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[wlug] graphics speed

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Oliver Jones oliver@d...
Tue Sep 23 10:15:30 NZST 2003

> And you'll probably find the AGP bridge isn't either, which may be another
> cause of slowdown.  Oh, and the XFree86 DRI stuff won't help at all either
> - the mainline X tree is *old* in this regard.  If you're feeling brave,
> check out the current XFree86 snapshots - ATI have been contributing a
> *lot* of code, and I think they've got 2D acceleration for the ATI IGP
> chipsets in there now (SWMBO has a laptop with one of these, and she's
> asking me to do this...).  If you're *really* lucky, the latest X snapshot
> will actually build!

I can't speak for the ATI support in XFree86.  But we have a bunch of
Intel 865 boards here that have integrated i830 video chips.  Their 3D
performance (for simple-ish stuff) is quite good using DRI/DRM.  Though
some of the GL screensavers tend to crash X.  And with the i8x0 chipsets
you're limited to 16bit colour if you want 3D.

> This tells me that your app doesn't use GLX, so it's probably doing all
> the rendering using the machine's CPU, RAM, $deity knows what else.  As
> the Duron 900 is the slowest machine you've got.
> I'm still boggled at anything over VNC being the fastest, though.  That's
> just bizarre.

The biggest problem historically with graphics has always been the
bandwidth between video memory and main memory.  Graphics are just plain
big and eat the data bandwidth something chronic.  So it is unsurprising
that VNC is faster than using a video card.  For 2 reasons:

1. Xvnc doesn't use the graphics hardware.  It renders to an in main
memory buffer.  No data is travelling on the AGP or PCI buses.  
2. The VNC protocol is a polling protocol that only refreshes the screen
when necessary and only transmits what has changed.  It is also
asynchronous to the video updates.  This means that only a fraction of
the video updates are being sent over the ethernet wire.


Oliver Jones » Director » oliver.jones@d... » +64 (21) 41
Deeper Design Limited » +64 (7) 377 3328 »
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