kernel param

[dhanish]

Can anybody let me know what does the parameter
dbc_max_pct is for .What it is default value and if i decrease it then what will be the effect.


thx

[George_Dodds · Accepted solution]

Check out the following

linkhttp://www.dbconnect.com/hpunix01.html

[Patrick Wallek]

dbc_max_pct is the maximum percentage of RAM that will be used for dynamic buffer cache. THe default is 50, so 50%, or one-half, of your RAM could possibly be used for dynamic buffer cache (dbc).

If you decrease then the maximum amount of RAM that could be used for dbc will be decreased. I do NOT recommend leaving dbc_max_pct at the default. I would recommend setting at a value that would equate to somewhere are 500MB or so, and that is probably being generous.

You could set both dbc_min_pct and dbc_max_pct to be the same value. This would cause the amount of memory used for dbc to be static, which is how I have my machines set up.

[Helen French]

Hi Nitin,

dbc_max_pct - Max Dynamic Buffer Cache Size as Percent of System RAM Size

dbc_min_pct - Min Dynamic Buffer Cache Size as Percent of System RAM Size

this may help you:

http://us-support.external.hp.com/cki/bin/doc.pl/sid=a480abbf10a986234e/screen=ckiDisplayDocument?docId=200000043976223

[Uday_S_Ankolekar]

Hi,

dbc_max_pct : This determines the percentage of main memory to which the dynamic filesystem buffer cache is allowed to grow (when nbuf and bufpages are zero) The default is 50 percent of memory. But this is a overkill.! If you want to really use of dynamic buffer cache start with dbc_max_pct with 10 %

-USA..

[Helen French]

Hi again,

See this for default Kernel parameters in 10.X:

http://us-support.external.hp.com/cki/bin/doc.pl/sid=a480abbf10a986234e/screen=ckiDisplayDocument?docId=200000011495574


HTH,
Shiju

[Don Morris_1]

dbc_max_pct and dbc_min_pct set (unsurprisingly) the maximum and minimum percent of memory that can be used by the Dynamic Buffer Cache -- which caches filesystem buffers as I recall. Default on 11i for max_pct is 50%, min is 5%. From everything I've heard here, DBC is a little piggy and will go to the max quickly - so you probably want it lower unless you're not seeing any paging activity currently (swapinfo -atm and look at % used for the entries excepting memory and reserve). As far as exactly what to set it to... that really depends on your memory usage for apps vs. filesystem access patterns (i.e. how much cache is really good for you). If you're experiencing paging, I'd try setting it to around 15 or 20% and see if filesystem performance suffers more than you gain in decreased paging.

Sorry I can't be more specific, but I hope this helps...

Don

[James R. Ferguson]

Hi:

See here:

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

Note that this is one of several related parameters that control the file buffer cache. It is frequently set far too high by default, needlessly allowing memory to be consumed by file buffers. For database engines (like Oracle) that manage their own buffers, too high a system buffer cache simply means that two entities are buffering I/O -- a needless waste.

Regards!

...JRF...

[dhanish]

Hi,
How do I see how much is configured at present and if want to set dbc_min_pct and max also to 5% how shud i do that ...i mean what value shud i give to this param

[Helen French]

Hi,

The bset way is through SAM. SAM-Kernel Configuration-Configurable parameters. You can view the current value and can change too.

# sysdef - will list you the kernel parameters.

HTH,
Shiju

[Sanjay_6]

Hi Nitin,

Try these links,

http://docs.hp.com/cgi-bin/otsearch/getfile?id=/hpux/onlinedocs/os/KCparam.DBCmaxPct.html&searchterms=dbc_max_pct&queryid=20020110-085245

http://us-support.external.hp.com/cki/bin/doc.pl/sid=019145ef1d96676cb3/screen=ckiDisplayDocument?docId=200000050018417

Hope this helps.

Regds

[dhanish]

Fine I got the answer how to find that ..thnks to u all

[James R. Ferguson]

Hi:

To see your values, you can use SAM or:

# sysdef (if 10.x)

# kmtune -q dbc_max_pct' if (11.x)

I'd advise you to use SAM to change the parameter, which will require a kernel regeneration and reboot.

Regards!

...JRF...