Performance Metrics
PLX Technology, Inc.
8.2.3
RAM and Queue Size
Table 8-1 defines the RAM and Queue Size built into the PEX 8532. The smallest unit of ingress and
egress packet RAM is defined as a beat, which can store 20 bytes of data. In the PEX 8532, the smallest
packet (12B header) takes 1 beat to store in a packet RAM, and the largest packet (16B header + 256B
payload + 4B digest = 276B) takes 14 beats to store in packet RAM.
In Table 8-1, the number 32 under the cell Ingress VOQ Entries, Per Port indicates the maximum
number of VOQ entries allocated to each ingress port. Each VOQ entry holds one Transaction Layer
packet. For ingress ports, the incoming packet can travel to two destination stations, with each station
containing up to four egress ports. Also, for egress ports, up to two VCs are supported, with each VC
potentially having three different packet types – Posted, Non-Posted, and Completion (P, NP, and Cpl,
respectively).
Each port on the egress side can contain up to six queues, to hold packets from two supported VCs and
three supported packet types. A queue that stores packets of a unique VC and a unique type is referred to
as a VC&T queue. Again, some queues can be completely empty and some queues can contain more
than one packet. The maximum number of packets held by a single egress port is limited by the number
of egress packet RAM beats allocated to that port.
It can be calculated from Table 8-1 that the total packet RAM size for the PEX 8532 is 136,960 bytes.
Assume the maximum Transaction Layer Packet (TLP) size is 276B with a Maximum Payload Size
(MPS) of 256B. Theoretically, the PEX 8532 can store up to 496 MPS packets.
Table 8-1. PEX 8532 Data Structure Size
Data Structure Name
Ingress Packet Name
Ingress VOQ Entries
Per Port
Programmable
32
Per Station
Overall
55,040 bytes
256
1,376 beats or
27,520 bytes
128
1,024 beats or
20,480 bytes for
1 to 2 ports;
2,048 beats or
40,960 bytes
Egress Packet RAM
81,920 bytes
48
512 beats or 10,240 bytes
for 3 to 4 ports
Egress VC&T Queues
6
24
100
ExpressLane PEX 8532AA/BA/BB/BC 8-Port/32-Lane Versatile PCI Express Switch Data Book
Copyright © 2007 by PLX Technology, Inc. All Rights Reserved – Version 1.6
PLX [ PLX TECHNOLOGY ]
