Workloads - IBM Power7 Optimization And Tuning Manual

Large page optimization
AIX allows translations of multiple memory page sizes within the same segment. Although 4
KB and 64 KB translations are allowed in the current version of AIX (Version 6.1 and greater),
Version 6.1 TL8 and Version 7.2 TL2 (4Q 2012) include dynamic 16 MB translation. For
workloads that use large chunks of data, using pages larger than the default size is useful
because the number of TLB/ERAT misses is reduced (For information about general page
size information and TLB/ERAT, see 2.3.1, "Page sizes (4 KB, 64 KB, 16 MB, and 16 GB)" on
page 25). DSO uses this new AIX feature to promote heavily used regions of memory to
16 MB pages dynamically, potentially improving the performance of workloads that use
those regions.
System V shared memory: In the current version, only System V shared memory is
eligible for dynamic 16 MB page optimization.
Memory prefetch optimization
Power Architecture provides a special purpose register (the DSCR) to control the
enablement, depth, and stride for hardware data stream prefetching (for more information,
see 2.3.7, "Data prefetching using d-cache instructions and the Data Streams Control
Register (DSCR)" on page 46). Setting this register appropriately can potentially benefit
workloads, depending on the workload data access patterns. DSO collects information from
the AIX kernel and PMU to dynamically determine the optimal setting of this register for a
specific period.

4.2.3 Workloads

For ASO/DSO to consider a workload for optimization, the workload must pass certain
minimum criteria, as described in this section.
Ideal workloads
Workload characteristics for each optimization are:
Cache affinity optimization and memory affinity optimization: Workloads must be
long-lived (the minimum lifetime varies with the type of optimization), multi-threaded, and
have stable CPU usage. The performance gain is higher for workloads that have a high
amount of communication between the threads in the workload.
Large page optimization: Workloads must use large System V memory regions, for
example, a database with a large shared memory region. Workloads can be either
multi-threaded or a group of single-threaded processes. DSO must be active when a
workload is started for this optimization to be applied.
Memory prefetch optimization: Workloads must have a large System V memory footprint,
high CPU usage, and a high context switch rate. Workloads can be either multi-threaded
or a group of single-threaded processes. This optimization is disabled if the DCSR register
is set manually at the system level (through the dscrctl command).
Eligible workloads
For ASO/DSO to consider a workload for optimization, it must pass certain minimum criteria.
These criteria are:
General requirements
Fixed priority. The ASO daemon runs with a fixed scheduler priority of 32. ASO does not
optimize a workload if it or any of its threads has a fixed priority more favorable
(numerically lower) than itself.
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Chapter 4. AIX