Configuring a Cache Replacement Policy

If a cache is dedicated to a table or an index, and has little or no buffer replacement when the system reaches a stable state, you can set the relaxed LRU (least recently used) replacement policy. This policy may improve performance for caches where there is little or no buffer replacement occurring, and for most log caches.

See Performance and Tuning Series: Basics > Memory Use and Performance.

To set relaxed replacement policy, use:

sp_cacheconfig pubs_log, relaxed

The default value is “strict.” The cache replacement policy and the asynchronous prefetch percentage are optional, but, if specified, they must have correct parameters or “DEFAULT”.

Note: Setting the cache replacement policy is static, requiring a restart of SAP ASE to take effect.

You can create a cache and specify its cache type and the replacement policy in one command. These examples create two caches, pubs_log and pubs_cache:

sp_cacheconfig pubs_log, "3M", logonly, relaxed

sp_cacheconfig pubs_cache, "10M", mixed, strict

Here are the results:

sp_cacheconfig

Cache Name                      Status    Type     Config Value Run Value
------------------------ --------- -------- ------------ ------------
default data cache       Active    Default      25.00 Mb     42.29 Mb
pubs_cache               Active    Mixed        10.00 Mb     10.00 Mb
pubs_log                 Active    Log Only      7.00 Mb      7.00 Mb
                                            ------------ ------------
                                     Total      42.00 Mb     59.29 Mb
=======================================================================
Cache: default data cache,   Status: Active,   Type: Default
     Config Size: 25.00 Mb,   Run Size: 42.29 Mb
     Config Replacement: strict LRU,   Run Replacement: strict LRU
     Config Partition:            1,   Run Partition:            1

IO Size  Wash Size Config Size  Run Size     APF Percent
-------- --------- ------------ ------------ -----------
    2 Kb   8662 Kb      0.00 Mb     42.29 Mb     10     
=======================================================================
Cache: pubs_cache,   Status: Active,   Type: Mixed
     Config Size: 10.00 Mb,   Run Size: 10.00 Mb
     Config Replacement: strict LRU,   Run Replacement: strict LRU
     Config Partition:            1,   Run Partition:            1

IO Size  Wash Size Config Size  Run Size     APF Percent
-------- --------- ------------ ------------ -----------
    2 Kb   2048 Kb      0.00 Mb     10.00 Mb     10
=======================================================================
Cache: pubs_log,   Status: Active,   Type: Log Only
     Config Size: 7.00 Mb,   Run Size: 7.00 Mb
     Config Replacement: relaxed LRU,   Run Replacement: relaxed LRU
     Config Partition:            1,   Run Partition:            1

IO Size  Wash Size Config Size  Run Size     APF Percent
-------- --------- ------------ ------------ -----------
    2 Kb   1432 Kb      0.00 Mb      7.00 Mb          10