@aeva well, suppose we're working in 32-thread waves internally (totally hypothetical number)

now those 32 invocations get (in the very first thread group) x=0,...,15 for y=0 and then y=1.

Say the image is R8G8B8A8 pixels and the internal image layout stores aligned groups of 4 texels next to each other and then goes to the next y, and the next 4-wide strip of texels is actually stored something like 256 bytes away or whatever.

@aeva so, x=0,..,3 y=0 are all good, these are all adjacent, straight shot, read 16 consecutive bytes, great.

x=0,...,3 y=1 in threads 16..19 are also good, these are the next 16 bytes in memory.

But if we have 256-byte cache lines (another Totally Hypothetical Number), well, those 32 bytes are all we get.

x=4,..,7 for y=0 and 1 are in the cache line at offset 256, x=8,...,11 for y=0,1 at offset 512, x=12,...,15 at offset 768.

@aeva And caches are usually built to have multiple "banks" that each handle a fraction of a cache line. Let's say our hypothetical cache has 16 16-byte banks to cover each 256B cache line.

Well, all the requests we get from that nice sequential load go into the first 2 banks and the rest gets nothing.

So that's lopsided and causes problems, and will often mean you lose a lot of your potential cache bandwidth because you only actually get that if your requests are nicely distributed over mem.

@aeva long story short, this whole thing with your thread groups being a row-major array of 16x16 pixels can kind of screw you over, if the underlying image layout is Not Like That.

This happens all the time.

Ordering and packing of PS invocations into waves is specifically set up by the GPU vendor to play nice with whatever memory pipeline, caches, and texture/surface layouts it has.

In CS, all of that is Your Job, generally given no information about the real memory layout.

Good luck!

@aeva If you do know what the real memory layout is, you can make sure consecutive invocations have nice memory access patterns, but outside consoles (where you often get those docs), eh, good luck with that.

The good news is that with 1D, this problem doesn't exist, because 1D data is sequential everywhere.

So as long as you're making sure adjacent invocations grab adjacent indices, your memory access patterns are generally fine.

(Once you do strided, you're back in the danger zone.)

@aeva I don't know what value Slice has with the sizes you pass in, but it would be really bad if Slice works out to be some medium to large power of 2.

The issue is that the "i" loop goes in sequential samples but invocation to invocation (which is the dimension that matters), the loads inside are strided to be "Slice" elements apart.

You really want that to be the other way round. Ideally sequential loads between invocations.

@aeva separately, don't want that % SizeA in there, that doesn't have to be bad but it can be, I don't know how good shader compilers are about optimizing induction variables like that

might want to keep that as an actual counter and just do (in the modified loop)

j += GROUP_SIZE;
j -= (j >= SizeA) ? SizeA : 0;

(you also need SizeA >= GROUP_SIZE now, but I don't think that changes anything in your case)

@aeva even on a GPU, if you do enough MADs per sample eventually you're going to be compute bound with this approach, but I'd be shocked if you were anywhere close to that right now.

First-order it's going to be all futzing with memory access.

@aeva I mean, you can literally do the math!

If you're on a GPU, then even on a mobile GPU from several years ago, you're in the TFLOP/s range by now for actual math.

So, ballpark 1e12 MADs per second.

48kHz stereo is ballpark 1e5 samples per second.

Math-wise, that means you can in theory do 1e7 MADs per sample, enough for brute-force direct convolution with a >3 minute IR. You're probably not doing that.

@aeva You can always do better convolution algs, but even for brute-force, the math is just not the problem for IR sizes you're likely using.

But as written in your code, you also have two loads for every MAD, and there's nowhere near that level of load bandwidth available, not even if it's all L1 hits.

Making it sequential across invocations should help noticeably. But beyond that, you'll need to save loads.

@aeva I don't know about ideal but there is definitely is some mileage to be had in loading one of the two into registers/shared memory in blocks, double-buffering the next fetch and having only one load via L1/tex in the inner loop.

That said the better FFT-based conv kinda nukes that.

Good news: FFT-based conv kinda automatically exploits all the sharing for you!

Bad news: that means you're now down to loading and using each IR FFT coeff exactly once.

@aeva It is work-efficient and gets both your load count and your mul-add count way down, but it also means what's left is kinda BW bound by construction and there's not much you can do about it.

(I mean you can make the block sizes small enough that you're still summing tons of terms again, but that's defeating the purpose.)

@aeva thanks! That does seem lower than what I remember getting…. though probably still means that for the amount of computation we usually have to do in a frame, it‘s CPU all the way

unless maybe in The Future we make the model a lot bigger

@aeva yeah, I think that‘s still the conclusion

which is unfortunate because I would really like to be paid to do that! but it is difficult to argue for when like, even if you implement everything very well the roundtrip already has highs that would (in presence of the rest of the audio stack) cause issues. like, right now our model runs, on a weakish desktop, with 2ms averages and 3 to 4ms highs, for a 10ms frame, and that’s already kind of as high as we dare going

@aeva noise / echo / reverb suppression and/or speech enhancement in various different configurations, for voice calling. so essentially „run smallish neural network on audio fast“

and yeah sure we can make the task arbitrarily more complex by making the model larger but then we need to do annoying things like justify the added compute by showing a major quality improvement. maybe requirements will do it for us eventually if someone decides we must have fullband stereo audio or something

@aeva doesn’t immediately file a bug, but when you press the uhhh idk what it looks like now they‘ve been messing with it but, thumbs down or below 4 stars or sad smiley face, whatever it is, button, ideally also with a Problem Token (the little what was actually bad questionaire) then at least for media (so calling a/v - can’t speak to how fronted or chat or whatever do it) in Teams/Skype it goes to a big database along with technical telemetry that is usually correlated with call quality (stuff like „percent of aborted calls“) which then feeds into a quite thorough A/B system, and if we spot regressions in what are considered Tier 1 metrics rollout stops (no statsig positive change is okay generally if you can justify why the change fixes a rare bug, adds a useful feature or whatever. Though of course if you can move a problem token or T1 metric in the right direction, that’s even better).

Mostly we catch issues before changes make it to public, though

@halcy the little reaction emoji thing has been broken for months, there's no overlay for it anymore so i don't see when people press it and i'm not sure if they see when i do. could you pass that along to whoever's problem that is?

also there doesn't seem to be an option to make it not use the GPU anymore which is somewhat problematic for me since i have to completely shut down teams when doing perf stuff