Re: [9fans] image/memimage speed

[erik quanstrom <quanstro@quanstro.net>]

> If you're still seeing bad performance it may be because you need to
> fix up the MTRR or GART settings. I've done this dance and have no
> memory at this point of what you do, but vague memory is that proper
> MTRR settings with a good PCIe card will give you far better bandwidth
> than the old AGP cards. There is nothing like the 60x assymetry.

i don't think this is the case.  if you recall from the original
post, i have used the pat registers to set up memory types on
a pcie card and i do see dramatic speedups for drawing to
the screen.  however, reading from the screen is just as slow
as before.

according to the intel's x86 arch guide vol 3a, §10-8, p. 466
speculative reads are allowed for WC/WT/WB memory.  so
i wouldn't think that it's a bus problem at all.

if you recall, the only difficulty in using subpixel
fonts a few years ago was the fact that for hold mode and
deselection, the the α draw was done with the new mask and
the on-screen image, which was read from the frame buffer.
not only did this result in a squared α, it was also sloooow,
especially on nvidia cards.  oddly, the via machines driven
in vesa mode were the fastest.  the speed up was at least a
factor of 10; you could easily see the speedup.

at this point, you probablly don't believe me.  so maybe
some numbers will help make the case:

; time dd -if /dev/wsys/1/screen -of /dev/null -bs 512k
0+1201 records in
0+1201 records out
0.00u 0.03s 4.04r 	 dd -if /dev/wsys/1/screen -of /dev/null ...
; time dd -if /dev/zero -of /dev/null -bs 512k -count 1201
1201+0 records in
1201+0 records out
0.00u 0.14s 0.14r 	 dd -if /dev/zero -of /dev/null ...

i've seen the same behavior on a number of different nvidia
cards of different generations.  newer cards seem to be worse
than older cards.  (if you have the programming interface
manuals, i'd be happy to double-check the settings.  ☺.)

can you explain what the downside of double-buffering
would be?  it's not like the days where we asked, hey buddy,
have you got 4 megs to spare?

- erik