Dynamic Buffer Cache

[Tiong-Lip Chua_1]

The dbc_max_pct and dbc_min_pct were assigned with value in order to activate dynamic buffer cache. I use HP OPENVIEW to monitor the memory usage and I notice that the percentile for the GBL_MEM_SYS_AND_CACHE_UTIL remains constant, even when the system is idle.

Any explaination?

Regards
Tiong-Lip Chua

[randy lee]

the buffer cache allocates so much memory, that is why it is constant.
ie: if dbc_min_pct=5 and dbc_max_pct=10
The buffer cache will stay at 10% of memory.

[Alan Riggs]

It is possible that you have simply filled all available buffer cache and thus see the same value (dbc_max_pct) every time. It is also bossible that there is a value set for either nbuf or bufpages in the kernel. If either of these is set to a non-zero value it will override dynamic caching.

[Navied Yousuf]

Tiong,
Dynamic Buffer Cache actually allocated on the system will vary between dbc_min_pct and dbc_max_pct bounds. If it stays constant it is good performance wise because the system does not have to adjust it to meet the other memory demands. However, you must make sure that nbuf is set to 0 and bufpages is also 0. Normally bufpages is set to 2*nbuf. These parameters if non zero will activate Static Buffer Cache.
Cheers.

[Jason Luginbuhl]

As Randy mentions, althought DBC is supposed to allow the buffer cache size to fluctuate between the lower limit (dbc_min_pct) and upper limit (dbc_max_pct) in most situations the buffer cache seems to gravitate towards the maximum. It takes a quite significant amount of "idle" time to see this level move back down toward the minimum. As such, spend a lot more time about tuning your dbc_max_pct then your dbc_min_pct because it will likely be at or near the maximum in most environments.

[Emil Velez]

The buffer cache will only decrease when you get low on memory that will be the only time when it needs to get smaller otherwise your buffer cache should always be around its dbc_max