On GPU architecture and why it matters

I had a nice conversation recently around the architecture of CPUs versus that of GPUs. It was so good that I still remember the day after, so it is probably worth writing down.

Note that a lot of the following are still several levels of abstraction away from the hardware, and this is in no way a rigorous discussion of modern hardware design. Still, from the software development point of view, they are adequate for everything we need to know.

It started out of the difference in allocating transistors to different components on the chip of CPU and GPU. Roughly speaking, on CPUs, a lot of transistors are reserved for the cache (several levels of those), while on GPUs, most of transistors are used for the ALUs, and cache is not very well-developed. Moreover, a modern CPU merely has a few dozen cores, while GPUs might have thousands.

Why is that? The simple answer is because CPUs are MIMD, while GPUs are SIMD (although modern nVidia GPUs are closer to MIMD).

The long answer is CPUs are designed for the Von-neumann architecture, where data and instructions are stored on RAM and then fetched to the chip on demand. The bandwidth between RAM and CPU is limited (so-called data bus and instruction bus, whose bandwidth are typically ~100 bits on modern computers). For each clock cycle, only ~100bits of data can be transfer from RAM to the chip. If an instruction or data element needed by the CPU is not on the chip, the CPU might need to wait for a few cycles before the data is fetched from RAM. Therefore, a cache is highly needed, and the bigger the cache, the better. Modern CPUs have around 3 levels of cache, unsurprisingly named L1, L2, L3… with higher level cache sits closer to the processor. Data and instructions will first be fetched to the caches, and CPU can read from the cache with much lower latency (cache is expensive though, but that is another story). In short, in order to keep the CPU processors busy, cache is used to reduce the latency of reading from RAM.

GPUs are different. Designed for graphic processing, GPUs need to compute the same, often simple, arithmetic operations on a large amount of data points, because this is what happens in 3D rendering where there are thousands of vertices need to be processed in the shader (for those who are not familiar with computer graphics, that is to compute the color values of each vertex in the scene). Each vertex can be computed independently, therefore it makes sense to have thousands of cores running in parallel. For this to be scalable, all the cores should run the same computation, hence SIMD (otherwise it is a mess to schedule thousands of cores).

For CPUs, even with caches, there are still chances that the chip requires some data or commands that are not in the cache yet, and it would need to wait for a few cycles for the data to be read from RAM. This is obviously wasteful. Modern CPUs have pretty smart and complicated prediction on where to prefetch the data from RAM to minimize latency. For example, when it enters a FOR loop, it could fetch data around the arrays being accessed and the commands around the loops. Nonetheless, even with all those tricks, there are still chances for cache misses!

One simple way to keep the CPU cores busy is context switching. While the CPU is waiting for data from RAM, it can work on something else, and this eventually keeps the cores busy, while also provides the multi-tasking feature. We are not going to dive into context switching, but basically it is about to store the current stack, restore the stack trace, reload the registers, reset the instruction counter, etc…

Let’s talk about GPUs. A typical fragment of data that GPUs have to work with are in the order of megabytes in size, so it could easily take hundreds of cycles for the data to be fetched to the cores. The question then is how to keep the cores busy.

CPUs deal with this problem by context switching. GPUs don’t do that. The threads on GPUs are not switching, because it would be problematic to switch context at the scale of thousands of cores. For the sake of efficiency, there is little of locking mechanism between GPU cores, so context switching is difficult to implement efficiently.
– In fact, the GPUs don’t try to be too smart in this regards. It simply leaves the problem to be solved at the higher level, i.e. the application level.

Talking of applications, GPUs are designed for a very specific set of applications anyway, so can we do something smarter to keep the cores busy? In graphical rendering, the usual workflow is the cores read a big chunk of data from RAM, do computation on each element of the data and write the results back to RAM (sounds like Map Reduce? Actually it is not too far from that, we can talk about GPGPU algorithms in another post). For this to be efficient, both the reading and writing phases should be efficient. Writing is tricky, but reading can be made way faster with, unsurprisingly, a cache. However, the biggest cache system on GPUs are read-only, because writable cache is messy, especially when you have thousands of cores. Historically it is called texture cache, because it is where the graphical application would write the texture (typically a bitmap) for the cores to use to shade the vertices. The cores cant write to this cache because it would not need to, but it is writable from the CPU. When people move to GPGPU, the texture cache is normally used to store constants, where they can be read by multiple cores simultaneously with low latency.

To summarize, the whole point of the discussion was about to avoid the cores being idle because of memory latency. Cache is the answer to both CPUs and GPUs, but cache on GPUs are read-only to the cores due to their massive number of cores. When cache is certainly helpful, CPUs also do context switching to further increase core utilization. GPUs, to the best of my knowledge, don’t do that much. It is left to the developers to design their algorithms so that the cores are fed with enough computation to hide the memory latency (which, by the way, also includes the transfer from RAM to GPU memory via PCIExpress – way slower and hasn’t been discussed so far).

The proper way to optimize GPGPU algorithms is, therefore, to use the data transfer latency as the guide to optimize.

Nowadays, frameworks like tensorflow or torch hide all of these details, but at the price of being a bit inefficient. Tensorflow community is aware of this and trying their best, but still much left to be done.


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