Chapter 3. Central processor complex system design 115
resolve all the memory it needs. Therefore, a long-running process can benefit even without
frequent memory access. Weigh the benefits of whether something in this category must use
large pages as a result of the system-level costs of tying up real storage. You must balance
the performance of a process using large pages with the performance of the remaining work
on the system.
On z13s servers, 1 MB large pages become pageable if Flash Express is available and
enabled. They are available only for 64-bit virtual private storage, such as virtual memory
above 2 GB.
It is easy to assume that increasing the TLB size is a feasible option to deal with TLB-miss
situations. However, this process is not as straightforward as it seems. As the size of the TLB
increases, so does the processor usage that is involved in managing the TLB contents.
Correct sizing of the TLB is subject to complex statistical modeling to find the optimal tradeoff
between size and performance.
3.6.2 Main storage
Main storage is addressable by programs, and storage that is not directly addressable by
programs. Non-addressable storage includes the hardware system area (HSA).
Main storage provides these functions:
Data storage and retrieval for PUs and I/O
Communication with PUs and I/O
Communication with and control of optional expanded storage
Error checking and correction
Main storage can be accessed by all processors, but cannot be shared between LPARs. Any
system image (LPAR) must have a main storage size defined. This defined main storage is
allocated exclusively to the LPAR during partition activation.
3.6.3 Expanded storage
Expanded storage can optionally be defined on z13s servers. Expanded storage is physically
a section of processor storage. It is controlled by the operating system, and transfers 4-KB
pages to and from main storage.
Storage considerations
Except for z/VM, z/Architecture operating systems do not use expanded storage. Because
they operate in 64-bit addressing mode, they can have all the required storage capacity
allocated as main storage. z/VM is an exception because even when it operates in 64-bit
mode, it can have guest virtual machines that are running in 31-bit addressing mode. The
guest systems can use expanded storage. In addition, z/VM uses expanded storage for its
own operations.
Defining expanded storage to a coupling facility image is not possible. However, any other
image type can have expanded storage that is defined, even if that image runs a 64-bit
operating system and does not use expanded storage.
z/VM 6.3: Starting in z/VM 6.3, it is no longer preferable to configure any of the memory as
expanded storage unless it is being attached to a virtual machine for testing or some
unique purpose. Generally, configure all the processor memory as main storage.
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