Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

Forwarding processor (FP) ASICs provide Ethernet MAC functions, queueing, and buffering, as well as store feature and forwarding tables for hardware-based lookup and forwarding decisions. 1G and 10G interfaces use different FPs.

All ports support eight queues, four for data traffic and four for control traffic. All eight queues are tunable.

Physical memory is organized into cells of 128 bytes. The cells are organized into two buffer pools — the dedicated buffer and the dynamic buffer.

• Dedicated buffer — this pool is reserved memory that other interfaces cannot use on the same ASIC or by other queues on the same interface. This buffer is always allocated, and no dynamic re-carving takes place based on changes in interface status. Dedicated buffers introduce a trade-off. They provide each interface with a guaranteed minimum buffer to prevent an overused and congested interface from starving all other interfaces. However, this minimum guarantee means that the buffer manager does not reallocate the buffer to an adjacent congested interface, which means that in some cases, memory is under-used.
• Dynamic buffer — this pool is shared memory that is allocated as needed, up to a configured limit. Using dynamic buffers provides the benefit of statistical buffer sharing. An interface requests dynamic buffers when its dedicated buffer pool is exhausted. The buffer manager grants the request based on three conditions:
  • The number of used and available dynamic buffers.
  • The maximum number of cells that an interface can occupy.
  • Available packet pointers (2k per interface). Each packet is managed in the buffer using a unique packet pointer. Thus, each interface can manage up to 2k packets.

Figure 1. Buffer Tuning Points