dbc_max_pct and device swap

[Franky Leeuwerck_1]

Hello,

It seems that our HP-UX 11 is swapping.
# swapinfo -tam
Mb Mb Mb PCT START/ Mb
TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME
dev 1024 230 794 22% 0 - 1 /dev/vg00/lvol2
reserve - 794 -794
memory 465 418 47 90%
total 1489 1442 47 97% - 0 -

Can I decrease the device swapping after reducing dbc_max_pct ?

I have a DBMS running with a lot of cache memory allocated. Instead of reducing cache memory, maybe reducing dbc_max_pct is a workaround for reducing the swapping activity.


Thanks in advance,
Franky

[T G Manikandan · Accepted solution]

you are right!
But with your server's amount of memory,you cannot reduce buffer cache for good results.

It can be done on high RAM installed servers.

The best option would be to go with increase of RAM.
DBMS definitely requires more RAM

[Franky Leeuwerck_1]

The context of my question can be read at :
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=227661

On a short term there is no more RAM available which leaves me either to
- reduce the DBMS cache
or to
- reduce OS buffer cache

In the discussion thread mentioned we came to the conclusion that

- reducing the swapping activity by reducing
the dbms cache

will result in better performance

- than allocating a lot of dbms cache resulting in more swapping.

There is apparently another way and that's reducing OS buffer cache which leaves me a large amount of DBMS cache allocated and yet a decrease in swapping activity.

Right ?

Franky

[Pete Randall]

Franky,

What is dbc_max_pct set to now? The normal rule of thumb is to set it to 300-400MB. However, with the limited amount of memory available to you, that's going to not be much less than 50% anyway. I'm afraid there is not much tuning you can do to relieve your memory pressure. You need to go out and purchase some more RAM or accept the performance you have now.


Pete

[Michael Steele_2]

You need to add swap as well:

lvcreate -L #### -n swap -C y -r n /dev/vg##
NOTE: -L = mb
swapon -f -p 1 /dev/vg##/swap
'swapon' may not complain if maxswapchucnks is OK. If it doesn't like maxswapchuncks then it will tell you.
maxswapchunks = total swap / 1024 * swchunk
sysdef | grep -i maxswapchuncks
sysdef | grep -i swchunck
/etc/fstab
/dev/vg##/swap ... swap pri=1 0 1

Note:

A) swap is contiguous
B) swap should be mirrored

[Franky Leeuwerck_1]

Thanks to your replies.

FYI : dbc_max_pct is now set to 25% with 640 MB RAM .

Regards,
Franky