Request Metering - Renesas IDT 89HPES48H12G2 User Manual

IDT Switch Core
Notes
PES48H12G2 User Manual
Ingress Ingress
Link Link
Speed Width
5.0 Gbps x8
x4
x2
x1
Table 3.5 Conditions for Cut-Through Transfers (Part 2 of 2)

Request Metering

Request metering may be used to reduce congestion in PCI express switches caused by a static rate
mismatch. Request metering is available on all PES48H12G2 switch ports but is disabled by default. A
static rate mismatch is a mismatch in the capacity of the path from a component injecting traffic into the
fabric (e.g., a Root Complex) and the ultimate destination (e.g., an Endpoint).
An example of a static rate mismatch in a PCIe fabric is a x8 root injecting traffic destined to a x1
endpoint. PCIe fabrics are typically no more than one switch deep. Therefore, static rate mismatches typi-
cally occur within a switch due to asymmetric link rates. Figure 3.3 illustrates the effect of congestion on
PCIe fabric caused by a static rate mismatch. In this example there are two endpoints issuing memory read
requests to a root. Endpoint A has a x1 link to the switch, while endpoint B and the root complex have a x8
link.
Memory read request TLPs are three or four DWords in size. A single memory read request may result
in up to 4 KB of completion data being returned to the requester. Depending on system architecture and
configured maximum payload size, this completion data may be returned as a single completion TLP or
may be returned as a series of small (e.g., 64B data) TLPs.
Consider an example where Endpoints A and B are injecting read request to the root at a high rate and
the root is able to inject completion data into the fabric at a rate higher than which may be supported by
endpoint A's egress link. The result is that the endpoint A's EFB and the root's IFB may become filled with
queued completion data blocking completion data to endpoint B.
Egress Egress
Link Link
Speed Width
2.5 x8, x4, x2, x1
5.0 x8, x4, x2, x1
2.5 x8, x4, x2, x1
5.0 x8
x4, x2, x1
2.5 x8
x4, x2, x1
5.0 x8
x4
x2, x1
2.5 x8
x4
x2, x1
5.0 x8
x4
x2
x1
3 - 8
Conditions for Cut-
Through
Always
Always
Always
At least 50% of packet is in IFB
Always
At least 50% of packet is in IFB
Always
At least 75% of packet is in IFB
At least 50% of packet is in IFB
Always
At least 75% of packet is in IFB
At least 50% of packet is in IFB
Always
Never (100% of packet is in IFB)
At least 75% of packet is in IFB
At least 50% of packet is in IFB
Always
April 5, 2013