Today it is a better library than it was when I stopped working on it. I don’t imagine there will be hundreds of archetypes, because the API discourages (via performance) adding or removing components ad-hoc from entities. Internally world creates the storage tables with each register and protects them with some synchronization logic. This wouldn’t be bad if the number of archetypes was controlled linearly with the component types, but it’s technically logarithmic. If you have entities with Foo and Bar and one with just Foo and Baz these are held in entirely different tables. When you are querying an archetype and you don’t need all of the values it contains it doesn’t pollute your cache. With higher saturation the performance delta returns to normal. Specs has evolved quite a bit since I last worked on it years ago, there are some very talented developers who were willing to put in the hard work to improve it. Like specs you have to create a world object to hold everything. One obvious choice is a Mandelbrot set, as it was simple and embarrassingly parallel. world.insert() does actually return a slice of entities, so the above example would be possible with some more steps. The answer is iteration performance. If you are re-learning Leatherworking, you can pick up Forgotten Patterns of the Broken Isles to learn all the Legion recipes … Unlike specs this is created from a Universe. An Archetype is a collection of similar entities, similar defined by sharing the same set of components. All benchmarking was done on a Windows 10 using a AMD Threadripper 3960x with clocks fixed at 3.8Ghz for consistency reasons. If they sound interesting I encourage you to try them out. Cora Sherratt. There are some awesome goodies that are also included, like tags which are like components that get shared by a number of elements. hibitset is a rather exhaustive data structure and transversing it is not as cheap as iterating though an array, or even a hashmap. Specs doesn’t care too much about how many arrangements of components there are unless it effects component density adversely. Each rank lets you craft an item more efficiently. How in that is that possible? I love the iterator API, and anything that takes advantage of it is awesome in my book. Well all ECS developers will claim it’s largely about performance, and show you a mountain of numbers to back it up. The dataset is not huge, but it isn’t trivial in size either. That’s crazy. That would have largely made this article pointless as it would just be questionable benchmarks of Specs being faster in every case. Specs and Legion both have areas that can cause poor performance. If you are just skipping to the end, fair enough. Specs has comparably less book keeping required to look up components and modify them. With a warm cache legion is at best 3x faster than specs. Think velocity & position, or transform and material. The performance delta is not small either, at peak its almost ~4 times faster. For one, the BTreeMap based storage is much slower than I would have expected. Just how bad is it? Now let’s show them in at their worst. 8x slower with a warm cache and 26x slower with a cold cache. If most of your types are similar then they will tend to share a single table. Legion is a hair behind specs, but given how cursed my sharing logic was, I’ll give it some wiggle room to say it’s multithreading is more then adequate. In specs adding or removing a component from an entity is an O(1) operation, it’s just a matter of adding or removing a value from a table. Tables are created on demand as new entities are created. That single function over 1.7M data points in the Mandbrot took about 2-3 minutes of prep time before I could get to the few seconds of actual benchmarking. Blacksmiths have no new weapons to craft, but in its place they have relics and a mount. In tight loops the overhead of searching can easily dominate that actual work you are doing. But it doesn’t if just a few systems start tacking on a components to entities for their own purpose I could imagine situations where the archetypes get scattered. It is interesting that the overhead form a cache miss is not enough to close the gap in the less than three component benchmarks. It is fast, but it is fastest when types are very homogenous. That’s because Legion doesn’t require the creation of tables for each component type like specs did. The more archetypes the worse the performance will be. Specs feels better on the api design front, it’s APIs compose a bit better with other libraries, exposing Rayon or supporting more of libstd’s traits would be helpful. It gets left up the programmer to hopefully do efficiently. Legion is arguably a simpler to user in fact. Looking at this benchmark in isolation would be enough to sell me on Legion. Iteration performance should be similar to iteration over a vector, since internally it is. The key here is that we are always pushing the data values onto the end of the data array. This removes all the expensive and complicated bitmap filtering that was required by Specs to filter entities and moves the process to a pattern of selecting the correct archetypes to transverse. Full disclosure, I worked on specs. Meaning components are pulled into the cached that won’t be read. You now unlocked the other Imbued patterns in which some are sold to you and others are achieved by completing a quest for the NPC holding the pattern. I personally like to think about an ECS in terms of a column store database. All benchmarks are shown with warm or cold, the difference is that a cold benchmark is done after we thrashed the cache. While a single lock is not expensive to lock, having to lock multiple components for each entity creation clearly has an adverse effect on performance. The performance at 1 is a side effect of the number of cache misses that are required before you can iterate over the 1st element. Specs and Legion are not too different to the API user. I did a quick benchmark where an entity with 8 components was created, I then timed how long it took to remove on of the entities. The first reason being that DenseVec internally has a lookup buffer that maps the entity index to the real index, this is a u32. Component storage in specs doesn’t perform the way my intuitive expectations would have been. Important takeaway from this little intro, Legion doesn’t require any the definition of a Storage type like specs did. All components get’s treated equally even if they are more common than other components. This is always referenced to know if a value is present or not in the table. When an entity is written in Specs, each component gets written to a different storage column. // You can also define a struct and `#[derive(SystemData)]`. If you are re-learning Tailoring, you can pick up Forgotten Patterns of the Broken Isles to learn all the Legion recipes you previously had from quests. This is the first and probably typical benchmark. In one micro-benchmark legion took 50 times longer for a single operation. Here we can see the performance advantage I expected. We expect warm to be faster, but this measures the effects of cache misses in worst case performance. A HashMap or DenseVec don’t largely waste much space, but they can suffer from cache locality issues during iteration as the mapping of an entity index to component is not always 1-to-1. You don’t need to, and they can be created by the systems themselves later if you like. My experiments were done with Vec because it has the least overhead, and I wanted the most favourable numbers for both. Now to make this all work, specs has some ritual that is involved to setup everything. Iteration speeds are really important and it sacrifices quite a bit to actually maximize iteration performance. I wanted to get a better quantification of it. Learning Blacksmithing Legion’s archetypes are basically small filtered tables of every entity that are pre-filtered. So we lose helpful methods like filter map count reduce etc. There are new ranks associated with most Legion Blacksmithing Recipes. This is to the point where in some cases specs overtakes and passes. Specs has a World object which is used to glue everything components/entities together. // These are the resources required for execution.
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