Old `@submodel` syntax is very slow within testsets

[penelopeysm]

Since DynamicPPL 0.31.1 (#696), using the now-deprecated @submodel syntax in testsets is very slow.

In the below timings, I ran sample(model, spl, iters) for:

1. demo_assume_observe_literal as a control;
2. the two demo models with submodels. Here, ..._old is the version which uses @submodel, and ..._new a version which uses to_submodel(). DynamicPPL@master is still using the old versions.

I tested with both DynamicPPL.SampleFromUniform() as well as a 'real' sampler NUTS(). All tests used Julia 1.11, 1 thread.

From the timings, it's clear that 0.31.1 introduced a significant slowdown when using @submodel. This is already very clear locally and on macOS CI (2-4x slowdown), but is even worse on Ubuntu CI (20-30x slowdown).

I don't know what's causing this, but I believe this is the root cause for why CI has been so ridiculously slow recently. Thus, to speed up the tests for both DPPL and Turing, it should be a matter of priority to replace any remaining instances of @submodel with to_submodel() and fix any associated changes in behaviour (such as variable prefixing).

Curiously, I can't replicate this slowdown with Chairmarks (see section below), so it's likely some interaction with the testset.

Testset timings

You can see the code that was run @ https://github.com/penelopeysm/Shaymin.jl/blob/main/test/runtests.jl

Ubuntu CI @ 0.31.1: https://github.com/penelopeysm/Shaymin.jl/actions/runs/12315810160/job/34374779974#step:6:563

macOS CI @ 0.31.1: https://github.com/penelopeysm/Shaymin.jl/actions/runs/12315810160/job/34374780294#step:6:558

Ubuntu CI @ 0.31.0: https://github.com/penelopeysm/Shaymin.jl/actions/runs/12315894358/job/34375022027?pr=20#step:6:555

macOS CI @ 0.31.0: https://github.com/penelopeysm/Shaymin.jl/actions/runs/12315894358/job/34375022380?pr=20#step:6:287

                                                          ------ DPPL 0.31.1 -------     ------ DPPL 0.30.0 ------- 
Test Summary:                                    | Total  Local  CI-ubuntu  CI-macOS     Local  CI-ubuntu  CI-macOS
DynamicPPL.jl                                    |     0
                                                 |
  demo_assume_observe_literal                    |     0
    SampleFromUniform @ 1000 iters               |     0   1.5s       2.9s      1.7s      1.5s       3.0s      2.8s
    NUTS @ 100 iters                             |     0   7.8s      15.6s      8.6s      8.6s      16.3s      9.7s
                                                 |
  demo_assume_submodel_observe_index_literal_old |     0
    SampleFromUniform @ 1000 iters               |     0   5.3s    1m00.8s      6.1s      2.7s       5.2s      3.2s
    NUTS @ 100 iters                             |     0  10.6s    1m23.3s     11.9s      7.9s      15.8s     10.4s
                                                 |
  demo_assume_submodel_observe_index_literal_new |     0
    SampleFromUniform @ 1000 iters               |     0   1.8s       3.7s      1.9s                      
    NUTS @ 100 iters                             |     0   5.8s      12.2s      6.6s                      
                                                 |
  demo_dot_assume_observe_submodel_old           |     0
    SampleFromUniform @ 1000 iters               |     0   2.9s      56.2s      3.1s      0.7s       1.5s      0.9s
    NUTS @ 100 iters                             |     0   5.3s    1m12.4s      6.2s      1.9s       4.0s      3.0s
                                                 |
  demo_dot_assume_observe_submodel_new           |     0
    SampleFromUniform @ 1000 iters               |     0   0.5s       1.2s      0.6s   
    NUTS @ 100 iters                             |     0   1.8s       3.5s      1.9s   

Chairmarks 0.31.1

julia> @be sample(DynamicPPL.TestUtils.demo_assume_submodel_observe_index_literal(), SampleFromUniform(), 1000; progress=false)
Benchmark: 17 samples with 1 evaluation
 min    5.080 ms (111500 allocs: 6.844 MiB)
 median 5.157 ms (111500 allocs: 6.844 MiB)
 mean   5.906 ms (111500 allocs: 6.844 MiB, 3.99% gc time)
 max    16.734 ms (111500 allocs: 6.844 MiB, 67.82% gc time)

julia> @be sample(DynamicPPL.TestUtils.demo_assume_submodel_observe_index_literal(), NUTS(), 100; progress=false)
Benchmark: 8 samples with 1 evaluation
 min    10.173 ms (197197 allocs: 13.027 MiB)
 median 11.206 ms (209143.50 allocs: 14.009 MiB)
 mean   12.884 ms (207954.62 allocs: 13.838 MiB, 7.18% gc time)
 max    26.406 ms (219314 allocs: 14.599 MiB, 57.44% gc time)

Chairmarks 0.31.0

julia> @be sample(DynamicPPL.TestUtils.demo_assume_submodel_observe_index_literal(), SampleFromUniform(), 1000; progress=false)
Benchmark: 11 samples with 1 evaluation
 min    5.264 ms (111500 allocs: 6.844 MiB)
 median 5.339 ms (111500 allocs: 6.844 MiB)
 mean   18.684 ms (111500 allocs: 6.844 MiB, 15.98% gc time)
 max    131.152 ms (111500 allocs: 6.844 MiB, 95.94% gc time)

julia> @be sample(DynamicPPL.TestUtils.demo_assume_submodel_observe_index_literal(), NUTS(), 100; progress=false)
Benchmark: 7 samples with 1 evaluation
 min    10.221 ms (193852 allocs: 12.863 MiB)
 median 11.400 ms (218755 allocs: 14.557 MiB)
 mean   14.288 ms (216068.14 allocs: 14.384 MiB, 9.04% gc time)
 max    32.843 ms (234745 allocs: 15.744 MiB, 63.25% gc time)

[devmotion]

IMO that's somewhat expected. Deprecations are massive performance killers. But they are disabled by default (--depwarn=no), so regular users are not affected by slowdowns caused by deprecations. However, tests are (by default, this can be changed) run with --depwarn=yes, so you'll see this effect when testing.

[penelopeysm]

@devmotion Ah! that's good to know, thanks – You wouldn't happen to know why it's so pronounced on Ubuntu? I think if not for that 20x slowdown the whole thing would've slipped under the radar.