Hitachi 1000 manual Intel Itanium I/O Expansion Module, L3 Cache Copy Tag

L3 Cache Copy Tag

The data residing in caches and main memory across Intel Itanium Server Blades are kept in sync by using a snooping cache coherency protocol. When one of the Intel Itanium processors needs to access memory, the requested address is broadcast by the Hitachi Node Controller. The other Node Controllers that are part of that partition (SMP) listen for (snoop) those broadcasts. The Node Controller keeps track of the memory addresses currently cached in each processor’s on-chip caches by assigning a tag for each cache entry. If one of the processors contains the requested data in its cache it initiates a cache-to-cache transfer. This reduces latency by avoiding the penalty to retrieve data from main memory and helps maintain consistency by sending the requesting processor the most current data. In order to save bandwidth on the processors’ front side bus, the Node Controller is able to use the L3 Cache Copy Tags to determine which memory address broadcasts its two local processors need to see. If a requested address is not in the processors’ cache, the Node Controller filters the request and does not forward the request to the local processors. This process is illustrated in Figure 10.

	Node 0		Node 1
Itanium2		Itanium2	Itanium2	Itanium2
L3 C		(1)	(2)	(3)’	L3 C
Copy	Node		Node		Copy
Tag					Tag
	Controller		Controller
			(4)	(3)
Memory		Memory	Memory	Memory
Controller		Controller	Controller	Controller
	Main Memory		Main Memory

(1)Cache consistency control within a local node

(2)Memory address

broadcasting

(3)	Parallel	(3)’ Cache
		consistency
Memory	Processing	control over
access		remote nodes

(4)Memory data transfer or

Cache data transfer

Figure 10. L3 cache copy tag process

Intel Itanium I/O Expansion Module

Some applications require more PCI slots than the two that are available per server blade. The Intel Itanium I/O Expansion Module provides more ports, without the expense of additional server blades. Using the Itanium I/O Expansion Module with the Intel Itanium Server Blade can increase the number of the PCI expansion-card slots that can be connected to the Intel Itanium Server Blade. The Itanium I/O expansion module cannot be used in with the Intel Xeon Server Blade.

The Intel Itanium I/O Expansion Module increases the number of PCI I/O slots to either four or eight slots depending on the chassis type. The type A chassis enables connection to four PCI I/O slots (Figure 11), and the type B chassis enables up to eight PCI I/O slots (Figure 12).

18 BladeSymphony 1000 Architecture White Paper www.hitachi.com