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Chapter 1. Technical description 19
1.6 IBM XceL4 Server Accelerator Cache
Integrated into each SMP Expansion Module is 32 MB of high-speed Level 4
cache (see Figure 1-10). This XceL4 Server Accelerator Cache provides the
necessary extra level of cache to alleviate the bottlenecks caused by memory
latency across the scalability port.
Cache memory is two-way interleaved 200 MHz DDR memory and is faster than
standard memory because it is directly connected to the memory controller and
does not have additional latency associated with the large fan-out necessary to
support the 16 DIMM slots.
Initial tests have shown the XceL4 cache has improved overall system
performance up to 20% on various applications.
1.7 System memory
The Xeon MP models of the x440 have 2 GB or 4 GB of RAM standard,
implemented as four PC133 ECC SDRAM DIMMs (four 512 MB or four 1 GB
DIMMs). There are 16 DIMM sockets (two ports of eight) in each of the two SMP
Expansion Modules for a total of 32 sockets. Using 2 GB DIMMs, this means that
each x440 can have up to 64 GB RAM.
See 3.1.2, “Memory” on page 65 for further discussion of how memory is
implemented in the x440 and what you should consider before an x440
installation.
There are a number of advanced features implemented in the x440 memory
subsystem, collectively known as
Active Memory:
Memory ProteXion
Memory ProteXion, also known as “redundant bit steering”, is the technology
behind using redundant bits in a data packet to provide backup in the event of
a DIMM failure.
Currently, other industry-standard servers use 8 bits of the 72-bit data packets
for ECC functions and the remaining 64 bits for data. However, because the
x440 uses four-way interleaved memory, it needs only 6 bits to perform the
same ECC functions, thus leaving 2 bits free (Figure 1-11 on page 20).