Can 10M Buffer Ceiling be lowere?

[Rob DeMarco]

On Wednesday 01 December 2004 07:34 am, Brian Bobowski wrote:
> Kris Kennaway wrote:
> >On Wed, Dec 01, 2004 at 03:27:11AM +0000, r.p.demarco at att.net wrote:
> >>     A technical question:
> >>
> >>     I have an old NEC computer (c. 1997) running 5.3-RELEASE with
> >>48M of RAM.  Getting a new computer isn't an option right now, but
> >>I would like to get as much out of my memory as possible.
> >>     My /boot/kernel/kernel file is about 3M, and from the initial
> >>boot:
> >>  real memory  = 50331648 (48 MB)
> >>  avail memory = 43896832 (41 MB)
> >>it appears this kernel takes up about 7M of memory with one screen saver
> >>kld loaded.  With a few unneeded services (cron, sendmail) disabled, I
> >>start off with about 26M free after a fresh reboot with just root logged
> >> in, running `top'.  Looking at top, I noticed:
> >>
> >>                Mem: 4320K Active, 15M Inact, 12M Wired, 10M Buf, 11M
> >> Free ^^^
> >>
> >>     From TOP(1):
> >>
> >>                Buf: number of pages used for BIO-level disk caching
> >>
> >>     Actually, the 10M is after some disk usage (it starts ~6M).
> >>It never gets above 10M.  Is there anyway to adjust this, to
> >>(say) a maximum of 5M?  Yes, a new 256 MB RAM system would be nice,
> >>but until then, I would like to avoid serious paging running xclock :)
> >>Thanks,
> >
> >There's no point, that memory will be used if demanded.  Note that you
> >still have 11M free in your example, so throwing away 6MB that is used
> >for caching would only *reduce* performance.
> >
> >Kris
>
> Stated otherwise, I recall reading that FreeBSD considers unused memory
> to be wasted memory. If you had other stuff taking up memory, it might
> not use that much for caching; as it is, though, it's only taking
> advantage of memory which is otherwise just sucking up power to keep
> ready. Its activity, based on this, is causing you no harm. (I don't
> know if my 32MB machine, which reports 440K free from top, is suffering
> as a result, but I've certainly not noticed any difficulty when loading
> web pages that couldn't be as adequately explained by the relatively
> slow 166MHz CPU.)

     Yeah, as I told Kris, I'm not so good at interpreting
the `top' output.  It just seemed strange to me that even when
I had a lot of processes running (and presumably needed some
of that "Buf" cache) it was still reporting 10M (I would have thought
`top' would show it freeing up some to accommodate them).

>
> Also note that cron is responsible for such things as the routine system
> checks; if you really insist on disabling cron, you might need to audit
> your system a bit more aggressively.
>
      Duly noted, though my requirements are modest.  I'm just
using this machine for my own personal use and thus have fewer
usage and security concerns than others on this list. (I think I
have pretty much disabled all of the internet services -- including
sendmail -- and I can always update my locate db anytime I
need to)

      Regards,

      -Rob

> -BB