Buffered Cache on Swap Memory

[Brian Ham]

Every week my buffered cache grows about 6%. How can I prevent buffered cache from growing so large?

[ramesh_6]

Try this document

[Uday_S_Ankolekar · Accepted solution]

Hi,

These three kernel parameters required to change for the proper buffer cache. Changes to made this parameter required a reboot.
dbc_min_pct, dbc_max_pct or bufpages.

If all three are set, then bufpages will
determine the size.

Look at this doc for more info

http://us-support.external.hp.com/cki/bin/doc.pl/sid=ad306a7611ec3e0d35/screen=ckiDisplayDocument?docId=200000058374588

Goodluck,

-USA..

[T. M. Louah]

The following is taken from "System Performance FaxPak" found in Kmine, Please consult the document for an overview & others for more info about Dynamic Buffer caching:

Tuning the File System Buffer Cache

The problem has been determined to be either a file system IO
bottleneck or a memory bottleneck. In either case, adjusting the size
of the buffer cache may resolve the problem. It is easiest to measure
the effectiveness of the buffer cache and the effectiveness of
changes if the buffer cache is of a fixed size, instead of the 10.X
default of a dynamic buffer cache that ranges in size from 5 to 50%
of main memory. The size of the buffer cache is controlled by the kernel
parameters bufpages, nbuf, dbc_max_pct, and dbc_min_pct. SAM help
can provide more information on these parameters, and the System
Administration Tasks manual has information on how to rebuild the
kernel. Historically, HP-UX has used 10% of main memory as a default
buffer cache size.

The goal of the buffer cache is to increase the speed of disk io
reads and writes. The trade off for that increased performance is
the usage of a percentage of main memory. After isolating the
resource that is slowing the system, the job is to adjust the size
of the buffer cache to get the best returns for this system.

# sar -b
It is desirable to see %rcache >= 90, and %wcache >= 70%. Some
system configurations (for example systems with raw partitions or
applications that perform random reads and writes) will never see
these buffer cache hit rates. The goal is to adjust the cache to
give the best performance for that particular system. Start with a
fixed buffer cache size, and adjust the cache larger or smaller
(ex. 2% at a time). Then measure the results: did the hit rate
increase/decrease, and was the result worth the change in memory
usage?

Two things to remember: if %rcache and %wcache do not change as
you are increasing/decreasing the buffer cache in small increments,
then the adjustment has neither helped nor hurt file system IO
performance. Also, every system will peak at different read/write
hit rates. Some systems will never see very high hit rates.