dbc_max_pct kernel paramter

[Aftab Sadiq]

We have N-Class Server with 2GB physical memory running HP-UX11.0(64-bit). I have changed the value of dbc_max_pct paramter to 20% from 50% due to low free memory. Now I see buffer cache 129.5mb which is about 5% of total memory. Before changing that parameter value of buffer cache was 1024mb which is exactly 50%. Here is output from glanceplus:

Total VM: 235.2mb phys mem: 2.00gb system mem: 141.5mb Active VM: 192.6mb buff cache: 128.5mb user mem: 384.4mb Free mem: 1.37gb

it seems that VM is used even lot of free memory is available. Do I need to set any other kernel parameter to correct buff cache value and VM usage?

some other kernel parameter setting:
dbc_min_pct=5
bufpages=0
nbuf=0
nproc=12820
ncallout=12836

Any help would be appreciated.

Thanks

[Andreas Voss]

Hi,

you don't have to be worried about that.
Buffer cache is filled up dynamically.
So after reboot the value is at the dbc_min_pct. If the system needs more buffer cache the value is increased until it reaches the dbc_max_pct.
So have a look at that after your machine goes busy. I bet the value grows dynamically.

Regards

[CHRIS_ANORUO]

To set the dbc_max_pct do (echo "bufpages"/D|adb -k /stand/vmunix /dev/kmem), this will give you the exact value of your system bufpages.
You should also set swapmem_on=1 (from kernel parameters) to get the effect of setting nbuf and bufpages to zero.

[Stefan Farrelly]

Here are the rules you seek;


nbuf sets system-wide number of file-system buffer and cache buffer headers.

The value for nbuf if selected 0 then it offers dynamic buffer cache.

Another parameter to control this is bufpages.

Relation betwn the 2 is as follows.

bufpages = 0, nbuf = 0: Enables dynamic buffer cache.
bufpages = X, nbuf = 0, Creates X/2 buffer headers & allocates X * 4 K of
buffer pool space at system boot.
bufpages = 0, nbuf = Y: Creates Y buffer headers & allocates Y*2 * 4K of
buffer pool space at system boot.
bufpages = X, nbuf = Y, Creates Y buffer headers & allocates X *4K of
buffer pool space at system boot.

sysdef shows the current value hence the difference.

However, I wouldnt recommend you run your buffer cache any higher than 400-500Mb as if its larger than this it actually slows down your server due to the effort required by HP-UX to administer such a large cache.

[Aftab Sadiq]

Buffer cache reached to its max when system get busy as Adreas mentioned it allocate dynamically. Why VM used when there is not much activity and free mem is available?

[Stefan Farrelly]

Follow this thread for more detailed info on VM and swap/paging;

http://www.dutchworks.nl/htbin/hpsysadmin?h=3&dn=45410&q=what%20is%20virtual%20memory&fh

(The above link may be wrapped on > 1 line in which case you need to paste it back to 1 line first)