Re: [9fans] nvidia scrolling performance

["Russ Cox" <rsc@swtch.com>]

> the only obvious difference i see between the plan 9 driver and the xf86 stuff is
> that the xf86 stuff tries to buffer dma access a bit.  is ths the key, or should i be looking
> at something else?

This isn't the nvidia driver.
The problem is that you're using greyscale fonts.
That requires dropping into the generic alpha code
instead of using the boolean alpha code that regular
character drawing uses.  The boolean alpha code
avoids some multiplies, which helps a little, and also
avoids reading from the frame buffer memory, which
helps a *lot*.

Frame buffer memory is very very slow to read from,
and not just on nvidia.  When I did some timings six years
ago, I found that reading from frame buffer memory
was slower than reading from disk.  I'm sure the situation
hasn't gotten better.  It's not on the fast path for any
other system, so the vendors just don't care.

Now that memories have gotten bigger, it might be
worth keeping a copy of the screen image like in
the X port, but it's not clear to me how to reconcile
that with using acceleration.

Greyscale fonts are slow and ugly.  For more than
occasional use, just don't do it.

Russ