# When to use sizehint! for Dict?

**URL:** https://discourse.julialang.org/t/when-to-use-sizehint-for-dict/28833
**Category:** Performance
**Created:** [September 16, 2019, 11:17pm UTC](https://discourse.julialang.org/t/when-to-use-sizehint-for-dict/28833 "2019-09-16T23:17:01Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![milesf](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/milesf/32/9289_2.png) [@milesf](https://discourse.julialang.org/u/milesf)
#### Post date: [September 16, 2019, 11:17pm UTC](https://discourse.julialang.org/t/when-to-use-sizehint-for-dict/28833/1 "2019-09-16T23:17:01Z")

</div>

Benchmarking shows that using `sizehint!` to match the expected number of Dict elements is best in almost all cases, except when the number of elements is at or near 10^5. Both 10^4 and 10^6 prefer `sizehint!`, but 10^5 strangely does not. This is a reproducible result on my system.

```julia
using BenchmarkTools

function bench_hint(n)
    @belapsed for i in 1:$n d[i] = i end setup = (
        d = Dict{Int, Int}();
        sizehint!(d, $n)
    )
end

function bench_no_hint(n)
    @belapsed for i in 1:$n d[i] = i end setup = (
        d = Dict{Int, Int}()
    )
end

#------- Interactive below ------------

# Run all benchmarks
julia> times = [(n, bench_no_hint(n), bench_hint(n)) for i in 1:8 for n = 10^i]
8-element Array{Tuple{Int64,Float64,Float64},1}:
 (10, 7.12299794661191e-8, 7.120739219712526e-8)     
 (100, 7.839081632653062e-7, 7.760636363636364e-7)   
 (1000, 1.2393333333333334e-5, 1.1199333333333335e-5)
 (10000, 0.000221074, 0.000163545)                   
 (100000, 0.002791825, 0.003224295) # <- Unexpected
 (1000000, 0.088250659, 0.066421847)                 
 (10000000, 1.313739977, 0.800849544)                
 (100000000, 17.57806693, 15.449179262)              

# Show performance ratio. hinting best for everything except 10^5
julia> [(t[3] / t[2], t[1]) for t in times]
8-element Array{Tuple{Float64,Int64},1}:
 (0.9996828965954626, 10)       
 (0.9899930536901236, 100)      
 (0.9036578805809575, 1000)     
 (0.7397749169961189, 10000)    
 (1.1549058411612476, 100000) # <- Unexpected
 (0.7526498697307179, 1000000)  
 (0.6095951695317863, 10000000) 
 (0.8788895459052619, 100000000)

# Reproduce the above result
julia> times = [(n, bench_no_hint(n), bench_hint(n)) for i in 4:6 for n = 10^i]
3-element Array{Tuple{Int64,Float64,Float64},1}:
 (10000, 0.000221071, 0.000163615)  
 (100000, 0.00277381, 0.003233641) # <- Unexpected
 (1000000, 0.063075762, 0.055881177)

julia> [(t[3] / t[2], t[1]) for t in times]
3-element Array{Tuple{Float64,Int64},1}:
 (0.7401015963197344, 10000)  
 (1.165775954373227, 100000) # <- Unexpected
 (0.8859374065112365, 1000000)

```