IBM z13s Technical Manual page 141

PR/SM has control over all physical memory, and can so make physical memory available to
the configuration when a CPC drawer is added.
In prior z Systems processors, memory allocation was striped across the available CPC
drawers because there was relatively fast connectivity between the drawers. Splitting the
work between all of the memory controllers allowed a smooth performance variability.
With the z13 and z13s memory design, the memory allocation algorithm has changed.
PR/SM tries to allocate memory to a single CPC drawer, striped between the two nodes.
Basically, the PR/SM memory and logical processor resources allocation goal is to place all
partition resources on a single CPC drawer, if possible.
The resources, like memory and logical processors, are assigned to the logical partitions at
the time of their activation. Later on, when all partitions are activated, PR/SM can move
memory between CPC drawers to benefit each LPARs performance, without OS knowledge.
This process was done on the previous families of z Systems only for PUs that used PR/SM
dynamic PU reallocation.
With z13 and z13s servers, this process occurs each time the configuration changes,
including these examples:
Activating an LPAR
Deactivating an LPAR
Changing the LPARs' processing weights
Upgrading the system through a temporary or permanent record
Downgrading the system through a temporary record
PR/SM schedules a global reoptimization of the resources in use. It does so by looking at all
the partitions that are active and prioritizing them based on their processing entitlement and
weights, creating a high and low priority rank. Then the resources, such as logical processors
and memory, can be moved from one CPC drawer to another to address the priority ranks
that have been created.
When partitions are activated, PR/SM tries to find a home assignment CPC drawer, home
assignment node, and home assignment chip for the logical processors that are defined to
them. The PR/SM goal is to allocate all the partition logical processors and memory to a
single CPC drawer (the home drawer for that partition). If all logical processors can be
assigned to a home drawer and the partition defined memory is greater than what is available
in that drawer, the exceeding memory amount is allocated on another CPC drawer. If all the
logical processors cannot fit in one CPC drawer, the remaining logical processors spill to
another CPC drawer and, when that happens, PR/SM stripes the memory, if possible, across
the CPC drawers where the logical processors are assigned.
The process of reallocating memory is based on the
was used to allow enhanced book availability (EBA) and concurrent book replacement (CBR)
in previous z Systems families. However, this process has been enhanced to provide more
efficiency and speed to the process without affecting system performance. PR/SM controls
the reassignment of the content of a specific physical memory array in one CPC drawer to a
physical memory array in another CPC drawer. To do that, PR/SM uses all the available
physical memory in the system. This memory includes the memory not in use by the system,
that is available but not purchased by the client, and the planned memory options, if installed.
Different sized DIMMs are available for use (16 GB, 32 GB, 64 GB, or 128 GB). On a z13s
server, the CPC drawer design allows for only same-sized DIMMs to be used in one particular
CPC drawer. However, two CPC drawer z13s machines can have DIMMs of a different size
installed in the second CPC drawer. If the memory fails, it is technically feasible to run a POR
of the system with the remaining working memory resources. After the POR completes, the
memory copy/reassign
Chapter 3. Central processor complex system design
function, which
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