Selective enablement for I/O

Selective enablement for I/O is a function that SRM uses to control the number of processors that are enabled for I/O interruptions. The intent of this function is to enable only the minimum number of processors needed to handle the I/O interruption activity without the system incurring excessive delays. That is, if one processor can process the I/O interruptions without excessive delays, then only one processor need be enabled for I/O interruptions.

At system initialization, one processor is enabled for I/O interruptions. To determine if a change should be made to the number of processors that are enabled, SRM periodically monitors I/O interruptions.

By comparing this value to threshold values, SRM determines if another processor should be enabled or if an enabled processor should be disabled for I/O interruptions. If the computed value exceeds the upper threshold, I/O interruptions are being delayed, and another processor (if available) will be enabled for I/O interruptions. If the value is less than the lower threshold (and more than one processor is enabled), a processor will be disabled for I/O interruptions. The installation can change the threshold values using the CPENABLE parameter in the IEAOPTxx parmlib member.

A processor that enters a wait state is always enabled for I/O interruptions, however, regardless of what you specify for the CPENABLE keyword.

In addition to enabling a processor when I/O activity requires it, SRM also enables another processor for I/O interruptions if one of the following occurs:

An enabled processor is taken offline.
An enabled processor has a hardware failure.
SRM detects that no I/O interruptions have been taken for a predetermined period of time and concludes that the enabled processor is unable to accept interrupts.

An installation can use the CPENABLE keyword to specify low and high thresholds for the percentage of I/O interruptions to be processed through the test pending interrupt (TPI) instruction. SRM uses these thresholds to determine if a change should be made to the number of processors enabled for I/O interruptions.

The following chart gives the internal names of the control variables and indicates their relation to the condition.

Table 1. Summary of variables used to determine if changes are needed to the number of processors enabled for I/O interruptions
Control variable and internal name	Percentage of I/O Interruptions		Keyword in OPT
Control variable and internal name	under	over	Keyword in OPT
Percentage of I/O interruptions through TPI instruction (ICVTPIP)	<ICCTPILO	>ICCTPIHI	CPENABLE

Table 2 relates SRM seconds to real time. The SRM constants that are shown in this table are merely generalizations and approximations. For more accurate comparisons of processors, see the internal throughput rate (ITR) numbers in Large Systems Performance Reference (LSPR), SC28-1187.

Table 2. Relating SRM seconds to real time
Processor Model	SRM Seconds/Real Seconds
Processors: zSeries® 990
zSeries 990 Models 301 - 332	508.1298
Processors: zSeries 900
zSeries 900 Models 101–109	269.3964
zSeries 900 Models 110–116, 1C1–1C9	281.5314
Processors: zSeries 890
zSeries 890 Models 110, 210, 310, 410	29.4117
zSeries 890 Models 120, 220, 320, 420	52.0399
zSeries 890 Models 130, 230, 330, 430	99.5222
zSeries 890 Models 140, 240, 340, 440	124.2544
zSeries 890 Models 150, 250, 350, 450	194.0993
zSeries 890 Models 160, 260, 360, 460	236.7423
zSeries 890 Models 170, 270, 370, 470	413.9071
Processors: zSeries 800
zSeries 800 Model 0E1	45.4545
zSeries 800 Model 0A1	90.8430
zSeries 800 Model 0X2	98.5804
zSeries 800 Model 0B1	130.4801
zSeries 800 Model 0C1	162.3376
zSeries 800 Model 0A2	160.2563
zSeries 800 Model 001 - 004	217.0138
Processors: S/390® 9672 G6 Models
S/390 Models 9672-X17 - 9672-XZ7	194.0993
S/390 Models 9672-Z17 - 9672-ZZ7	224.8200
Processors: S/390 9672 G5 Models
S/390 Model 9672-R16	129.9376
S/390 Model 9672-R26	129.9376
S/390 Models 9672-R36 - 9672-R96	141.0834
S/390 Models 9672-RA6, 9672-RB6	97.9623
S/390 Models 9672-RC6, 9672-RD6	129.9376
S/390 Models 9672-RX6, 9672-T16, 9672-T26	141.0834
S/390 Models 9672-Y16 - 9672-YX6	168.4635
Processors: S/390 3000
S/390 3000 Model A10	41.6666
S/390 3000 Model A20	39.9872
Processors: S/390 2003
S/390 2003 Model 107	27.8520
S/390 2003 Model 124 (All Models)	30.3988 (per CPU)
S/390 2003 Model 1C5 (All Models)	37.6279 (per CPU)
S/390 2003 Model 2X7 (All Models)	46.0914 (per CPU)
S/390 2003 Model 203	6.1814
S/390 2003 Model 204	10.3203
S/390 2003 Model 205	14.4375
S/390 2003 Model 206	18.5680
S/390 2003 Model 207	27.8520
S/390 2003 Model 215	33.3333
S/390 2003 Model 216	41.6666
S/390 2003 Model 224 (All Models)	30.5325 (per CPU)
S/390 2003 Model 225 (All Models)	39.9872 (per CPU)
S/390 2003 Model 246 (All Models)	41.1455 (per CPU)
S/390 2003 Model 2C5 (All Models)	37.6279 (per CPU)