Re: useful ram on voodoo(2) cards?

Kurt Garloff (garloff@kg1.ping.de)
Thu, 17 Dec 1998 23:43:04 +0100

On Thu, Dec 17, 1998 at 10:31:55AM -0500, Mike A. Harris wrote:
> > 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? If that is the case, I want a video card with a hard
> disk instead of video memory. ;o)

It seems the VGAs are not able to write the contents of their memory via
busmastering into main memory. Maybe you know the ctcm (c't cache monitor)
program which tests your emeory and cache speeds. It's a great testing tool.
(Look at http://www.heise.de/ct Software zum Heft ctcm16n.zip).
You can also test video mem speed and while writing to it gives you 33MB/s
or evene higher speeds, reading is relatively slow: 6MB/s.

(These numbers are from memory. BX board with MGA Mill I)

It's still better than most disks if you consider that there is no seek time
at all.

-- 
Kurt Garloff <K.Garloff@ping.de>  (Dortmund, FRG)
PGP key on http://student.physik.uni-dortmund.de/homepages/garloff


 There is  something  frustrating  about  the quality  and  speed  of  Linux
 development.  I.e. the quality  is too high  and the speed  is too high, in
 other words,  I  can implement  this XXXX feature,  but I bet  someone else
 has already done it and is just about to release his patch to Linus soon...
    [From a posting of Tigran Aivazian to linux-kernel, XXXX = disk stat]


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