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From: Mikael Karpberg <karpen@ocean.campus.luth.se>
Message-Id: <199610030805.KAA25133@ocean.campus.luth.se>
Subject: Re: nbuf in buffer cache
To: dyson@FreeBSD.org
Date: Thu, 3 Oct 1996 10:05:45 +0200 (MET DST)
Cc: bde@zeta.org.au, heo@cslsun10.sogang.ac.kr, freebsd-fs@FreeBSD.org
In-Reply-To: <199610020511.AAA00199@dyson.iquest.net> from "John S. Dyson" at "Oct 2, 96 00:11:16 am"
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Hi!

> memory as possible.  If (and it is a very big if) there is free (spare)
> memory, the system will provide it in the form of the merged VM object cache.
> Note also the system prefers to keep metadata in the cache, and to push
> file data to the VM objects.  It is then the best of both worlds.
> 
> So, to be precise, limiting the number of buffers keeps the freedom
> maximized.  The larger the number of buffers, the greater the chance that
> there will be too much wired memory for an application.  I have found that
> the knee for gcc appears to be about 2M (plus or minus.)  And it is very
> sharp.  If you restrict the amount of memory even by 100K-200K, compile
> times go through the roof.
> 
> Additionally, the issue of MSDOS having a very small cache size isn't valid,
> and is limited by the total amount of available memory.

Umm... hold on a second, here... :-)
I always thought Linux etc used all free memory for disk caching, and
that the BSD's used a formula (basically something like some percentage of
the available memory) to determine the size of a static buffer, used as
disk cache. Now... it makes sense if this changes when you use a merged
disk cache and VM system. Someone let me in on how things work? :-)

  /Mikael