Re: What to do with slow Chip/ST RAM

[David Brownlee <abs%NetBSD.org@localhost>]

On Wed, 11 Mar 2009, Izumi Tsutsui wrote:

> abs%NetBSD.org@localhost wrote:
>
> >         Both amiga and atari have the concept of 'fast' and 'slow' RAM.
> >         For a system with a reasonable amount of fast RAM using the
> >         slow RAM can actually slow things down, though it seems like
> >         a shame to waste it...
>
> I guess ST is on 16 bit bus and TT is on 32 bit bus on atari,
> and both could be handled by pmap/uvm with proper priorities
> by MI uvm(9) API. IIRC, mvme68k has some code which handles
> slower (no cache) VME bus memory.

        Aha, that would be the free_list param to uvm_page_physload()?
        Is memory allocated strictly from the freelists in ascending
        order?

        So the 'slow' ram on amiga and atari is probably not 'slow enough'
        to be worth switching to swap, though it would be beneficial to
        put them on separate free lists, so fast ram is consumed first?

> >         Jared has written a driver for the sgimips O2 to allow it to
> >         access some of its extra memory banks as a block device, which
> >         means it can use it as a RAM swap device. (The issue with the O2
> >         is different, as the current NetBSD kernel cannot use that extra
> >         memory directly, but the principle is related).
>
> Our mips pmap can't handle physical memory located at
> PA higher than 256MB, so his driver just provides alternative
> usage of memories which can't be handled by uvm/pmap,
> as >4GB PAE memory on i386.

        I thought PAE allowed i386 to usefully use more than 4GB of RAM,
        though each process would still be limited to a 4GB address space?

> On the other hand, x68k has the similar RAM disk device (bmd)
> for optional memory on the Nereid board.

        There is always prior art :) Would it make sense for sgimips and
        x68k to use a common driver for their add on ram?


--
                David/absolute       -- www.NetBSD.org: No hype required --