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:
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 you see a high conflict count for LRU queue locks in MONITOR MPLOCK command output, increase NLRUQ and NLRUQG.