Please help: Poor performance, what am I missing?

General Usage
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1

Hello clever people,

I’m having trouble dissecting a performance problem.

MWE below. The computation in the 3rd benchmark is the composition of the computations in the first 2 benchmarks. I expect the 3rd benchmark to take about the total time taken by the first 2 benchmarks. But it’s 2x as long and there’s an unexpected allocation. Clearly I’m missing something fundamental. What’s happening here?


using BenchmarkTools

"Sum the numeric values, count the non-numeric values"
function processcollection(t)
    total = 0.0
    ncat  = 0
    for x in t
        total, ncat = processvalue(x, total, ncat)
    end
    total, ncat
end

processvalue(x::Real, total, ncat) = total + x, ncat
processvalue(x, total, ncat) = total, ncat + 1

d = Dict("a" => (1,2), "b" => ("a", "b", "c"), "c" => (1.1, 2.2, 3.3, 4.4, 5.5, 6.6))

k = "c"
v = d[k]
@benchmark $d[$k]                     # 20ns,  0 bytes
@benchmark processcollection($v)      #  2ns,  0 bytes
@benchmark processcollection($d[$k])  # 45ns, 32 bytes, 1 alloc
2

If you move the lvalues into a function

function main()
    d = Dict("a" => (1,2), "b" => ("a", "b", "c"), "c" => (1.1, 2.2, 3.3, 4.4, 5.5, 6.6))
    k = "c"
    @benchmark processcollection($d[$k])
end
main()

I get 27ns

3

In general this will still perform a dynamic dispatch — even if the types of d and k are known by the compiler (because you interpolated them), it doesn’t know the type of d[k] (because your dictionary is heterogeneous).

4

Doesn’t seem to help - I’m still getting 45ns on my machine.

5

Thanks Steven.
I still don’t get why the 3rd benchmark is slower than the sum of the first two. The dynamic dispatch should happen in both the 1st and the 3rd benchmarks right?

6

No, the dynamic dispatch is determining (at runtime) which compiled method of processcollection to call based on the type of d[k], and this only happens in the third benchmark.

7

Ah ok, so we’re essentially talking about the distinction between
@benchmark processcollection($v) and
@benchmark processcollection(v) , which use compile-time dispatch and run-time dispatch respectively, and the latter is the same as the 3rd benchmark.

Thanks again, most helpful.
Jock