

8.13.7 Enabling/Disabling Lossless TCP

This feature is controlled using the registry key DelayDropTimeout that enables Lossless TCP

capability in hardware and by Set

OID OID_MLX_DROPLESS_MODE which triggers transition to/

from Lossless (poll) mode.