On Monday 15 May 2006 9:58 am, Perry Lorier wrote:
Generally larger alignment is faster. Also as you start getting closer to the hardware you start getting stricter requirements for alignment. Early DMA controllers for instance had to be "page" aligned (16 bytes IIRC). I have no idea if this is still true today.
The downside of large alignment is that it uses more memory, and if you end up having to touch that alignment padding then you end up wasting resources. The trick is trading off this "Wasted" space, vs the speedup you get from the alignment.
Right, pretty much matches what I was thinking. I will look at dma stuff next. I should probably start doing some profiling to see what the objective results are, rather than subjective ;-p. might look into ck series kernels to see differences. My understanding is that P4 chips have quite bad latency for some common io instructions, which is where AMD makes ground. For example more here... http://answers.google.com/answers/threadview?id=321522 I traced the movsl mask back to L1_CACHE_BYTES, which makes sense.
What exactly are you trying to discover here? Are you trying to figure out how to deglitch some audio?
Not really, just using that as an example (although ac97 drivers do still have a bad io related bug). I'm hacking round in i386 arch trying to learn a bit more about how io is handled and increasing my knowledge of system programming at the same time. Been attempting to absorb Intel docs on this. Kinda hobby type thing. So far recent testing versions built created about 5% decrease in core code size (subtle bugs aside) using gcc 3.4.6, just by manually optimizing kernel code for a p4 2.6 (stepping 9) which I'm running. Building with 'march=pentium4' has worked nicely so far. Also the MB (Abit IS7) appears to have really good IO subsystems, which fascinates me :). Learning what kernel devs do when things break has been fun. I realise that newer 64bit offerings do many things differently, but this is what I have to play with for now. -- Whether you can hear it or not, The Universe is laughing behind your back. -- National Lampoon, "Deteriorata"