HiperDispatch
Encyclopedia
HiperDispatch is a workload dispatching feature found in the newest IBM mainframe
models (the System z10 machines and presumably their successors) running recent releases of z/OS
. HiperDispatch was introduced in February, 2008.
One of the engineering challenges with large SMP
server designs is to maintain near-linear scalability
as the number of CPUs increases. Performance and throughput do not double when doubling the number of processors. There are many overhead factors, including contention for cache and main memory access. These overhead factors become increasingly difficult to mitigate as the number of CPUs increases. The design goal for delivering maximum performance is to minimize those overhead factors. Each new mainframe model supports a higher maximum number of CPUs (up to 64 main processors in a single System z10 mainframe for example), so this engineering challenge becomes ever more important.
HiperDispatch helps address the problem through a combination of hardware features, z/OS dispatching, and the z/OS Workload Manager. In z/OS there may be tasks waiting for processing attention, such as transaction programs. Each task often requires access to memory. In a large SMP design such as System z, some CPUs are physically "closer" with faster access to cache memory that might hold supporting data for particular tasks. HiperDispatch exploits this fact and steers tasks to the CPUs most likely to have the fastest access to relevant data already in cache. If that particular CPU is busy, HiperDispatch will, at first, wait for it to finish its other task, even if another less favorable CPU is idle. However, there are limitations to how patient HiperDispatch will be, as governed by Workload Manager goals. If z/OS Workload Manager senses that there's a risk the pending task will miss its service level (responding within a certain number of milliseconds to a user request for example), Workload Manager and HiperDispatch will send the task over to an idle CPU for processing, even if that CPU must fetch data from slower main memory.
HiperDispatch offers very little benefit on System z10 mainframes configured with a relatively small number of CPUs. However, the feature does help quite a lot as the CPU count increases. IBM mainframe capacity tables (and thus its software pricing) are all based on the assumption that HiperDispatch is active.
Workload Manager (WLM) must be configured correctly for HiperDispatch to work well. Some mainframe users have latent problems with their WLM goal settings which are only exposed with HiperDispatch, so there is an option to disable HiperDispatch in those cases where mainframe users do not want to correct those issues right away.
IBM mainframe
IBM mainframes are large computer systems produced by IBM from 1952 to the present. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare...
models (the System z10 machines and presumably their successors) running recent releases of z/OS
Z/OS
z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...
. HiperDispatch was introduced in February, 2008.
One of the engineering challenges with large SMP
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
server designs is to maintain near-linear scalability
Scalability
In electronics scalability is the ability of a system, network, or process, to handle growing amount of work in a graceful manner or its ability to be enlarged to accommodate that growth...
as the number of CPUs increases. Performance and throughput do not double when doubling the number of processors. There are many overhead factors, including contention for cache and main memory access. These overhead factors become increasingly difficult to mitigate as the number of CPUs increases. The design goal for delivering maximum performance is to minimize those overhead factors. Each new mainframe model supports a higher maximum number of CPUs (up to 64 main processors in a single System z10 mainframe for example), so this engineering challenge becomes ever more important.
HiperDispatch helps address the problem through a combination of hardware features, z/OS dispatching, and the z/OS Workload Manager. In z/OS there may be tasks waiting for processing attention, such as transaction programs. Each task often requires access to memory. In a large SMP design such as System z, some CPUs are physically "closer" with faster access to cache memory that might hold supporting data for particular tasks. HiperDispatch exploits this fact and steers tasks to the CPUs most likely to have the fastest access to relevant data already in cache. If that particular CPU is busy, HiperDispatch will, at first, wait for it to finish its other task, even if another less favorable CPU is idle. However, there are limitations to how patient HiperDispatch will be, as governed by Workload Manager goals. If z/OS Workload Manager senses that there's a risk the pending task will miss its service level (responding within a certain number of milliseconds to a user request for example), Workload Manager and HiperDispatch will send the task over to an idle CPU for processing, even if that CPU must fetch data from slower main memory.
HiperDispatch offers very little benefit on System z10 mainframes configured with a relatively small number of CPUs. However, the feature does help quite a lot as the CPU count increases. IBM mainframe capacity tables (and thus its software pricing) are all based on the assumption that HiperDispatch is active.
Workload Manager (WLM) must be configured correctly for HiperDispatch to work well. Some mainframe users have latent problems with their WLM goal settings which are only exposed with HiperDispatch, so there is an option to disable HiperDispatch in those cases where mainframe users do not want to correct those issues right away.
See also
- HiperSocketHiperSocketHiperSockets is an IBM technology for high-speed communications between partitions on a server with a hypervisor. The term is most commonly associated with zSeries, System z9 and System z10 mainframes which can provide in-memory TCP/IP connections between and among LPARs running several different...
- LPARLPARA logical partition, commonly called an LPAR, is a subset of computer's hardware resources, virtualized as a separate computer. In effect, a physical machine can be partitioned into multiple logical partitions, each hosting a separate operating system....
- Non-Uniform Memory AccessNon-Uniform Memory AccessNon-Uniform Memory Access is a computer memory design used in Multiprocessing, where the memory access time depends on the memory location relative to a processor...
- IBM Parallel SysplexIBM Parallel SysplexIn computing, a Parallel Sysplex is a cluster of IBM mainframes acting together as a single system image with z/OS. Used for disaster recovery, Parallel Sysplex combines data sharing and parallel computing to allow a cluster of up to 32 systems to share a workload for high performance and high...