Intel 7xx Servers, 170 Servers, AS/400 RISC Server manual POWER6 520 Memory Considerations

20.5 POWER6 520 Memory Considerations

Because of the design of the Power6 520 system, there are some key factors with the memory subsystem that one should keep in mind when sizing this system. The Power6 520, unlike the Power6 570, has no L3 cache, which does have an effect on memory sensitive workloads, like Java applications for instance. Having no L3 cache makes memory speed, or the bandwidth rating in megabytes per second, even more critical for memory sensitive workloads. The Power6 520 has 8 memory DIMM slots, which are positioned in groups of four behind each of the Power6-SCM modules and each group of four will be referred to as a quad for this discussion. The available number of active memory slots depends on the Processor Feature Code of the system.

When only one SCM module is installed, only one quad of memory is active and all slots must contain DIMMs of the same size and speed. When two SCM modules are installed (except in the case of the 4-way capable Capacity-on-Demand model with only one module enabled, which activates both memory quads), both quads of memory are active. When both are active, it is important to note that the first and second modules are separate and independent. So this means that even though the size and speed of memory DIMMs behind each module have to be the same, the size and speed of memory DIMMs behind the first module do not have to match the memory DIMMs behind the second module. For DIMMs ranging from 512 MB to 4 GB, the speed is 667 Mbps (PC2-5300). The 8 GB DIMMs are different however, with a speed of 400 Mbps (PC2-3200). This decrease in speed for 8 GB DIMMs can have a negative effect on performance with memory sensitive workloads. This effect, along with the fact that there is no L3 cache, should be considered when planning for current and future growth and also LPAR configurations.

To test the performance difference of 4 GB DIMMs versus 8 GB DIMMs (essentially testing the difference in speed) and what occurs when the DIMMs of different sizes are “mixed”, we used a Power6 520 (9408-M25) F/C 5635 (a fully enabled system) with one partition using all the available resources.

“Mixed” here means the DIMMs in one quad behind a module are 4 GB and the DIMMs in the opposite quad are 8 GB. We started with a baseline consisting of all 4 GB DIMMs behind both modules, which is the best performing case. Then switched to all 8 GB DIMMs behind both modules and ran the same tests again. The performance of the workloads that were memory sensitive followed suit with the decrease in memory speed, which was expected. This is very important to consider when considering the amount of memory needed for a system. Deciding to go with the larger capacity 8 GB DIMMs does reduce your memory’s speed and can have a negative performance effect on your workload. Of course each workload will behave differently based on its sensitivity to memory.

Next we placed 4 GB DIMMs behind one module and 8 GB DIMMs behind the opposite module. Because the one module had the faster 4 GB DIMMs behind it, the same workloads produced results that ranged between the best case, all 4 GB DIMMs, and the worst case, all 8 GB DIMMs. Again, we used only one partition that utilized all the available resources, but there are other factors to consider when using LPAR.

LPAR, or Logical Partitioning, increases flexibility, enabling selected system resources like processors, memory and I/O components to be utilized by various partitions, either in a shared or dedicated environment, on the same system. In the “mixed” environment previously described, it is possible to have one partition utilizing memory on 4 GB DIMMs and a second partition, configured with exactly the same amount of resources, utilizing memory on 8 GB DIMMs. This can cause an application to have different performance characteristics on the partitions. It is also possible for partitions to be assigned a mix of memory from different DIMMs, depending on how the memory is allocated at partition

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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