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Date:      Wed, 24 Apr 2002 16:38:08 -0700
From:      Terry Lambert <tlambert2@mindspring.com>
To:        Steve O'Hara-Smith <steve@sohara.org>
Cc:        scrappy@hub.org, freebsd-current@freebsd.org, freebsd-stable@freebsd.org
Subject:   Re: FreeBSD 4.5-STABLE not easily scalable to large servers ... ?
Message-ID:  <3CC741E0.2BB79E0C@mindspring.com>
References:  <20020422091602.O1721-100000@mail1.hub.org> <3CC40A62.B40E816F@mindspring.com> <20020424191849.3a81b126.steve@sohara.org>

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Steve O'Hara-Smith wrote:
> On Mon, 22 Apr 2002 06:04:34 -0700
> Terry Lambert <tlambert2@mindspring.com> wrote:
> 
> TL> FreeBSD doesn't currently support bank selection.  Peter was
> TL> working on it, last time I heard.  Linux supports it, at an
> TL> incredible performance penalty.
> 
>         This inspired an off the wall thought that may be insane. Would
> it be possible (on a >4Gb system) to address 4Gb of RAM and write a driver
> to make the rest appear as a device, which could then be used for a
> preferred or (even neater) first level swap.

Only if you reserved a window for it.  Say 1G of KVA, though last
I checked the bank selection granularity wasn't fine enough for
that.  Memory in the window can *never* be a target for DMA, and
should *probably* never be used for kernel structures.

If you ever programmed graphics on a TI 99/4A, which has a 4k
visible window onto screen memory, or programmed code on the
Commodore C64 to use the 32K of RAM-under-ROM, or programmed
in DOS or CP/M using Overlays, then you'll be familiar with the
problem.

-- Terry

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