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    SubjectRe: useful ram on voodoo(2) cards?
    On Thu, 17 Dec 1998, Mike A. Harris wrote:

    > On Mon, 14 Dec 1998, Jon M. Taylor wrote:
    > >> [Such a driver would presumably have to prevent simultaneous access to the
    > >> card for graphics purposes or we would have some funky displays whilst the
    > >> machine hosed itself.]
    > >>
    > >> And a subsidary question: given a), is this idea insufficiently ambitious
    > >> and could/should such RAM be usable as real system memory, not just fast
    > >> swap?
    > >
    > > It probably can't even be used as fast swap. I suggested such an
    > >idea in the old GGI FAQ, and someone later told me that the VRAM+memory
    > >controller logic on standard video cards is designed around the assumption
    > >that writing to video memory will happen much, much more often than reading
    > >from video memory. I don't remember the particulars, but the upshot is that
    > >bandwith reading from VRAM is maybe 1/4 of bandwidth writing to VRAM. As
    > >such, VRAM reads will be so much slower than VRAM writes that any gains you
    > >might have gotten from the extra RAM evaporate. If you have "normal" VRAM
    > >I/O and maybe accels and DMA too (i.e. something is actually using the video
    > >card for its intended purpose) going on simultaneously the situation gets
    > >even worse.
    > Are you saying that swap to VRAM would be slower than swap to
    > hard disk?

    No. I am saying that you'd better not be wanting to do any
    serious video work with the video card if you do use it for swap. And
    normal system [S]DRAM is cheaper and sits on a faster bus with no
    contention from other I/O devices. AGP and/or UDMA architectures will
    render this idea obsolete anyway.


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