Thursday, 18 October 2012

NEON, timing, object detection

Well it's been an all-NEON week and together with some very poor sleep, a hangover, and bad hayfever ... i'm pretty much over it.

But I kind of have some interesting code that manages to coax a bit of performance out of the little beagleboard-xm i'm using for coding at the moment; memory is the biggest bottleneck there. It's not the target platform, but it provides a well defined minimum baseline. I also finally hit a very measurable problem with the small cache on it ... processing 512x512 images as floats just happens to be a bad size when you go over 16 rows in a vertical span - worse than 2x performance loss for some of the code.

I also found a new (to me) fft library for NEON that I will post more on once I get it to compile.

I was looking up something and (re)came across the nice little cortex-a8 static code analyser and put it through it's paces (somehow I missed that it was just a web application last time). Learnt a bit about cycle timing and dual issue, it's got a couple of bugs but works pretty well. The display of the SPU timing tool would be nice though.

Anyway for some reason I thought vext only worked on double's (probably since vtbl does), but on seeing it worked for quads due to something else I was poking at, and how quads interacted with each other (the ARM documentation on cycle timing isn't as clear as it could be), I thought i'd go revisit the LBP object detector I wrote as I remembered I didn't use quads there. I ran a whole bunch of variations of the inner loop through the tool and shaved about 7 cycles off the time. Not bad when it was only 33 to start with. Unfortunately I ran out of NEON registers (they were already all used) and had to resort to swapping 2 individual byte constants through q15 - all because vand.u8 doesn't support a general 8 bit immediate ... otherwise i could've gone 2 better (sad face).

Anyway, I gave it a go on my 'cheapie' tablet for the first time. It's fairly comparable in performance to the Galaxy Note I used last time, it's only a bit slower.

Cut a long story short, 130ms average for the original version, and now it's down to around 114ms (friggan timing stuff is all shot on this machine so it jumps around a lot, might be scheduler related, or debug output from android). Dunno if i added some bugs - it seems to work 'at all' which means they can't be big if they are there. Not quite as good as the timing analyser suggested - but then that is modelling an A8 and this tablet has an A9 which changed some of the features I was using, and I only analysed part of the loop - but all-in-all still worth the effort for something which I thought was already as tight as possible. Actually I thought it was a bit better than that at first because when I changed back to a 17x17 search window I forgot to adjust the search parameters, so it was detecting faces at about 35fps vs 30fps (search range of 6x to 20x, rather than 2x to 12x as my original test - minimum face size of (17*6)x(17*6) - which is still reasonably small on a webcam).

The better-than-1 instruction classifier!

Now i've got the classifier down to well under 1 instruction ... I should really work on improving the detector quality. I would really like to know if it can be made into a decently robust detector or not - because if it is it could be quite useful: it's trivial to train, has tiny classifiers, and can be made to run very fast on modern hardware because it is fully parallelisable down to the SIMD level, relatively cache friendly, and even has a deterministic running-time.

I've a few ideas but need to set time aside.

I've also been switching around a lot at work lately, and that's throwing me around a bit (not to mention the hayfever). When I switch between diverse areas such as NEON coding, Android applications, OpenCL, Java applications, JNI, C, jjmpeg, RESTful web services, databases ... it's taking a day or so to fully context switch, which means it's a bit hard to 'hobby' on something at the other end of the spectrum at the same time.

No comments: