IBM Power7 Optimization And Tuning Manual page 31

Power dedicated LPARs
Dedicated LPAR deployments generally use larger partitions, ranging from just one POWER7
core up to a partition that includes all of the cores and memory in a large symmetric
multi-processor (SMP) system. A smaller partition might run a single application, and a larger
partition typically runs multiple applications, or multiple instances of a single application. A
common example of multiple instances of a single application is in deployments of
WebSphere Application Server.
With larger partitions, one of the most important performance considerations is often which
cores and memory are allocated to a partition. For partitions of eight cores or less, the
POWER Hypervisor attempts to allocate all cores for the partition from a single POWER7 chip
and attempts to allocate only memory local to the chip that is used. Those partitions generally
and automatically have good cache and memory affinity. However, it might not be possible to
obtain resources for each of the LPARs from a single chip.
For example, assume that you have a 32-core system with four chips, each with eight cores. If
five partitions are configured, each with six cores, the fifth LPAR would spread across three
chips. Start the most important partition first to obtain resources from a single chip. (The order
of starting partitions is one consideration in obtaining the best performance for high
priority workloads.).
Another example is when the partition sizes are mixed. Here, starting smaller partitions
consumes resources that are spread across many chips, resulting in larger partitions that are
spread across multiple chips, which might be contained on a chip if the larger partitions are
started first. It is a preferred practice to start higher priority partitions first, so that there is a
better opportunity for them to obtain good affinity characteristics in their core and memory
allocations. The affinity of the cores and memory that is allocated to a partition can be
determined by running the AIX lssrad -va command. For more information about partition
resource allocation and the lssrad command, see Chapter 3, "The POWER Hypervisor" on
page 55.
For partitions larger than eight cores, the partition always spans more than one chip and has
a mixture of local and remote memory. For these larger partitions, it is often useful to
manually force good affinity for an application. Manual affinity can be forced by binding
applications so that they can run only on particular cores, and by specifying to the operating
system that only local memory can be used by the application.
Consider an example where you run four instances of WebSphere Application Server on a
partition of 16 cores on a POWER7 system that is running in SMT4 mode. Each instance of
WebSphere Application Server would be bound to run on four of the cores of the system.
Because each of the cores has four SMT threads, each instance of WebSphere Application
Server is bound to 16 logical processors. Good memory and cache affinity on AIX can
therefore be ensured by completing the following steps:
1. Set the AIX MEMORY_AFFINITY environment variable, typically to the value MCM. This
setting tells the AIX operating system to use local memory when an application thread
requires physical memory to be allocated.
2. Start the four instances of WebSphere Application Server by running the following
execrset commands, which bind the execution to the specified set of logical processors:
– execrset -c 0-15 -m 0 -e
Server instance>
– execrset -c 16-31 -m 0 -e
Server instance>
Chapter 1. Optimization and tuning on IBM POWER7 and IBM POWER7+
<command to start first WebSphere Application
<command to start second WebSphere Application
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