Why accessing a Float property allocates memory?

General Usage Performance
1

Hello community!

I’m running a simulation which requires millions of iterations, and the memory consumption (even temporary) is huge and I would like to reduce it. Pursuing this, I found something that I wasn’t expecting.

I have my own struct defined, with some basic attributes and I noticed using @time that every time that I want to access a Float64 (or any Float) property it allocates 16bytes but if the property is an Int, it doesn’t.

Here is my type definition

abstract type AbstractVoltmeter end

struct EChTVoltmeter <: AbstractVoltmeter
  EChTVoltmeter() = new()
end

mutable struct ECTVoltmeter <: AbstractVoltmeter
  pulse_acum::Float64
  phase::UInt8
  I::Float64 # Current

  ECTVoltmeter() = new(0.0, 1, 1.0)
end

And here is the output

julia> z = ECTVoltmeter()
ECTVoltmeter(0.0, 0x01, 1.0)

julia> @time z.phase
  0.000004 seconds
0x01

julia> @time z.I
  0.000005 seconds (1 allocation: 16 bytes)
1.0

I know this won’t change much on my memory-leak problem, but I want to understand what is happening.
I notice something similar using Tuples, which always allocates memory when you access one element (not expected by me at all)

Thank you!

2

Use BenchmarkTools.jl and you should find that there is no allocation.

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3

To expand slightly on @dpsanders correct answer, in your example you’re doing what’s called “benchmarking in global scope”. In other words, the allocation you’re seeing is due to the fact that you’re running @time z.I in global scope and not because running z.I (which is equivalent to getproperty(z,:I) allocates. See the section of the performance tips manual section on global variables.

As @dpsanders mentioned, you can use the BenchmarkTools package to do careful benchmarking:

julia> using BenchMarkTools
julia> @benchmark getproperty($z,:I)
BenchmarkTools.Trial: 
  memory estimate:  0 bytes
  allocs estimate:  0
  --------------
  minimum time:     0.015 ns (0.00% GC)
  median time:      0.017 ns (0.00% GC)
  mean time:        0.018 ns (0.00% GC)
  maximum time:     0.031 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     1000
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