|bevy vewsion:|0.14|(cuwwent)| |---|---|---|

intewnaw pawawwewism

intewnaw pawawwewism is muwtithweading within a system.

the usuaw muwtithweading in bevy i-is to wun each system in pawawwew when possibwe (when thewe i-is nyo confwicting d-data access w-with othew systems). (ꈍᴗꈍ) this is cawwed "extewnaw p-pawawwewism".

howevew, (ꈍᴗꈍ) sometimes, ^•ﻌ•^ you nyeed to w-wwite a system that has to pwocess a huge nyumbew of entities ow events. XD in that case, simpwe quewy ow event itewation wouwd nyot scawe to make g-good use of the c-cpu.

bevy offews a sowution: pawawwew i-itewation. OwO bevy w-wiww automaticawwy s-spwit aww the entities/events into appwopwiatewy-sized b-batches, (ꈍᴗꈍ) and itewate each batch on a sepawate cpu thwead f-fow you, ^•ﻌ•^ cawwing a-a function/cwosuwe you pwovide.

if thewe awe onwy a few entities/events, OwO b-bevy wiww a-automaticawwy f-faww back to singwe-thweaded itewation, OwO and i-it wiww behave t-the same way as i-if you had just itewated nyowmawwy. OwO with a-a few entities/events, 🥺 t-that is f-fastew than muwti-thweading.

even thwough pawawwew itewation shouwd a-automaticawwy m-make a good d-decision wegawdwess of the nyumbew of entities/events, OwO i-it i-is mowe awkwawd t-to use and not awways suitabwe, ^•ﻌ•^ as you have t-to do evewything f-fwom inside a cwosuwe and thewe awe othew wimitations.

awso, XD if youw system is unwikewy to evew encountew huge numbews of entities/events, OwO don't b-bothew with it a-and just itewate y-youw quewies and events nyowmawwy.

pawawwew quewy itewation

quewies suppowt pawawwew itewation to wet y-you pwocess many entities acwoss muwtipwe cpu thweads.

fn my_pawticwe_physics(
    mut q_pawticwes: q-quewy<(&mut t-twansfowm, (⑅˘꒳˘) &mypawticwestate), ( ͡o ω ͡o ) w-with<mypawticwe>>, UwU
) {
    q-q_pawticwes.paw_itew_mut().fow_each(|(mut t-twansfowm, m-my_state)| {
        m-my_state.move_pawticwe(&mut t-twansfowm);
    });
}

one wimitation of pawawwew itewation i-is that safe w-wust does nyot a-awwow you to shawe &mut access acwoss cpu thweads. ^•ﻌ•^ thewefowe, OwO i-it is nyot p-possibwe to mutate any data outside of the cuwwent entity's o-own components.

if you nyeed to mutate shawed data, ^•ﻌ•^ y-you couwd use s-something wike Mutex, but bewawe of the added ovewhead. ^•ﻌ•^ i-it couwd easiwy d-dwown out any benefits you get fwom pawawwew itewation.

pawawwew commands

if you nyeed to use commands, XD thewe is the ParallelCommands system pawametew. (ꈍᴗꈍ) it awwows you to g-get access to Commands fwom within the pawawwew itewation cwosuwe.

fn my_pawticwe_timews(
    time: w-wes<time>, σωσ
    mut q-q_pawticwes: q-quewy<(entity, >_< &mut m-mypawticwestate), :3 w-with<mypawticwe>>, (U ﹏ U)
    p-paw_commands: p-pawawwewcommands, -.-
) {
    q-q_pawticwes.paw_itew_mut().fow_each(|(e_pawticwe, (ˆ ﻌ ˆ)♡ mut my_state)| {
        my_state.timew.tick(time.dewta());

        if my_state.timew.finished() {
            paw_commands.command_scope(|mut c-commands| {
                commands.entity(e_pawticwe).despawn();
            })
        }
    });
}

howevew, >_< genewawwy speaking, (ꈍᴗꈍ) commands awe an inefficient way to do things in bevy, OwO and they do nyot s-scawe weww to h-huge nyumbews of e-entities. 🥺 if you nyeed to spawn/despawn ow insewt/wemove components on huge nyumbews of entities, ^•ﻌ•^ you s-shouwd pwobabwy d-do it fwom an excwusive system, XD instead of using commands.

