Reliable Change Detection #54
Closed
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
|
Closes #52 |
…reliable-change-detection
alice-i-cecile
force-pushed
the
reliable-change-detection
branch
from
5d6cc41
to
cb5030f
Compare
|
Todo: add ring buffer diagram. |
alice-i-cecile
commented
May 6, 2021
|
Slides: https://docs.google.com/presentation/d/1gggEX4dfOaBdKcZJySm8i7TmxIJkyModEcl8dDkrqQw/edit?usp=sharing Also make sure to link to the talk |
|
Steal simpler explanation of the algorithm from the talk. |
✅ Deploy Preview for leafwing-studios ready!
To edit notification comments on pull requests, go to your Netlify site settings. |
TimJentzsch
reviewed
Aug 12, 2022
There was a problem hiding this comment.
Not sure if you're still working on this, but I did a first pass.
Definitely an interesting write-up, especially since I tried to find out just recently if it only works until the end of the frame or if it "wraps around". (Of course the docs had the answer though :D)
Comments are mostly suggestions to link to the docs where it makes sense.
I purposefully used links to the 0.8 docs, so that the links don't break (or stop making sense) in future Bevy versions.
| draft = true | ||
| +++ | ||
|
|
||
| **An abridged version of this post was also presented as a 10 minute talk for the Rust East Coast meetup group ([slides](https://docs.google.com/presentation/d/1gggEX4dfOaBdKcZJySm8i7TmxIJkyModEcl8dDkrqQw/edit#slide=id.p), [video](TODO: add link))*. |
There was a problem hiding this comment.
Don't forget the TODO here :)
There was a problem hiding this comment.
The Leafwing logo on the Docs is the old one, but it's probably weird to update the slides after the talk
|
|
||
| **An abridged version of this post was also presented as a 10 minute talk for the Rust East Coast meetup group ([slides](https://docs.google.com/presentation/d/1gggEX4dfOaBdKcZJySm8i7TmxIJkyModEcl8dDkrqQw/edit#slide=id.p), [video](TODO: add link))*. | ||
|
|
||
| Over the past half-year or so, I've dedicated much of my time to improving [Bevy](https://bevyengine.org/), an ECS-first game engine in Rust. |
There was a problem hiding this comment.
The "past half-year" is probably not accurate anymore
Comment on lines
+29
to
+31
| // TODO: WRITE EXAMPLE | ||
| // Text-only | ||
| // Spawning system, changes creation |
There was a problem hiding this comment.
Don't forget the TODO here :)
| Today, I want to talk about one of the features I helped build and love using: **reliable change detection**. | ||
|
|
||
| For those of you who've never used Bevy before, all of the game logic occurs in **systems**: ordinary Rust functions that are automatically scheduled to run each frame. | ||
| These receive their data from the central ECS data store by requesting access to specific data, typically via **queries**, which carefully slice the data and request only the specific data needed. |
There was a problem hiding this comment.
Consider linking to Query here:
Suggested change
| These receive their data from the central ECS data store by requesting access to specific data, typically via **queries**, which carefully slice the data and request only the specific data needed. | |
| These receive their data from the central ECS data store by requesting access to specific data, typically via [**queries**](https://docs.rs/bevy/0.8.0/bevy/prelude/struct.Query.html), which carefully slice the data and request only the specific data needed. |
| As we build out our game, we can combine these systems to spectacular effect, carefully slicing our data and performing elaborate feats of automatically-parallelized behavior-first execution. | ||
|
|
||
| By default though, queries fetch data from *all* of the entities with the appropriate components. | ||
| This can be quite expensive when we only need to operate on a subset, even if we carefully slice our queries using `With` or `Without` **query filters**. |
There was a problem hiding this comment.
Consider linking to the docs here:
Suggested change
| This can be quite expensive when we only need to operate on a subset, even if we carefully slice our queries using `With` or `Without` **query filters**. | |
| This can be quite expensive when we only need to operate on a subset, even if we carefully slice our queries using [`With`](https://docs.rs/bevy/0.8.0/bevy/prelude/struct.With.html) or [`Without`](https://docs.rs/bevy/0.8.0/bevy/prelude/struct.Without.html) **query filters**. |
| To do so, we store timers in three separate locations: | ||
|
|
||
| * On the `World` as a global indicator of current time. Whenever a system begins execution, [this increments once](https://github.com/bevyengine/bevy/pull/1471/files#diff-5499ffc12ee4011981ff6d5069a1986197d824648c65ed24056e6b2772b3cd81R146). | ||
| * On each piece of `Resource` or `Component` data, [marking when the data was last changed](https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) in terms of the global timer. |
There was a problem hiding this comment.
