Setting of dbc_max_pct and dbc_min_pct

[Pavel Krumnikl]

Hi all,

We have our application on HP-UX consuming more CPU than we would expect. I am looking for options to optimize the performance. Currently I'm looking into dbc_max_pct and dbc_min_pct parameters. We have set to 2/50.
We have 8 GB RAM.

SAR output is:

$ sar -b 1 10

HP-UX VTC3OMG1 B.11.11 U 9000/800 12/01/09

09:17:13 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
09:17:14 0 210 100 18 198 91 0 0
09:17:15 0 145 100 20 179 89 0 0
09:17:16 0 162 100 21 197 89 0 0
09:17:17 0 150 100 25 182 86 0 0
09:17:18 0 182 100 18 211 91 0 0
09:17:19 0 161 100 19 194 90 0 0
09:17:20 0 154 100 29 188 85 0 0
09:17:21 0 164 100 12 208 94 0 0
09:17:22 0 163 100 27 191 86 0 0
09:17:23 0 184 100 103 223 54 0 0

Average 0 167 100 29 197 85 0 0

Should we modify the buffer cache parameters?
Thanks a lot
Pavel

[Rita C Workman]

I am going to presume that 2/50 means min=2 and max=50.

We have written about this subject many times, so you may want to search for other threads....

50 is the default setting for dbc_max and it is one of the first parms you should modify.

If you have 8GB of ram, try around 12-15 for dbc_max and maybe set dbc_min=5.

There are other parms you may want to adjust, but we'd have to know where you are before going farther.

Regards,
Rita

[Patrick Wallek]

I definitely would. I would set dbc_max_pct to a value between 5 and 10, which would give you 400 MB to 800 MB of buffer cache rather than 4 GB.

[Dennis Handly]

>We have our application on HP-UX consuming more CPU than we would expect. I am looking for options to optimize the performance.

Why do you think that reducing the buffer cache will help CPU performance?
Reducing the buffer cache will help if you can use the memory for something else.

[Taifur]

Hi,

During file-system I/O operations, data is stored in a buffer cache, the size of which can be fixed or dynamically allocated. When the parameters dbc_min_pct and dbc_max_pct are different, the size of the buffer cache grows or shrinks dynamically depending on competing requests for system memory.

The value of dbc_max_pct specifies the maximum percentage of physical memory that may be consumed by the dynamic buffer cache. The minimum amount of memory represented by the related tunable parameter dbc_min_pct is permanently consumed by the buffer cache. As needed, the dynamic buffer cache will consume memory to grow up to the maximum limit represented by dbc_max_pct if the physical memory is available, depending on competing requests for physical memory.

By setting both dbc_min_pct and dbc_max_pct to the same value the kernel creates a fixed size buffer cache that uses exactly that percentage of physical memory and does not change in size.

Check below link,
http://docs.hp.com/en/B2355-60105/dbc_max_pct.5.html

Regards//
Taifur