Donald K. Burleson
Oracle RAC Tips
Cluster Interconnect Products
Most cluster vendors utilize very competitive technology. Many of the
interconnect products approach the latency levels of the SMP bus.
Technologies like Memory Channel, SCI, and Myrinet support virtual shared
memory space by means of internodal memory address space mapping. Connections
between nodes are established by mapping part of the nodes virtual address
space interconnect interface. Because of the memory-mapped nature of the
interface, transmitting or receiving overhead is similar to an access of
local main memory. This mechanism is the fundamental reason for the low
latency levels seen in Memory Channel, SCI, and Myrinet. Table 3.2 summarized
the various interconnect capabilities.
Let’s examine some of the popular interconnect products and technologies.
- Memory Channel - The memory channel interconnect is a high-speed
network interconnect and provides applications with a cluster-wide address
space. Applications map portions of this address space into their own
virtual address space as 8 Kbyte pages, and then read and write from this
address space just like normal memory. This is available for Alpha-based HP
(now Compaq) clusters.
- Myrinet - Myrinet is a cost effective, high performance packet
communication and switching technology. It is widely used in Linux
Clusters. Myrinet software supports most common hosts and operating
systems. The software is supplied ‘open source’. Myrinet implements host
interfaces that execute a control program to interact directly with host
processes (‘OS bypass’) for low latency communication, and directly with
networks to send, receive, and buffer packets.
- Scalable Coherent Interconnect - SCI is Sun's best performing
cluster interconnect because of its high data rate and low latency.
Applications that stress the interconnect will scale better using SCI,
compared to using lower performing alternatives. Sun SCI implements Remote
Shared Memory (RSM), a feature that bypasses the TCP/IP communication
overhead of Solaris. This improves cluster performance. SCI is available
for the Sun Fire 4800 and 6800 servers.
- Veritas - Database Edition / Advanced Cluster communications
consists of LLT (low level transport) and GAB (Group Membership Services).
LLT provides kernel-to-kernel communications and functions as a high
performance replacement for the IP stack. The use of LLT rather than IP
reduces the latency and overhead associated with the IP stack.
- HP Hyper Fabric HMP - HP Hyper-Fabric supports standard TCP/UDP
over IP and HP’s proprietary Hyper Message Protocol. Hyper Fabric extends
the scalability and reliability of TCP/UDP by providing transparent load
balancing of connection traffic across multiple network interface cards.
HMP, coupled with OS bypass capability and the hardware support for
protocol offload, provides low latency and extremely low CPU utilization.
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