Consider linking to the docs here:
Suggested change
| * On each piece of `Resource` or `Component` data, [marking when the data was last changed](https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) in terms of the global timer. | |
| * On each piece of [`Res`ource](https://docs.rs/bevy/0.8.0/bevy/prelude/struct.Res.html) or [`Component`](https://docs.rs/bevy/0.8.0/bevy/prelude/trait.Component.html) data, [marking when the data was last changed](https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) in terms of the global timer. |
|
|
||
| * On the `World` as a global indicator of current time. Whenever a system begins execution, [this increments once](https://github.com/bevyengine/bevy/pull/1471/files#diff-5499ffc12ee4011981ff6d5069a1986197d824648c65ed24056e6b2772b3cd81R146). | ||
| * On each piece of `Resource` or `Component` data, [marking when the data was last changed](https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) in terms of the global timer. | ||
| * On each `System`, marking when the system last ran in terms of the global timer. |
There was a problem hiding this comment.
Consider linking to the docs here:
Suggested change
| * On each `System`, marking when the system last ran in terms of the global timer. | |
| * On each [`System`](https://docs.rs/bevy/0.8.0/bevy/prelude/trait.System.html), marking when the system last ran in terms of the global timer. |
|
|
||
| 1. **Ergonomic.** Yes, all of the magic happens behind-the-scenes. | ||
| 2. **Automatic.** Yes, there's no need to opt in. | ||
| 3. **Low memory overhead.** Yes, we're only storing a single u32 per component, no matter how many systems we have. |
There was a problem hiding this comment.
Formatting:
Suggested change
| 3. **Low memory overhead.** Yes, we're only storing a single u32 per component, no matter how many systems we have. | |
| 3. **Low memory overhead.** Yes, we're only storing a single [`u32`](https://doc.rust-lang.org/std/primitive.u32.html) per component, no matter how many systems we have. |
| No matter the scenario, this solution detects each change *exactly* once per system if the data has been mutably accessed at least once since the system last ran. | ||
|
|
||
| What's that? Oh right, I must apologize: there is one case where a change may be missed with a warning. | ||
| If, while your system is asleep, more than [`u32::MAX * 3 / 4`](https://github.com/bevyengine/bevy/pull/1471#issuecomment-787009753) (3221225471) systems have run, you will miss changes that occurred. |
There was a problem hiding this comment.
Formatting:
Suggested change
| If, while your system is asleep, more than [`u32::MAX * 3 / 4`](https://github.com/bevyengine/bevy/pull/1471#issuecomment-787009753) (3221225471) systems have run, you will miss changes that occurred. | |
| If, while your system is asleep, more than [`u32::MAX * 3 / 4`](https://github.com/bevyengine/bevy/pull/1471#issuecomment-787009753) (3,221,225,471) systems have run, you will miss changes that occurred. |
There was a problem hiding this comment.
Also, why is it 3 / 4?
| * On each `System`, marking when the system last ran in terms of the global timer. | ||
|
|
||
| If the data's change tick is greater than the system's change tick, the data has changed since it was last processed. | ||
| We use [wrapping subtraction]((https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) to determine what "greater than" means, giving us **ring buffer** behavior, allowing our change detection to continue functioning indefinitely rather than running out of space as time goes on. |
There was a problem hiding this comment.
Typo:
Suggested change
| We use [wrapping subtraction]((https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) to determine what "greater than" means, giving us **ring buffer** behavior, allowing our change detection to continue functioning indefinitely rather than running out of space as time goes on. | |
| We use [wrapping subtraction](https://github.com/bevyengine/bevy/pull/1471/files#diff-0e94997025571f709abd7cac97f03bb4e8ec8bf29650068a7e5ec07170011892R137) to determine what "greater than" means, giving us **ring buffer** behavior, allowing our change detection to continue functioning indefinitely rather than running out of space as time goes on. |
|
Thanks for the feedback! I think I'm going to close this out though; not sure it has enough value at this point. |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Rendered
A technical post on solving reliable change detection for Bevy.