Re: What to do with slow Chip/ST RAM

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

On Wed, 11 Mar 2009, Frank Wille wrote:

> >   Would anyone be interested in adding support to use chip/ST
> >   RAM on the amiga/atari in a similar fashion (obviously some of
> >   it is likely to be reserved for device DMA).
>
> I doubt that it would work very well for Amiga, because (I think) the amount
> of available Chip RAM is not constant. There may be allocations for floppy
> disk, audio or video DMA buffers at any time.
>
> The altmem device works with a fixed contiguous memory region?

        It would probably be best to allocate most of the slow RAM to the
        altmem device on boot and leave just enough to work as DMA space
        for the devices. The important question is how *much* slower is
        the slow ram. tsutsui@ commented that mvme86k already handles
        split between fast and *very* slow (uncached!) ram by just
        assigning priorities in uvm() so the fast ram is all used first.
        Maybe that is a better approach for amiga and atari (maybe they
        are already doing that?)

        An interesting benchmark would be to run a system with the
        memory bank priorities set appropriately, and then run it
        again with the slow ram unsued apart from dma allocations
        and see if having more ram (albeit slower) offsets the
        speed penalty of accessing it.

        In an ideal world the system could be tunable, so those with
        large quantities for fast ram would not use slow at all, and
        those with very little fast ram would...


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