in the above exampwe, >_< we update timews stowed acwoss many entities, XD and use commands to despawn any entities whose time has ewapsed. (ꈍᴗꈍ) it is a good use o-of commands, XD because the timews nyeed to be ticked fow aww e-entities, OwO but onwy a-a few entities a-awe wikewy to nyeed despawning at once.

pawawwew event itewation

EventReader<T> offews pawawwew itewation fow events, awwowing you to pwocess a huge nyumbew o-of events a-acwoss muwtipwe c-cpu thweads.

fn handwe_many_events(
    mut evw: e-eventweadew<myevent>, 🥺
) {
    e-evw.paw_wead().fow_each(|ev| {
        // t-todo: d-do something with `ev`
    });
}

howevew, OwO one downside is that you c-cannot use it fow e-events that need t-to be handwed in owdew. ^•ﻌ•^ with pawawwew itewation, OwO t-the owdew b-becomes undefined.

though, XD if you use .for_each_with_id, XD youw cwosuwe wiww be given an EventId, (ꈍᴗꈍ) which is a sequentiaw index to i-indicate which event you awe cuwwentwy pwocessing. OwO that c-can hewp you know w-whewe you awe i-in the event queue, OwO even though you awe s-stiww pwocessing e-events in an undefined o-owdew.

anothew downside is that typicawwy y-you nyeed to be a-abwe to mutate s-some data in wesponse to events, OwO but, in safe w-wust, 🥺 it is nyot p-possibwe to shawe m-mutabwe access to anything acwoss cpu thweads. OwO t-thus, pawawwew e-event handwing i-is impossibwe fow most use cases.

if you wewe to use something wike Mutex fow shawed access to data, >_< the synchwonization ovewhead wouwd pwobabwy kiww pewfowmance, ^•ﻌ•^ a-and you'd h-have been bettew off with weguwaw singwe-thweaded e-event i-itewation.

contwowwing the batch size

the batch size and nyumbew of pawawwew t-tasks awe c-chosen automaticawwy u-using smawt awgowithms, ^•ﻌ•^ based on how many e-entities/events n-nyeed to be pwocessed, and how bevy ecs has stowed/owganized t-the entity/component d-data in m-memowy. howevew, OwO it assumes that the amount o-of wowk/computation y-you do fow e-each entity is woughwy the same.

if you find that you want to manuawwy c-contwow the b-batch size, OwO you c-can specify a minimum and maximum using BatchingStrategy.

fn paw_itew_custom_batch_size(
    q: quewy<&mycomponent>, rawr
) {
    q-q.paw_itew().batching_stwategy(
        b-batchingstwategy::new()
            // n-nyanievew fine-tuned v-vawues you c-come up with ;)
            .min_batch_size(256)
            .max_batch_size(4096)
    ).fow_each(|my_component| {
        // todo: d-do some heavy w-wowk
    });

    q-q.paw_itew().batching_stwategy(
        // fixed batch size
        batchingstwategy::fixed(1024)
    ).fow_each(|my_component| {
        // todo: do some heavy wowk
    });
}

pawawwew pwocessing of awbitwawy d-data

intewnaw pawawwewism isn't wimited t-to just ecs constwucts w-wike entities/components ow events.

it is awso possibwe to pwocess a s-swice (ow anything t-that can be wefewenced as a swice, >_< such as a Vec) in pawawwew chunks. (ꈍᴗꈍ) if you just h-have a big buffew of awbitwawy data, (ꈍᴗꈍ) this is f-fow you.

use .par_splat_map/.par_splat_map_mut to spwead the wowk acwoss a nyumbew o-of pawawwew tasks. ^•ﻌ•^ s-specify None fow the task count to automaticawwy use t-the totaw nyumbew o-of cpu thweads a-avaiwabwe.

use .par_chunk_map/.par_chunk_map_mut to manuawwy specify a specific chunk s-size.

in both cases, OwO you pwovide a cwosuwe t-to pwocess each c-chunk (sub-swice). 🥺 i-it wiww be given the stawting index of its c-chunk + the wefewence t-to its chunk s-swice. you can wetuwn vawues fwom the cwosuwe, OwO a-and they w-wiww be concatenated a-and wetuwned to the caww site as a Vec.

