Setting static parameter types in benchmarks

Question

There is very little information to be found on static parameter types. All I could find is a docstring that says: In testing, StaticArgument can be altered later: StaticArgument::set(8).

I need to alter a parameter in benchmarking code to be able to benchmark a specific path Pallet config:

#[pallet::config]
pub trait Config: frame_system::Config {
  type HeadersToKeep: Get<u32>;
}

In the runtime code the parameter is declared as static:

parameter_types! {
    pub static HeadersToKeepArgument: u32 = 1000;
}

Passed to the config:

impl my_pallet::Config for Runtime {
    type HeadersToKeep = HeadersToKeepArgument;
}

Now when I want to set it's value to execute a benchmark code:

benchmarks! {

  benchmark_with_pruning {

    let _ = T::HeadersToKeep::set(3);

    ...

  }: foo(RawOrigin::Signed(caller.clone()))

  verify {
   ...
  }

}

75 |         let _ = T::HeadersToKeep::set(3);
   |                                   ^^^ function or associated item cannot be called on `<T as pallet::Config>::HeadersToKeep` due to unsatisfied trait bounds
   |
   = note: the following trait bounds were not satisfied:
           `<T as pallet::Config>::HeadersToKeep: frame_support::storage::generator::StorageValue<_>`
           which is required by `<T as pallet::Config>::HeadersToKeep: frame_support::StorageValue<_>`
           `&<T as pallet::Config>::HeadersToKeep: frame_support::storage::generator::StorageValue<_>`
           which is required by `&<T as pallet::Config>::HeadersToKeep: frame_support::StorageValue<_>`
           `&mut <T as pallet::Config>::HeadersToKeep: frame_support::storage::generator::StorageValue<_>`
           which is required by `&mut <T as pallet::Config>::HeadersToKeep: frame_support::StorageValue<_>`

Answer 1

Other than Oliver's answer, I want to point out that you are likely having the wrong mental model here.

Your benchmarking code receives T, including T::HeadersToKeep and all the other configurations as input. Your benchmarking code must behave and tested without needing to alter these values.

For example, your mental model should be like this: You should write your benchmarking code such that if HeadersToKeep has an impact on the weight, in one runtime where it is configured to a higher value, it is captured.

Nowhere in this scenario you should need to alter HeadersToKeep within a benchmark. Again, T is the input of your benchmark, and it a way for you to know the final configuration of the runtime, not to alter it.

Lastly, as noted here, set is only available in std, meaning that while you could mis-use it in native execution of benchmarks, you cannot use it in WASM at all.

Answer 2

In your case, if you want to know how the benchmark result goes after you change the constant, you can just change that, and then benchmark this pallet.

Because when you modify the constant and deploy the new runtime code, you also need to do this benchmark again, right?

Answer 3

I don't completely understand your problem here, but you can do:

benchmark_with_pruning {
    // Components must be named with a single char. _ does not work.
    let c in 0 .. 3;
    HeadersToKeepArgument::set(c);
} …

Note that you have to define the parameter_types in the benchmarking and then use it in your mock file as super::benchmarking::HeadersToKeepArgument. The other way around does not work as the benchmarking cannot access the mock.