Asymmetric Multiprocessing

Enhanced Definition

Asymmetric Multiprocessing (AMP) is a system architecture where different processors are assigned specific, specialized tasks and are not interchangeable. In the mainframe context, this often means one or more processors are dedicated to particular functions like I/O handling or system control, while others manage general-purpose computing. This specialization aims to optimize resource utilization and ensure critical system functions are handled efficiently.

Key Characteristics

- Specialized Processor Roles: Each processor or group of processors is dedicated to a specific function (e.g., I/O processing, system management, general computation).
- Master/Slave Relationship (Historical): Often involved a master processor controlling the system and dispatching tasks to slave processors, which performed specific functions.
- Dedicated Workloads: Processors are optimized for particular types of workloads, leading to efficient execution of those specific tasks.
- Reduced Contention: By separating workloads, contention for shared resources like memory or I/O channels can be minimized.
- Complexity in Configuration: Requires careful system design and configuration to ensure tasks are correctly routed to the appropriate specialized processor.
- Precursor to SMP: More prevalent in earlier mainframe architectures before the widespread adoption of highly symmetric multiprocessing (SMP).

Use Cases

- Early Mainframe I/O Processing: Historically, dedicated channel processors or I/O processors were used to manage complex I/O operations, offloading this overhead from the main CPU.
- Specialty Engines in z/OS: Modern z/OS environments utilize specialty engines like zIIP (z Integrated Information Processor) which are a form of asymmetric processing, dedicated to specific workloads (e.g., Java, XML, DB2 queries) to offload them from general-purpose CPs.
- System Control Functions: In some architectures, a processor might be dedicated solely to operating system overhead, interrupt handling, or critical system management functions to ensure stability.

Related Concepts

Asymmetric Multiprocessing stands in contrast to Symmetric Multiprocessing (SMP), where all processors are identical and can execute any task. While modern z/OS systems are primarily SMP for general-purpose workloads, the concept of AMP persists through Specialty Engines (like zIIPs), which represent a targeted form of asymmetric processing. The z/OS Dispatcher and Workload Manager (WLM) must understand and manage these specialized processors to effectively schedule and dispatch tasks to the appropriate engine.

Best Practices:

Leverage Specialty Engines: For modern z/OS, configure and optimize applications to utilize zIIP engines for eligible workloads to reduce general-purpose CPU consumption and associated costs.
Workload Segregation: Design applications and system configurations to effectively segregate workloads that can benefit from specialized processing.
Monitor Processor Utilization: Continuously monitor the utilization of both general-purpose and specialty engines to ensure optimal load balancing and identify potential bottlenecks.
Understand Hardware Capabilities: Be thoroughly familiar with the specific roles and capabilities of each processor type within an AMP or hybrid (AMP/SMP) system.
System Tuning: Regularly tune the system and application parameters to ensure that tasks are correctly assigned and executed efficiently on their designated processors.