performance work

[Jordan Crouse]

Neil Graham wrote:
> On Mon, 2008-12-22 at 15:36 -0700, Jordan Crouse wrote:
> 
>> You might want to re-acquire the numbers with wireless turned off and 
>> the system in a very quiet state.  If you want to be extra careful, you 
>> can run the benchmarks in an empty X server (no sugar) and save the 
>> results to a ramfs backed directory to avoid NAND. 
> 
> 
> The XO Numbers were recorded from a fairly inactive state.  Wireless was
> active but there shouldn't have been any traffic.  I did launch X with
> just an xterm, so sugar shouldn't be in play at all.  I didn't think of
> the speed of nand writes however.
> 
> 
>>  2) The accel path requires reading from video memory (which is 
>> very slow)
> 
> I'm curious as to why reads from video memory are so slow,  On standard
> video cards it's slow because there is quite a division between the CPU
> and the video memory,  but on the geode isn't the video memory shared in
> the same SDRAM as Main memory. 

It is, in that they share the same physical RAM chips, but they are 
controlled by different entities - one is managed by the system memory 
controller and the other is handled by the GPU.   At start up time, the 
memory is carved up by the firmware, and after the top of system RAM is 
established, video and system memory behave for all intents and purposes 
like separate components.  Put simply, there is no way to directly 
address video memory from the system memory.  Access to the video memory 
has to happen via PCI cycles, and for obvious reasons the active video 
region has the cache disabled, accounting for relatively slow readback.

That said, the read from memory performance is still worse then you 
might expect - I never really got a good answer from the silicon guys as 
to why.  If Tom Sylla is still reading this list, he might know more.

> There's a separate 2 meg for DCON memory, but I was under the impression
> that was just to remember the last frame.
> 
> Do I have that all wrong?   

No - thats right, there is a completely separate bank of chips just for 
the DCON.

Jordan