IDT 89HPES48T12G2, User Manual

Page: 47 / 276

IDT Switch Core
PES48T12G2 User Manual 3 - 7 April 5, 2013
Notes
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 PES48T12G2 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.
5.0 Gbps x8 2.5 x8, x4, x2, x1 Always
5.0 x8, x4, x2, x1 Always
x4 2.5 x8, x4, x2, x1 Always
5.0 x8 At least 50% of packet is in IFB
x4, x2, x1 Always
x2 2.5 x8 At least 50% of packet is in IFB
x4, x2, x1 Always
5.0 x8 At least 75% of packet is in IFB
x4 At least 50% of packet is in IFB
x2, x1 Always
x1 2.5 x8 At least 75% of packet is in IFB
x4 At least 50% of packet is in IFB
x2, x1 Always
5.0 x8 Never (100% of packet is in IFB)
x4 At least 75% of packet is in IFB
x2 At least 50% of packet is in IFB
x1 Always
Ingress
Link
Speed
Ingress
Link
Width
Egress
Link
Speed
Egress
Link
Width
Conditions for Cut-
Through
Table 3.5 Conditions for Cut-Through Transfers (Part 2 of 2)