Draft Document for Review April 7, 2004 6:15 pm

6947ch07.fm

CSS Priority Queueing uses different priorities calculated in a different way from the I/O priorities used for UCB and control unit queueing.

Value of Channel Subsystem Priority Queueing

The benefits proved by Channel Subsystem Priority Queueing include the following:

￿Improved performance

I/O from work that is not meeting its goals may be given priority over I/O from work that is meeting its goals, providing Workload Manager with an additional method for adjusting I/O performance. Channel Subsystem Priority Queueing is complimentary to UCB priority queueing and control unit priority queueing, each addressing a different queueing mechanism that may affect I/O performance.

￿Reduced skills required to manage z/OS

Monitoring and tuning requirements are reduced because of the self-tuning abilities of the Channel Subsystem.

7.5.4WLM and Channel Subsystem priority

WLM assigns the highest to lowest CSS priority as given in Table 7-4. It assigns eight priority levels.

Table 7-4 WLM-assigned CSS I/O priorities

Workload type

Priority

 

 

System work

FF

 

 

Importance 1 and 2 missing goals

FE

 

 

Importance 3 and 4 missing goals

FD

 

 

Meeting goals. Adjust by ratio of connect time to elapsed time.

F9-FC

 

 

Discretionary

F8

 

 

Work that is meeting its WLM target is assigned CSS priorities between F9 and FC, depending on its execution profile. Work that has a light I/O usage has its CSS priority moved upwards.

When an I/O operation is started by a CP on the Server, it can be queued by the Channel Subsystem for several reasons, including Switch port busy, Control unit busy, Device busy, and All channel paths busy. Queued I/O requests are started or restarted when an I/O completes or the Control unit indicates the condition has cleared. Where two or more I/O requests are queued in the Channel Subsystem, the CSS LIC on the zSeries selects the requests in priority order. The LIC also ages requests to ensure that low priority requests are not queued for excessive periods.

In the LPAR image profile for the z/OS image there are two specifications that relate to the Channel Subsystem I/O Priority Queueing. They are:

￿The range of priorities that will be used by this image

￿The default Channel Subsystem I/O priority

For images running operating systems that do not support Channel Subsystem priority, the customer can prioritize all the Channel Subsystem requests coming from that image against the other images by specifying a value for the default priority.

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IBM 990 manual WLM and Channel Subsystem priority, Value of Channel Subsystem Priority Queueing, Workload type Priority
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990 specifications

The IBM 990 series, often referred to in the context of IBM's pioneering efforts in the realm of mainframe computing, represents a unique chapter in the history of information technology. Introduced in the late 1960s, the IBM 990 series was designed as a powerful tool for enterprise-level data processing and scientific calculations, showcasing the company's commitment to advancing computing capabilities.

One of the main features of the IBM 990 was its architecture, which was built to support a wide range of applications, from business processing to complex scientific computations. The system employed a 32-bit word length, which was advanced for its time, allowing for more flexible and efficient data handling. CPUs in the IBM 990 series supported multiple instructions per cycle, which contributed significantly to the overall efficiency and processing power of the machines.

The technology behind the IBM 990 was also notable for its use of solid-state technology. This provided a shift away from vacuum tube systems that were prevalent in earlier computing systems, enhancing the reliability and longevity of the hardware. The IBM 990 series utilized core memory, which was faster and more reliable than the magnetic drum memory systems that had been standard up to that point.

Another defining characteristic of the IBM 990 was its extensibility. Organizations could configure the machine to suit their specific needs by adding memory, storage, and peripheral devices as required. This modular approach facilitated the growth of systems alongside the technological and operational demands of the business environments they served.

In terms of software, the IBM 990 series was compatible with a variety of operating systems and programming environments, including FORTRAN and COBOL, enabling users to access a broader array of applications. This versatility was a significant advantage, making the IBM 990 an appealing choice for educational institutions, research facilities, and enterprises alike.

Moreover, the IBM 990 was engineered to support multiprocessing, which allowed multiple processes to run simultaneously, further increasing its effectiveness in tackling complex computing tasks.

In summary, the IBM 990 series represents a significant advancement in computing technology during the late 20th century. With a robust architecture, versatile configuration options, and a focus on solid-state technology, the IBM 990 facilitated substantial improvements in data processing capabilities, making it a cornerstone for many businesses and academic institutions of its time. Its impact can still be seen today in the continued evolution of mainframe computing.
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