alt Hacker NewsI agree that optimizing for lower occupancy can yield significant performance gains in specific cases, especially when memory latencies are the primary bottleneck. Leveraging ILP and storing more data in registers can indeed help reduce the need for higher occupancy and lead to more efficient kernels. The examples in the GTC2010 talks highlighted that quite well. However, I would argue that occupancy still plays an important role, especially for scalability and general-purpose optimization. Over-relying on low occupancy and fewer threads, while beneficial in certain contexts, has its limits.
The first thing to consider is the register pressure. Increasing the number of registers per thread to optimize for ILP can lead to register spilling when the register file is exhausted, which drastically reduces performance. This becomes more pronounced as problem sizes scale up (the talk examples avoids that problem). Many real-world applications, especially compute-bound kernels, need high occupancy to fully utilize the GPU’s resources. Focusing too much on minimizing thread counts can lead to underutilization of the SM’s parallel execution units. An standard example will be inference engines.
Also, while low-occupancy optimizations can be effective for specific workloads (e.g, memory-bound kernels), designing code that depends on such strategies as a general practice can result in less adaptable and robust solutions for a wide variety of applications.
I believe there is a balance to strike here. low occupancy can work for specific cases, higher occupancy often provides better scalability and overall performance for more general use cases. But you have to test for that while you are optimizing your code. There will not be a general rule of thump to follow here.
> The first thing to consider is the register pressure. Increasing the number of registers per thread to optimize for ILP can lead to register spilling when the register file is exhausted
Kernels should almost never use local memory (except in arcane cases where you are using recursion and thus a call stack that will spill where an alternative non-recursive formulation would not really work).
> Many real-world applications, especially compute-bound kernels, need high occupancy to fully utilize the GPU’s resources
> while low-occupancy optimizations can be effective for specific workloads (e.g, memory-bound kernels)
I think this is almost exactly backwards, performant high compute intensity kernels (on a (fl)op/byte of memory traffic basis) tend to uniformly have low occupancy; look at a ncu trace of many kernels in cuBLAS or cuDNN for instance. You need a large working set of arguments in registers or in smem to feed scalar arithmetic or especially MMA units quickly enough as gmem/L2 bandwidth alone is not sufficient to achieve peak performance in many case. The only thing you need to do is to ensure that you are using all SMs (and thus all available scalar arithmetic or MMA units) which does not by itself imply high occupancy (e.g., a kernel that has 1 CTA per SM).
The simplest way to write a memory-bound kernel is to simply spawn a bunch of threads and perform load/stores from them and it isn't too hard to achieve close to peak this way, but even then depending upon the warp scheduler to rotate other warps in to issue more load/stores is inferior to unrolling loops, and you can also get close to peak mem b/w by using not too many SMs either through such unrolling, so even these need not have high occupancy.
(I've been Nvidia GPU programming for around 11 years and wrote the original pytorch GPU backend/tensor library, the Faiss GPU library, and contributed some stuff to cuDNN in its early days such as FFT convolution.)