Chapter 11. Flow Control; Throughput Of Posted Writes - Altera Cyclone V User Manual

November 2011
1101
Throughput analysis requires that you understand the Flow Control Loop, shown in
"Flow Control Update Loop" on page
Loop and strategies to improve throughput. It covers the following topics:
■

Throughput of Posted Writes

■ Throughput of Non-Posted Reads
Throughput of Posted Writes
The throughput of posted writes is limited primarily by the Flow Control Update loop
shown in
and the completer consumes the data as quickly as possible, then the Flow Control
Update loop may be the biggest determining factor in write throughput, after the
actual bandwidth of the link.
Figure 11–1
communicating PCI Express ports:
■ Write Requester
■ Write Completer
As the
in this case, maintains a Credit Limit Register and a Credits Consumed Register.
The Credit Limit Register is the sum of all credits issued by the receiver, the write
completer in this case. The Credit Limit Register is initialized during the flow
control initialization phase of link initialization and then updated during operation by
Flow Control (FC) Update DLLPs. The Credits Consumed Register is the sum of all
credits consumed by packets transmitted. Separate Credit Limit and Credits
Consumed Registers exist for each of the six types of Flow Control:
■ Posted Headers
■ Posted Data
Non-Posted Headers
■
■ Non-Posted Data
■ Completion Headers
■ Completion Data
November 2011 Altera Corporation
Figure 11–1. If the write requester sources the data as quickly as possible,
shows the main components of the Flow Control Update loop with two
PCI Express Base Specification 2.1
11. Flow Control
11–2. This chapter discusses the Flow Control
describes, each transmitter, the write requester
Cyclone V Hard IP for PCI Express