tuneable parameters differ when listed using sysdef and sam

[SURI HUNJAN]

I configured a paramater - dbc_max_pct to a 1 through sam and the kernel rebuilt itself and system rebooted successfully.
When re-viewing the parameter thro' sam - it was set to 1 but when viewed thro' sysdef - it was still listed as 2 ( default)
This is on a N -Class - N4000 running hpux-11.
Subsequently I have found that the minimum value should be a 2.
But why does SAM and sysdef listing differ.

[Praveen Bezawada]

Hi
try using absolute debugger

echo "dbc_max_pct/D" | adb -k /stand/vmunix /dev/kmem

This shoud give you the correct value. We had this problem once and the value displayed in SAM turned out to be the correct one.

...BPK...

[linuxfan]

Hi,

On 11.0 you can use kmtune to determine the value of a kernel parameter (kmtune -l -q dbc_max_pct) to determine the default and current value of dbc_max_pct. or you could use the absolute debugger(adb).

dbc_max_pct defines the maximum percentage of memory to be used by dynamic buffer cache.

The minimum value for dbc_max_pct is 2 and the maximum is 90, the default is 50.

http://docs.hp.com/hpux/onlinedocs/os/11i/kcparams/KCparam.DBCmaxPct.html

-Regards
Ramesh

[Marc Ahrendt]

i have seen this as well and called HP but it was respect to other parameters
...the bottom line is that "sysdef" is not reliable, and HP is aware of this

i usually never use "sam", but find myself using sam for seeing kernel parameter settings. yet, as already mentioned, "adb" is also another option that has never given me bad output like "sysdef"

bottom line is that i do not use "sysdef" at all, only "sam" and "adb"

[linuxfan]

Hi,

Do you have the sysdef patch PHCO_22035, i know it definitely fixes the issue with max*siz parameters. Also like others have already said, i generally don't rely on sysdef.


-Regards
Ramesh

[Tom Danzig]

I believe SAM get the parameters directly from the kernel, however, sysdef get the values defined in the /stand/system file.

[David Lieberman_1]

S,

A bit of a tangent-

I am curious as to why you are setting dbc_max_pct so low. Are you trying to simply eliminate the DBC, or do you have a very large amount of RAM?

As you may or may not know, besides the "50%" problem with default installs, the DBC algorithm contends with the HPUX Memory Manager vhand() and really chews up CPU cycles - particularly when free RAM is in short supply. If you'd like an explanation, just ask; it's not a book, but it's not exactly a short story either.

Anyway, you may wish to get rid of the DBC altogether and set a fixed-size filesystem buffer cache (same function, just no overhead and it doesn't thrash, er, resize) by setting max_dbc_pct, min_dbc_pct, and nbuf to 0, and setting bufpages to something like 76800 (300MB) unless it's an NFS, ftp, web server etc. One of HP's Senior Technical Consultants - Stephen Ciullo provided me some great information on this earlier this year. It's worked well for us.

Regards,
David Lieberman

[SURI HUNJAN]

Thanks to everyone for their replies - has cleared some points.
1: The PATCH listed above is not installed - but will look into this.
2: system has 3GB RAM hence the reason for such a low dbc_max_pct value.
3: I have checked the system config again -
According to SAM value=1
According to sysdef value=2 (default)
According to adb value=2
According to Kmtune value=1
Value in /stand/system = 1
Hence it points to the fact that sysdef and adb seem to be giving the correct value.
SAM seem to be reading the value off the /stand/system file.
Is it true to say that the kernel will not configure an unacceptable value (e.g. 1) but will substitute it with the minimum acceptable value.

cheers
Surinder

[linuxfan]

Hi,

Both sam and kmtune are getting the information from /stand/system. you can give arguments (-S) to kmtune to look at different system files though.

what is your dbc_min_pct set to?


-Ramesh

[Michael Tully]

Hi,

Also have a look at /var/sam/boot.config This file will display information like this:

KC_PARAM {
KC_PARAM_NAME = "dbc_min_pct"
KC_PARAM_STATUS = "5"
KC_PARAM_DEFAULT = "5"
KC_PARAM_MAX = ""
KC_PARAM_MIN = ""
KC_PARAM_CLASS = "File System"
KC_PARAM_DESC = "Min Dynamic Buffer Cache Size as Percent of System RAM Size"
KC_PARAM_IN_MASTER = 1
KC_PARAM_HELP = "KCparam.DBCminPct.sdl"
KC_PARAM_MODULE = "-"

HTH
-Michael