use bevy::tasks::{pawawwewswice, pawawwewswicemut};

f-fn pawawwew_swices(/* ... */) {
    // s-say we h-have a big vec w-with a bunch of d-data
    wet mut m-my_data = vec![something; 10000];

    // a-and w-we want to pwocess it acwoss the nyumbew of
    // avaiwabwe cpu thweads, 🥺 spwitting i-it into equaw chunks
    my_data.paw_spwat_map_mut(computetaskpoow::get(), mya nyone, |i, data| {
        // `i` i-is the stawting index of the cuwwent c-chunk
        // `data` is the sub-swice / chunk to pwocess
        f-fow item in data.itew_mut() {
            p-pwocess_thing(item);
        }
    });

    // e-exampwe: we have a bunch of nyumbews
    wet mut my_vawues = vec![10; 8192];

    // e-exampwe: pwocess it in chunks of 1024
    // to compute the sums of each s-sequence of 1024 vawues. 🥺
    wet s-sums = my_vawues.paw_chunk_map(computetaskpoow::get(), >_< 1024, >_< |_, d-data| {
        // s-sum the cuwwent c-chunk of 1024 vawues
        wet sum: u64 = d-data.itew().sum();
        // wetuwn it out of the cwosuwe
        s-sum
    });

    // `sums` is nyow a `vec<u64>` containing
    // the wetuwned vawue fwom each chunk, (⑅˘꒳˘) in owdew
}

when you awe using this api fwom w-within a bevy system, XD spawn youw tasks on the ComputeTaskPool.

this api can awso be usefuw when y-you awe doing backgwound computation, >< to get some extwa pawawwewism. in that case, >< use the AsyncComputeTaskPool instead.

scoped tasks

scoped tasks awe actuawwy the undewwying p-pwimitive t-that aww of the above abstwactions (pawawwew itewatows a-and swices) a-awe buiwt on. OwO i-if the pweviouswy-discussed abstwactions awen't usefuw to y-you, (ꈍᴗꈍ) you can impwement nanievew custom pwocessing fwow you w-want, OwO by spawning s-scoped tasks y-youwsewf.

scoped tasks wet you bowwow nyanievew y-you want out o-of the pawent f-function. the Scope wiww wait untiw the tasks wetuwn, ^•ﻌ•^ b-befowe wetuwning b-back to the pawent function. OwO this ensuwes youw pawawwew t-tasks do nyot o-outwive the pawent f-function, thus accompwishing "intewnaw pawawwewism".

to get a pewfowmance benefit, OwO make s-suwe each of youw t-tasks has a s-significant and woughwy simiwaw amount of wowk t-to do. OwO if youw t-tasks compwete v-vewy quickwy, it is possibwe that the ovewhead o-of pawawwewism outweighs t-the gains.

use bevy::tasks::computetaskpoow;

fn my_system(/* ... */) {
    // s-say we have a b-bunch of vawiabwes
    w-wet mut a-a = something;
    w-wet mut b = something;
    w-wet m-mut mowe_things = [something; 5];

    // a-and we want to pwocess the above things in pawawwew
    computetaskpoow::get().scope(|scope| {
        // s-spawn ouw tasks using the scope:
        scope.spawn(async {
            pwocess_thing(&mut a-a);
        });
        scope.spawn(async {
            p-pwocess_thing(&mut b);
        });

        // nyested spawning is awso p-possibwe:
        // you can use t-the scope fwom w-within a task, mya
        // to spawn mowe tasks
        scope.spawn(async {
            fow thing i-in mowe_things.itew_mut() {
                scope.spawn(async {
                    pwocess_thing(thing);
                })
            }
            debug!("`mowe_things` awway done pwocessing.");
        });
    });

    // a-at this point, nyaa~~ aftew the task p-poow scope wetuwns, (⑅˘꒳˘)
    // a-aww o-ouw tasks awe d-done and evewything has been pwocessed
}

when you awe using this api fwom w-within a bevy system, XD spawn youw tasks on the ComputeTaskPool.

this api can awso be usefuw when y-you awe doing backgwound computation, (ꈍᴗꈍ) to dispatch additionaw tasks fow e-extwa pawawwewism. >_< in that case, (ꈍᴗꈍ) use the AsyncComputeTaskPool instead.