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 NLRUQ greater than 1 has no benefit in a non-MP run. In fact, in all cases, a setting greater than 1 might result in a slight increase in disk I/O, owing to a loss in precision in the least-recently-used algorithm for managing the below the bar (BTB) disk buffer pool. With NLRUQ greater than 1, it would be possible for a page to be stolen from the BTB buffer pool before one that was less recently used.

Note: As of version 7.7 of Model 204, whether BTB buffers are used at all depends entirely on the setting of NUMBUFG, which activates buffer storage above the bar.

• If NUMBUFG is greater than 0, all buffer pool buffers are forced above the bar — no buffers are below the bar. The MINBUF and NUMBUF parameters are forced to 0, and the other parameters that affect BTB storage (MAXBUF, SPCORE, LDKBMWND, and NLRUQ) are ignored.
• If NUMBUFG is set to 0, all buffers are below the bar, and the settings of the BTB-related parameters are respected and calculated as in pre-7.7 versions.

If BTB buffers are used

When BTB buffers are being used (that is, NUMBUFG is 0 under version 7.7 or higher, or NUMBUFG is any value under Model 204 prior to 7.7):

• NLRUQ=1 preserves the single reuse queue model used in releases of Model 204 before V6R1.0.
• NLRUQ is used as a multiplier to determine the minimum number of BTB disk buffers used in an Online:

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

• A reasonable guess for a proper NLRUQ value adds two for each MP subtask, so a setting of NMPSUBS=1 suggests NLRUQ=3, and NMPSUBS=7 suggests NLRUQ=15.
• If NLRUQ is set greater than 1, the values of LDKBMWND, MAXBUF, and MINBUF are all rounded up to a multiple of NLRUQ. The maximum setting for NLRUQ is 255.
• If a MONITOR MPLOCK command output reveals a high conflict count for LRU queue locks, increase NLRUQ and NLRUQG.