Re: Savage DRI memory requirements compared with MS Windows 2000

[email protected] wrote:
> Hi.
> 
> According to my /var/log/Xorg.log, 8844 kB of Videoram is needed for 3D at a
> resolution of 1400x1050 at 16 bpp. My IBM Thinkpad T21 has an onboard "S3 Inc.
> 86C270-294 Savage/IX-MV" which only contains 8192kB of video ram, so I can't
> get 3D at this resolution/colour-depth. MS Windows 2000, however, seems to
> handle direct rendering OK at this setting (in fact, I even get the option to
> run at 24-bit colour).
> 
> Is the larger memory requirement a by-product of Xorg's DRI implementation, or
> is there something I can do (as an end-user) to reduce the memory requirement?
> Apologies if this mail seems like I'm trolling - it's just an honest question.
You could say it's a by-product of the xorg dri implementation. The 
problem is, at this resolution and bit depth, your framebuffer is 
already 3MB. For 3d, you also need (for "common" 3d apps at least) a 
z-buffer and a back buffer, which gets you the 9MB requirement (if 
z-buffer is 16bit too). You don't need any space for textures at least, 
as these can be read from agp memory. There are basically two things a 
driver can do against those memory requirement.
1) Depending on the chip, it may be possible to put backbuffer (maybe 
even depth buffer) into apg memory instead of local video ram. Not sure 
if the savage you got could do that (if in windows you can actually run 
3d apps at the full 1400x1050 screen resolution then it can probably do 
it). Even if the chip could do it there would probably be a performance 
penalty for doing so.
2) Don't allocate unified full-resolution back/depth buffers. If your 3d 
app only renders in a 640x480 window you don't really need the full 
resolution back/depth buffer. You could either allocate per-application 
back/depth buffers or do even some advanced schemes like unified 
back/depth buffers but only as large as currently needed. I think there 
is some work in this area, but currently the savage driver is not the 
only one suffering from the full-resolution static back/depth buffer 
allocation problem, there are for instance quite some mobility radeons 
which share the same fate (non-mobility usually have more local video ram).

Roland

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