i/o polling process count

i/o polling process count specifies the maximum number of processes that SAP ASE can run before the scheduler checks for disk and network I/O completions. Tuning i/o polling process count affects both the response time and throughput of SAP ASE.

Summary Information
Default value	10
Range of values	1 – 2147483647
Status	Dynamic
Display level	Comprehensive
Required role	System administrator
Configuration group	SQL Server Administration

Note: i/o polling process count functions only when you configure SAP ASE for process kernel mode; it is nonfunctional for threaded kernel mode.

SAP ASE checks for disk or network I/O completions:

If the number of tasks run since the last time SAP ASE checked for I/O completions equals the value for i/o polling process count, and
At every SAP ASE clock tick.

As a general rule, increasing the value of i/o polling process count increases throughput for applications that generate a lot of disk and network I/O. Conversely, decreasing the value improves process response time in these applications, possibly at the risk of lowering throughput.

If your applications create both I/O and CPU-bound tasks, tuning i/o polling process count to a low value (1 – 2) ensures that I/O-bound tasks get access to CPU cycles.

For OLTP applications (or any I/O-bound application with user connections and short transactions), tuning i/o polling process count to a value in the range of 20 – 30 may increase throughput, but may also increase response time.

When tuning i/o polling process count, consider these other parameters:

sql server clock tick length, which specifies the duration of the SAP ASE clock tick in microseconds.
time slice, which specifies the number of clock ticks the the SAP ASE scheduler allows a user process to run.
cpu grace time, which specifies the maximum amount of time, in clock ticks, a user process can run without yielding the CPU before SAP ASE preempts it and terminates it with a timeslice error.

Use sp_sysmon to determine the effect of changing i/o polling process count. See the Performance and Tuning Series: Monitoring SAP Adaptive Server with sp_sysmon.