[Vorbis] Problem with CPU usage on Playstation 2 when using OggVorbis.

[Dave Mercier]

>>I get CPU usage that looks like this (when i say frames i mean actually 
>>rendering frames on the
>>Playstations 2 and you have 50 or 60 frames per second)

>>cpu usage:        ~100%         ~0%         ~0%          0%         ~100%
>>current frame:        0                1             2               3     
>>            4

The fundamental problem is Vorbis can use window sizes larger than the 
amount of samples than naturally want to be processed each 60 Hz frame draw. 
E.g. at 48 Khz, about 800 samples would be processed each frame draw. Vorbis 
doesn't use window sizes of 800. If it did (in a dream world), you wouldn't 
be having issues.

I believe Vorbis will often use window sizes of 2048. So your peak CPU hit 
in a frame draw will be 2048/800 percent above the average CPU load.

One thing you could try doing is modifying the encoder to only use frame 
sizes as large as 1024. This will cut your peak CPU in half. You could go to 
512, but I don't think it would help the worst peaks as you will have to 
decode 2 512 size windows sometimes. Even 256 wouldn't help the peak as you 
would still need 4 of them. 128 would start to see some returns as you would 
need at most 7. But at that point you are reducing the efficiency of the 
compression considerably, and you probably eat any CPU savings back 
processing those small windows. The smaller numbers below 512 only get you a 
more even CPU distribution between frames (which doesn't do much good in 
video games as it's the peak that kills you). Keep in mind limiting to 1024 
means you can only play your own Vorbis files, if that's an issue. You also 
reduce the amount of compression possible.

Other than that, looking at the numbers I think you want to do a pile of 
optimization as it looks like you are using at least 25% CPU anyway. Unless 
your PS2 application can afford that CPU (that would be surprising), load 
balancing improvements are a bit pointless unless you optimize as well. I'm 
pretty sure libvorbis would run dog slow on a PS2 out of the box.

The idea of running Vorbis on a separate thread at a lower priority is also 
interesting, although perhaps a bit messy. The biggest issue on PS2 is it is 
cooperative, so you would have to implement a watchdog interrupt of some 
sort (e.g. the VBL) that could force the low priority Voribis thread(s) to 
yield when the CPU is needed again for other threads.

I've always thought it would be pretty easy to produce a version of Vorbis 
that is more friendly to video games. It would encompass at least a few 
changes, including the window sizes. Out of the box, Vorbis is not the most 
friendly format for in game music/SFX in video games (e.g. MP3 slides into 
video games easier, at least on a technical level). Not trying to push MP3, 
I'm just saying I think it would be very easy to remove the obstacles in 
Vorbis to the point where it would behave similar to MP3 in regards to CPU, 
memory, and usage.

Thanks,
Dave.