Add parallel iteration benchmarks

[alice-i-cecile]

ElliotB256/bevy_bench#2 reveals that while Bevy can be quite competitive with the performance of other Rust ECS backends it's parallel iteration performance is quite poor.

This is likely to be particularly true with unpredictable workloads.

Once we have a good benchmark suite for this we can work on improving the performance in other issues.

[mockersf]

bench from https://github.com/ElliotB256/bevy_bench/blob/master/examples/bevy.rs looks not that far from https://github.com/cart/ecs_bench_suite/blob/bevy-benches/src/bevy/heavy_compute.rs, are there specific things to add?

results also seems to be in the same ballpark as those reported
from ecs_bench_suite:

bench	legion (*)	legion 0.2.4	bevy	specs
heavy_compute	0.701ms(0.723ms)	4.34ms	1.06ms	0.995ms

from ElliotB256:

ECS	create entities	run loop
bevy	0.01	1.11
specs	0.03	0.54
legion	0.82	0.70

[alice-i-cecile]

Adding that benchmark suite to Bevy itself will help reduce the need for this issue for sure.

The other concern that I have is that perfectly predictable and uniform workloads like those shown in both of those examples are not always representative and will underestimate the potential gains of more sophisticated strategies.

Parallel pathfinding might be a good example of an unpredictable but highly parallelizable task to supplement the example you linked.

[ElliotB256]

Is it worth noting that my bench is using a simpler operation - e.g. multiplying a couple of floats, instead of inverting a matrix 100 times? Each parallel task is much smaller.

[alice-i-cecile]

Yep: ideally we could vary the weight of the tasks to try and assess the relative overhead in each engine.

[ElliotB256]

I'd be happy to make a PR for adding some benchmarks for this issue

[ElliotB256]

Apologies I was away from this for so long.

I've added an example of a parallel_light_compute benchmark to https://github.com/ElliotB256/ecs_bench_suite/tree/parallel_light_compute. This benchmark is for the case where some very short task must be performed for each entity - a realistic example would be updating position of an entity based on velocity. It compares to the existing heavy_compute benchmark as follows:

Benchmark	Description
heavy_compute	1,000 entities. Parallel iter inverts a matrix 100 times.
parallel_light_compute	10,000 entities. Parallel iter inverts a matrix once.

The numbers on my PC are as follows:

Benchmark	time
parallel_light_compute/legion	106.60 us
parallel_light_compute/legion (packed)	112.58 us
parallel_light_compute/bevy	1.0066 ms
parallel_light_compute/hecs	100.08 us
parallel_light_compute/shipyard	142.53 us
parallel_light_compute/specs	108.00 us

[ElliotB256]

The batch size used for bevy in the above tests was 64. Changing to a batch size of 1024 offers an improvement to 907.58us, but still behind the other libraries.

For different batch sizes:

Batch Size	Time
8	1.9473ms
64	1.0444ms
256	1.0048ms
1024	960.13us
4096	1.0296ms
10,000	1.2633ms

It's not clear to me what the equivalent batch size is for the other tests. legion and specs (maybe the others too?) use rayon for parallel iteration, but I'm unfamiliar with the internals. Some information claims by default it splits every time (see e.g. this PR and this SO - but that doesn't seem quite right, and the Rayon page says:

Parallel iterators take care of deciding how to divide your data into tasks; it will dynamically adapt for maximum performance.

[ElliotB256]

@mockersf pointed out on discord that my example was creating a new task pool every iteration. Changing it so the task pool is initialised before hand, the results are:

Batch Size	Time
8	1.177ms
64	234.13us
256	149.48us
1024	130.48us
4096	207.13us
10,000	485.55us

This is much better. It is still not quite as good as the others, but at optimum batch sizes bevy is at least competitive.

[ElliotB256]

ElliotB256/ecs_bench_suite@0ad6a44

[ElliotB256]

By chance I just saw on the discord:

iamseb: I'm trying to run query.par_for_each across a large set of entities, and noticing that my CPU is only engaging half my available cores. I tried running https://github.com/bevyengine/bevy/blob/main/examples/async_tasks/async_compute.rs and setting the sleep timer to 1 sec for all tasks and noticed the same thing, with the example taking twice the expected time to run. In previous versions of bevy that used rayon it would max out the CPU. Is this a design decision, or is there some config I'm missing for the task scheduler?
MinerSebas: Bevy splits has three diferent Thread pools which together use the whole CPU .(https://github.com/bevyengine/bevy/blob/main/crates/bevy_core/src/task_pool_options.rs#L56-L81).
You can adjust the used ratios by inserting the DefaultTaskPoolOptions Resource before adding the default plugins.

Does anyone know if this is relevant for the benchmark?