5. Performance > Performance Measurements
CPS-1848 User Manual
98
June 2, 2014
Formal Status
This document is confidential and is subject to an NDA.
Integrated Device Technology
5.3.2
Store-and-Forward or Cut-Through Mode
The CPS-1848 supports two buffer management modes: Store-and-Forward and Cut-Through. Store-and-Forward mode
ensures an incoming packet is completely received before it is forwarded, while the use of Cut-Through mode forwards the
incoming packet as soon as possible.
The Store-and-Forward mode is the default mode. Use Cut-Through mode only if all active ports have the same capacity (for
example, all ports are 2x with a lane rate of 5 Gbps). The selection of Store-and-Forward and Cut-Through mode for all input
ports is made using CUT_THRU_EN in the
.
5.3.3
Transmitter-Controlled or Receiver-Controlled Flow Control Mode
Receiver- and transmitter-controlled flow control are the most basic flow control functions supported by RapidIO. One of these
functions is always active on a link. The choice between receiver- and transmitter-controlled flow control is made automatically
as part of the
Port and Lane Initialization Sequence
5.3.3.1
Transmitter-Controlled Flow Control
Transmitter-controlled flow control is the default mode for each port. In this mode, the transmitter sends packets only when the
link partner has indicated that it has available input buffer space to receive them.
Buffer watermarks are used to manage the link-partners receive buffers. It restricts the transmission of lower priority packets to
the advantage of higher priority packets. Watermark settings directly affect throughput and indirectly latency and latency
variation. For more information on watermarks, see
Port {0..17} Watermarks Register
The default watermark settings should be used for the fair share configuration for both S-RIO ingress buffer management and
S-RIO egress buffer management.
The default watermark settings are aligned with the RapidIO specification as follows:
• Last buffer is reserved for priority 3 packet
• Second last buffer is reserved for a priority 2 or 3 packet
• Third last buffer is reserved for a priority 1, 2 or 3 packet
• All other buffers may be used by packets of any priority
5.3.3.2
Receiver-Controlled Flow Control
In receiver-controlled flow control, the receiver informs the transmitter when it cannot accept a packet by issuing a retry
(usually because of a lack of resources). The transmitter may resume packet transfer with a packet of higher priority than the
one that was retried, if such a packet is available.
The watermark settings need to be configured based on the link partner's buffers and buffer
management.