Clarify global variable interpolation in benchmarking.

[nathanrboyer]

I've never understood why benchmarking requires interpolation, but running the code normally doesn't. I don't think the explanation here was satisfactory. Hoping someone can rework this sentence to explain a little better.

"Additionally, by using $ to interpolate external values, you remove the overhead caused by global variables."

julia> using BenchmarkTools

julia> @btime sum_abs(v);
98.725 ns (1 allocation: 16 bytes)

julia> @btime sum_abs($v);
60.469 ns (0 allocations: 0 bytes)

It's especially confusing given the earlier statement:

"Typically, the most common beginner pitfall is the use of untyped global variables without passing them as arguments."

In the example, v is clearly passed as an argument, so I don't see why there is suddenly a performance issue.

---

Also, apparently Chairmarks.jl drops the need to interpolate, so maybe the entire benchmarking section would be clearer if flipped to show examples with Chairmarks.jl and just mention BenchmarkTools.jl as the alternative option?

[gdalle]

That's a good point, and I've never really understood interpolation either. However, BenchmarkTools remains the most common option in the ecosystem, so I think it remains important to at least mention the dollar sign syntax so that people don't get confused?

[nathanrboyer]

Modern Julia workflows would probably switch to Chairmarks for speed and ergonomics though right, so shouldn't it be the first suggestion? Why point new users to the older, more complicated option first? 🤷‍♂

BenchmarkTools and its need for $ could just be mentioned in an admonition as something you may come across. There is already an admonition on the same page discussing older packages users may come across but shouldn't necessarily choose; a similar but softer version could be written to mention BenchmarkTools.jl and its need for $ interpolation:

Advanced: Some widely used parallel programming packages like LoopVectorization.jl (which also powers Octavian.jl) or ThreadsX.jl are no longer maintained.

[gdalle]

I've given it some thought and I think you're right, Chairmarks is more actively maintained and should probably be recommended instead of BenchmarkTools. Do you want to open a PR?

[nathanrboyer]

Sure. It may be a few weeks before I get to it though.