Buffer cache & bufpages

[GUAZZI,DAVIDE]

Hello everyone,

I set bufpages and nbuf equal to zero on my
HPUX 11.0 system.
I want to use dinamyc buffer cache with two
parameters dbc_max_pct & dbc_min_pct.
On my system I have 4GB of Ram and max_pct=50
and min_pct=5.

SAM shows to me bufpages=0
Kmtune shows to me bufpages=(NBUF*2) according
to the file /usr/conf/master.d/core-hpux
Kmtune shows to me nbuf=0

If I tried to check this value with adb I found
different value. Why?

Output of:
echo bufpages/D |adb -k /stand/vmunix /dev/kmem

shows bufpages=523776 and nbuf=524748

also sysdef shows the same situation, but
is an obsolete tool for HPUX 11.0

Could you help me to understand this behaviour.

Thanks Davide

[Stefan Farrelly]

Have a read of this;

http://docs.hp.com/cgi-bin/otsearch/getfile?id=/hpux/onlinedocs/939/KCParms/KCparam.Nbuf.html&searchterms=nbuf&queryid=20031118-023700

There is a relationship between nbuf, bufpages and dbc_mac/min_pct

Normal procedure is to leave nbuf=0 in /stand/system, bufpages=(NBUF*2) and let dbc_max_pct and dbc_min_pct control buffer size. Normally buffer cache should only be a max size of around 400MB so set dbc_mac_pct accordingly depending on your physical ram size.

[Stefan Farrelly]

oops, forgive my typos, dbc_mac_pct should read dbc_max_pct.

[GUAZZI,DAVIDE]

Hello Stefan,

I know perfectly the metod to set these
parameters. My question is regarding
the diffrent output from kmtune and adb or
sysdef.
On my system bufpages is equal to zero
(NBUF*2) nbuf is equal to zero.

Why adb shows bufpage equal to 523776 and
nbuf equal to 524748?
Thanks
Davide

[Stefan Farrelly]

Hi Davide,

bufpages shows the current size of the buffer cache - as controlled by dbc_max and dbc_min_pct (if set to 0 in the system file). You can see it go up and down, ie. boot your server, adb will show bufpages nice and low, and it will slowly grow to the dbc_max_pct size (ie. RAM=4096, dbc_mac_pct = 10%, bufpages=104857 *4= 429MB). nbuf is similar, but im not sure of the relationship size wise to bufpages in this case.