NLRUQ parameter

# of DKBM LRU queues

Summary

Default value

1, also the minimum

Parameter type

System

Where set

Not resettable

Related products

All

Introduced

Model 204 V6.1

Description

The NLRUQ parameter reduces contention on the MP lock that protects the reuse queues.

This MP lock is a primary source of MP overhead. Setting this parameter to greater than 1 has no benefit in a non-MP run. In fact, in all cases a setting of greater than 1 might result in a slight increase in disk I/O, due to a loss in precision in the least-recently used algorithm for managing the disk buffer pool. With NLRUQ greater than 1, it would be possible for a page to be stolen from the buffer pool before one that was less recently used.

NLRUQ=1 preserves the single reuse queue model used in releases of Model 204 before V6R1.0.

In the following formula NLRUQ is used as a multiplier to determine the minimum number of disk buffers used in an Online.

MINUF >= NLRUQ * ((NSERVS + NSUBTKS) * MAXOBUF + 15)

A reasonable guess for a proper NLRUQ would be adding two for each MP subtask so that a NMPSUBS=1 would suggest NLRUQ=3 and NMPSUBS=7 would suggest NLRUQ=15.

If NLRUQ is set greater than 1, then the values of LDKBMWND, MAXBUF, and MINBUF are all rounded up to a multiple of NLRUQ. The maximum setting for NLRUQ is 255.

A high conflict count for LRU queue locks that would appear in MONITOR MPLOCK command output indicates that you should increase NLRUQ and NLRUQG.