Altera Stratix V Avalon-MM Interface for PCIe Solutions User Manual
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Nevertheless, maintaining maximum throughput of completion data packets is important. Endpoints
must offer an infinite number of completion credits. Endpoints must buffer this data in the RX buffer
until the Application Layer can process it. Because the Endpoint is no longer managing the RX buffer for
Completions through the flow control mechanism, the Application Layer must manage the RX buffer by
the rate at which it issues read requests.
To determine the appropriate settings for the amount of space to reserve for completions in the RX buffer,
you must make an assumption about the length of time until read completions are returned. This
assumption can be estimated in terms of an additional delay, beyond the FC Update Loop Delay, as
discussed in the section Throughput of Posted Writes. The paths for the read requests and the completions
are not exactly the same as those for the posted writes and FC Updates in the PCI Express logic. However,
the delay differences are probably small compared with the inaccuracy in the estimate of the external read
to completion delays.
With multiple completions, the number of available credits for completion headers must be larger than
the completion data space divided by the maximum packet size. Instead, the credit space for headers must
be the completion data space (in bytes) divided by 64, because this is the smallest possible read
completion boundary. Setting the RX Buffer space allocation—Desired performance for received
completions to High under the System Settings heading when specifying parameter settings configures
the RX buffer with enough space to meet this requirement. You can adjust this setting up or down from
the High setting to tailor the RX buffer size to your delays and required performance.
11-4
Throughput of Non-Posted Reads
UG-01097_avmm
2014.12.15
Altera Corporation
Throughput Optimization