SUMMARY : about bufhwm in /etc/system

From: Jacques Beigbeder (
Date: Tue Jun 21 1994 - 03:09:09 CDT

Thanks to the only answer: sunven!emsca!lsp@Venezuela.Sun.COM.

        Q: How do I find the value used on my running system?:
        A: You can use an adb script like this:

           -------> cut here <---------
           #! /bin/sh
           # Buffer Cache values in the Solaris 2.3 Kernel
           adb -k /dev/ksyms /dev/mem << EOF
           ="Buffer Cache Parameters"
           *nbuf=D"How many page sized buffers have been allocated"
           *nhbuf=D"How many buffers headers"
           *p_nbuf=D"How many new buffers are allocated in one go"
           *bufhwm=D"Maximum amount of memory allocated to the buffer (Kbytes)"
           -------> cut here <---------

        Q: What is this parameter bufhwm?

        A: The variable bufhwm controls the maximum amount of memory allocated
           to the buffer and is specified in Kbytes. By default up to 2% of
           the system memory is used, this can be increased to 20%,and it will
           be usually need to be increased to 10% for a dedicated NFS server.

        Q: Is it worth changing the default value of this parameter?

        A: Buffer Cache can be monitored using "sar -b", this reports a read
           and a write hit rate for the buffer cache. The buffer cache size
           (bufhwm) should be increased if there are significant ( say more
           than 50 ) number of reads and writes per second and the read hit
           rate falls below 90% or the write hit rate falls below 65%.

           Also you can use and undocumented option of the netstat command
           "netstat -k" and calculate the hist rate in this form:

               % Hit rate = buffer cache hits/buffer cache lookups

Jacques Beigbeder | Internet:
Service de Prestations Informatiques |
Ecole Normale Superieure |
45 rue d'Ulm | Tel : (33-1) 44-32-37-96
F75230 Paris Cedex 05 | Fax : (33-1) 44-32-20-80

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