6 ms·
Yes... But you don't need to memorize all this, and it leaves out a simple rule: vectors (or arrays) outperform everything else if the dataset is small. Small i
by harrid 3y ago
Yes... But you don't need to memorize all this, and it leaves out a simple rule: vectors (or arrays) outperform everything else if the dataset is small. Small is usually in the order of 100-300, but can vary wildly.
Also note that all <algorithm>s have built-in fully automatic parallelism via <execution>, a massively underused feature. In typical CPP fashion though, their newer views:: counterparts lack those overloads for the moment.
- varjag 3y agoNot quite. Many other datastructures can be shoehorned into contiguous, cache efficient representations.
- csjh 3y agoNever heard of `execution`, great tip. Is there some big caveat or just generally unknown?
- harrid 3y agoYou need to be aware that these invocations are going to blast your cores full throttle as you obviously don't have fine grained control. But as long as your data is easily parallelized on a vector with computations that don't depend on each other, it's a game changer. I use it all the time to multithread things with literally a single line of code.
- lionkor 3y agoI looked into it (my video, probably too long https://youtu.be/9oh66SF91LA?si=azDCSOAJKA9Gpzim https://youtu.be/9oh66SF91LA?si=azDCSOAJKA9Gpzim), and the general result was that they make sense for non-small datasets and are a solid way to to parallelize something without having to pull in OpenMP or something.
- coffeeaddict1 3y agoThey're only supported in MSVC and GCC (for the latter you need to link against Intel's TBB to make it work). Support in libc++ (Clang) is work in progress.
- pca006132 3y agoClang does support parallel stl already (requires either TBB or OpenMP). Our project https://github.com/elalish/manifold https://github.com/elalish/manifold made use of this to speed up mesh processing algorithms a lot.
- coffeeaddict1 3y agoDoes it? You mean if you link against libstdc++ instead of libc++?
- pca006132 3y agoI remember it works for libc++ (partially, see https://libcxx.llvm.org/Status/PSTL.html https://libcxx.llvm.org/Status/PSTL.html), but forgot when I linked against libc++ last time...
- cyber_kinetist 3y agoIncomplete support from Clang’s STL (especially in Apple Clang).
- 01100011 3y agoI rarely meet a CS major who will accept that small things will always stay small. They will frequently talk you into more complex data structures with the assurance that you are just too stupid to realize your problem will suddenly need to scale several orders of magnitude. Do they teach you in CS-101 to interpret '+' as the exponential operator? It often feels that way.
- baq 3y agoThere’s computer science and there’s software engineering. The best developers are good at both. …but in order for this to really matter, communication is required, since even the best developers don’t scale.
- pca006132 3y agoAre they fresh graduates? It is very important to understand the workload distribution for any optimization. Even if small things can sometimes get large, optimizing for the small case can often yield large gain as they may occur frequently. And complex data structures are usually worse in the small case...
- papichulo2023 3y agoiirc, on msvc, execution parallel delegates to the OS (windows) to decide how many threads to create, at it is usually more than the total number of vcpu contrary to the usual recommendation.
- harrid 3y agoIt's very much implementation defined yes. I'm currently using this for something that runs for about 10 seconds, and even music playback and mouse cursor movement gets affected. (But I'm about to move it to GPU)
- Arelius 3y agoYes, but actually small can often be much larger then 100-300 depending on the specifics. Programmers often vastly underestimate how fast cache, and prefetch can go compared to complicated data structures.
- jcelerier 3y ago... Or much smaller, I remember a benchmark for a case I had a few years ago, the cutoff I had for a map being faster than std::vector/array & linear probing was closer to N=10 that 100. (Not std::map, at the time it must have been something like tsl:: hopscotch_map). Note also that nowadays for instance boost comes with state-of-the-art flat_map and flat_unordered_map which gives both the cache coherency for small sizes and the algorithmic characteristics of various kinds of maps
- gpderetta 3y agoExactly. I measured vector vs an efficient linear probing hash map very recently and the cutoff was single digit. Even against unordered_map or plain std::map the cutoff was surprisingly low (although in this case I would trust a synthetic benchmark significantly less).
- Arelius 3y agoSure, the point here is with all the specific context specific details, your most likely comparing apples versus oranges. So simple complexity analysis or a general rule without benchmarks and a good understanding of the system and how it interacts with your details is not going to solve your problem.
- menaerus 3y agoSo you're saying that if I had had to store 100 elements in the memory, I would be better off using hashmap instead of vector/array? What type of elements did you use in your experiment or how large they were and what was your access pattern?
- 3y ago
- elromulous 3y agoIt's even more true for larger collections. Stroustrup gave a talk on this back at GoingNative2012. Tl;Dr: "Use a Vectah" -Bjarne https://youtu.be/YQs6IC-vgmo https://youtu.be/YQs6IC-vgmo
- CyberDildonics 3y agoA vector should be people's default data structure, but this presentation is bizarre, because it is based on looping through every element of a vector or a list to find the element you want. This is never a scenario that should happen, because if you are going to retrieve an arbitrary element it should be in a hash map or sorted map.
- maldev 3y agoIsn't that because view performs the procedure and access when the data is accessed? It's mainly meant so that you don't have to load all this memory in or stall when accessing the view, and if you don't need that capability, the regular algorithm stl is better.
- CyberDildonics 3y agovectors (or arrays) outperform everything else if the dataset is small. Small is usually in the order of 100-300, but can vary wildly. This is a very poor way to choose a data structure. How many items you want to store is not what someone should be thinking about. How you are going to access it is what is important. Looping through it - vector. Random access - hash map. These two data structures are what people need 90% of the time. If you are putting data on the heap it is already because you don't know how many items you want to store.