[asdfman123]

Can anyone simply explain the gist of how matrix multiplication is optimized? I know a lot of is farmed out to the GPU (if you've got a good GPU), but what's the essence of it? Caching? Some kind of clever mathematical tricks? All of the above?

[dragontamer]

> Caching?

That's the first step. If you have a 64kB cache, then you want to fill that cache ONCE, calculate everything you can with that 64kB chunk of data. Save off the result, and then load a new 64kB chunk. This is called "tiling".

Its actually kinda tricky to do just right, but once you know the concept, you basically spend effort ensuring that main-memory hits are minimized.

GPUs have many different memory regions: global Memory, L2, L1, "Shared" memory, and finally registers. Maximizing your math and minimizing your memory-moves is one big part of optimization.

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There are a ton of other optimization tricks: GPUs are more efficient if they access memory in certain ways. "Shared Memory" in GPUs are typically banked (on modern NVidia and AMD GPUs).

If you have 64-threads, it is more efficient if thread#0 accesses X+0. Thread#1 accesses X+1. Thread#2 accesses X+2. (etc. etc.) Thread#63 accesses X+63. In fact, a GPU can perform all 64-memory loads simultaneously.

However, if the 64-threads all access memory location X+4, you have a "bank conflict". The 64-threads can only read this memory location one-at-a-time, resulting in a massive slowdown.

There are a huge number of tricks in "tricking" the for loops to access memory optimally across your many, many threads that are calculating the multiplication.

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> Some kind of clever mathematical tricks?

The clever mathematical tricks have been solved and are known. LU decomposition, etc. etc. Its important to know them, but that's not generally what people mean by "optimization".

[yarg]

I'd hope that those conflicts only occur on writes and not reads?

[dragontamer]

> I'd hope that those conflicts only occur on writes and not reads?

They happen on reads and writes.

The best way to describe it is... GPUs internally not only have tons of cores and threads (albeit "SIMD" threads, not true threads...)... they also have multiple memory banks.

I know AMD's architecture better. So let me talk about GCN. The smallest "cohesive" structure of an AMD GCN GPU is the execution unit. An execution unit has its own L1 data-cache, a 64kB "shared-memory" region, and finally the 256 vALUs (vector ALUs). There are 4-instruction pointers (64-simd threads per instruction pointer).

I'll focus on the high performance "64kB Shared Memory" region.

The 64kB shared memory is organized into 32-channels. In effect, channel 0 handles all addresses ending in XXXX00. Channel 1 handles all addresses ending in XXXX04. Channel 2 handles all addresses ending in XXXX08. Etc. etc. Channel 31 handles all addresses ending in XXXX7C.

Each channel can serve ONE thread per clock cycle. So if all 256-threads of the execution unit try to grab address 400000 (ending in 00, so channel 0), they all hammer channel 0. Channels 1 through 31 don't do anything, because no one asked for data from them. In this case, ONLY one channel is working, while 31-channels are sitting around doing nothing.

In this case, the last thread will be waiting for ~256 clock cycles, because its got 255 threads in front of it trying to grab data.

If you instead wrote your program so that all the threads asked for data "equally" between all 32-channels, then your code would be 32x faster (because all 32-channels would be working). Each channel has 8 requests (32x8 == 256 reads), and the 256 threads all get their data in just 8 clock cycles.

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This isn't a problem in CPU world, because your L1 cache on Intel / AMD systems only has one channel per core. Actually, AMD offers TWO L1 channels per core (!!), and Intel offers 3x channels per core (2x reads + 1x write). So your one thread can do 2-reads + 1x write per clock tick.

If you have a massive 32-core CPU, you still have 2x reads + 1x write per core. So total of 64-reads + 32 writes across the whole system. This makes CPUs simple.

GPUs however, have a compromise. The 256-simd threads (or really, up to 2560-threads on an AMD system per EU) share the same 32 read/write channels.

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Technically speaking, there are "channels" and "banks", two different memory organization schemes in a GPU. In practice, you can just pretend that only channels exist, because a bank conflict more or less acts the same as a channel conflict.

[yarg]

Thanks for the reply. It makes me wonder how much of a slowdown GPU accelerated neural nets will get due to the mass reading of shared input values.

[dragontamer]

Broadcasts can be done efficiently on AMD systems. I dunno about NVidia, but I would assume NVidia PTX has some kind of low-level broadcast mechanism too.

A lot of optimization is just knowing all of the special ways you can move memory around. Broadcast was common enough that they've given AMD GPUs a special instruction just for it.

So in the case of neural networks all reading from the same input, you'd want to do it through the broadcast instructions, instead of through shared memory. Shared memory would create bank conflicts.

[eindiran]

On the CPU, matrix multiplication follows the same procedure you'd use to multiply matrices by hand. But GPUs are good at performing the same operation on a bunch of data at the same time. Any operation that is embarrassingly parallel is a good fit for doing on the GPU and often large matrix multiplications are. So the premise is that you do the steps that don't need to be done sequentially in parallel on the GPU.

Most approaches to doing matrix multiplication on the GPU benefit from doing operations in a way that plays off the behavior described above, make good use of caching, and respond to how the data in your matrix actually looks (e.g. is it sparse, etc).

To learn how you'd do matrix multiplication on the GPU, you might want to look up how its done via CUDA since many applications that make use of the GPU do so via CUDA and it doesn't require specific knowlege of graphics programming. This seems like a good introduction: https://www.shodor.org/media/content/petascale/materials/UPM...

[keerthiko]

You can look at the basic algorithm on wikipedia[0], and you'll see that a lot of it is parallelizable (non-sequential dependency of calculations) which by default is easily (time-) optimized via GPU pipes.

After that, there are some caching optimizations to be had by iterating over the nested loops to minimize cache misses. Another optimization is to split up the matrices if their shapes are appropriate and use compounding matrix operations to recombine them, allowing for further use of parallelization in phases.

There are a few fancier algorithms out there which are optimized for certain assumptions -- extremely large matrices, several consecutive operations, matrices with many 0s or many identical entries or following certain patterns for values such as the identity matrix or eigenvectors, etc.

[0] https://en.wikipedia.org/wiki/Matrix_multiplication

[johndough]

The answer is: No one really knows because cuBLAS is closed source.

But to get within the same order of magnitude, tiling the workload for better cache utilization is usually the most important step. This article [1] explains it quite well and also lists a few other tricks.

In addition, there's also the fast Fourier transform for large filter kernels and Winograd convolutions [2] for small filter kernels.

[1] https://cnugteren.github.io/tutorial/pages/page1.html

[2] https://arxiv.org/pdf/1509.09308.pdf

[tntn]

Not entirely true - Scott Gray knows: https://github.com/NervanaSystems/maxas/wiki/SGEMM

IIRC his kernels shipped in cuBLAS at some point.

[gnulinux]

Things I used in my project were:

> (vectorization, OpenMP, handwritten assembly, automatically optimized code, various optimizations, better algorithm.....)

also in addition to these, I used caching. Also GPU programming, but it's a trade-off.