tim2CF
Elixir structs vs Erlang records
Hello!
Does anyone know why Elixir structures are implemented on top of Erlang maps, but not on top of Erlang records (tuples)? For me, Erlang records are looking more natural then maps for basement of Elixir structs, and besides in some cases it seems performance of records is better (for example in pattern matching), take a look:
iex(2)> defmodule Hello do
...(2)> defstruct [:foo]
...(2)> end
{:module, Hello,
<<70, 79, 82, 49, 0, 0, 5, 144, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 0, 182,
0, 0, 0, 18, 12, 69, 108, 105, 120, 105, 114, 46, 72, 101, 108, 108, 111, 8,
95, 95, 105, 110, 102, 111, 95, 95, 7, ...>>, %Hello{foo: nil}}
iex(3)> struct = %Hello{foo: 123}
%Hello{foo: 123}
iex(4)> fn -> 1..1000000 |> Enum.each(fn(_) -> %Hello{foo: foo} = struct; foo end) end |> :timer.tc
{13719674, :ok}
iex(5)> record = {Hello, 123}
{Hello, 123}
iex(6)> fn -> 1..1000000 |> Enum.each(fn(_) -> {Hello, foo} = record; foo end) end |> :timer.tc
{565886, :ok}
iex(7)>
Of course, you can say - “If you like Erlang records, just use it”, but Elixir infrastructure is very coupled with structs/maps - they are used everywhere (Ecto, Phoenix, Plug, Elixir standard libraries). So if I’m using Elixir, I don’t really have a choice - I have to use structs/maps
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kokolegorille
There is a good answer from Jose here about the choice of struct vs record.
michalmuskala
Please, please, don’t benchmark in the shell. The shell runs an interpreter and not the compiled code. The result will probably will be wildly different.
OvermindDL1
Indeed, here it is in Benchee, I tried to prevent certain optimizations from happening by interning the test data in different modules than that which is accessed so things like the record macro’s don’t get optimized out and so forth (records were a lot faster than structs before I made that change).
Code struct_record_bench.exs:
defmodule AStruct1 do
defstruct [a: 1]
def news1(), do: %__MODULE__{}
end
defmodule AStruct9 do
defstruct [a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9]
def news9(), do: %__MODULE__{}
end
defmodule ARecords do
import Record
defrecord :aRecord1, [a: 1]
defrecord :aRecord9, [a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9]
def newr1(), do: aRecord1()
def newr9(), do: aRecord9()
end
defmodule StructRecordBench do
import AStruct1
import AStruct9
import ARecords
def classifiers(), do: [:get, :put]
def time_mult(_), do: 2
def inputs(_) do
nil
end
def actions(:get) do
%{
"Struct1" => fn -> news1().a end,
"Struct9-first" => fn -> news9().a end,
"Struct9-last" => fn -> news9().i end,
"Record1" => fn -> aRecord1(newr1(), :a) end,
"Record9-first" => fn -> aRecord9(newr9(), :a) end,
"Record9-last" => fn -> aRecord9(newr9(), :i) end,
}
end
def actions(:put) do
%{
"Struct1" => fn -> %{news1() | a: 42} end,
"Struct1-opt" => fn -> %AStruct1{news1() | a: 42} end,
"Struct9-first" => fn -> %{news9() | a: 42} end,
"Struct9-first-opt" => fn -> %AStruct9{news9() | a: 42} end,
"Struct9-last" => fn -> %{news9() | i: 42} end,
"Struct9-last-opt" => fn -> %AStruct9{news9() | i: 42} end,
"Record1" => fn -> aRecord1(newr1(), a: 42) end,
"Record9-first" => fn -> aRecord9(newr9(), a: 42) end,
"Record9-last" => fn -> aRecord9(newr9(), i: 42) end,
}
end
end
Results:
╰─➤ mix bench struct_record
Benchmarking Classifier: get
=============================
Operating System: Linux"
CPU Information: AMD Phenom(tm) II X6 1090T Processor
Number of Available Cores: 6
Available memory: 15.67 GB
Elixir 1.7.4
Erlang 21.1.1
Benchmark suite executing with the following configuration:
warmup: 2 s
time: 2 s
memory time: 2 s
parallel: 1
inputs: none specified
Estimated total run time: 36 s
Benchmarking Record1...
Benchmarking Record9-first...
Benchmarking Record9-last...
Benchmarking Struct1...
Benchmarking Struct9-first...
Benchmarking Struct9-last...
Name ips average deviation median 99th %
Record9-last 23.29 M 0.0429 μs ±107.83% 0.0400 μs 0.0700 μs
Record9-first 22.06 M 0.0453 μs ±5.87% 0.0440 μs 0.0550 μs
Record1 21.61 M 0.0463 μs ±10.12% 0.0450 μs 0.0640 μs
Struct9-first 18.14 M 0.0551 μs ±19.80% 0.0560 μs 0.0660 μs
Struct1 17.93 M 0.0558 μs ±8.76% 0.0560 μs 0.0700 μs
Struct9-last 17.46 M 0.0573 μs ±6.31% 0.0560 μs 0.0720 μs
Comparison:
Record9-last 23.29 M
Record9-first 22.06 M - 1.06x slower
Record1 21.61 M - 1.08x slower
Struct9-first 18.14 M - 1.28x slower
Struct1 17.93 M - 1.30x slower
Struct9-last 17.46 M - 1.33x slower
Memory usage statistics:
Name Memory usage
Record9-last 72 B
Record9-first 72 B - 1.00x memory usage
Record1 72 B - 1.00x memory usage
Struct9-first 72 B - 1.00x memory usage
Struct1 72 B - 1.00x memory usage
Struct9-last 72 B - 1.00x memory usage
**All measurements for memory usage were the same**
Benchmarking Classifier: put
=============================
Operating System: Linux"
CPU Information: AMD Phenom(tm) II X6 1090T Processor
Number of Available Cores: 6
Available memory: 15.67 GB
Elixir 1.7.4
Erlang 21.1.1
Benchmark suite executing with the following configuration:
warmup: 2 s
time: 2 s
memory time: 2 s
parallel: 1
inputs: none specified
Estimated total run time: 54 s
Benchmarking Record1...
Benchmarking Record9-first...
Benchmarking Record9-last...
Benchmarking Struct1...
Benchmarking Struct1-opt...
Benchmarking Struct9-first...
Benchmarking Struct9-first-opt...
Benchmarking Struct9-last...
Benchmarking Struct9-last-opt...
Name ips average deviation median 99th %
Record1 18.38 M 0.0544 μs ±691.86% 0.0500 μs 0.120 μs
Record9-first 15.52 M 0.0644 μs ±497.05% 0.0600 μs 0.140 μs
Record9-last 15.45 M 0.0647 μs ±539.09% 0.0600 μs 0.150 μs
Struct1 15.13 M 0.0661 μs ±647.84% 0.0600 μs 0.130 μs
Struct1-opt 14.48 M 0.0691 μs ±327.02% 0.0600 μs 0.110 μs
Struct9-first 14.31 M 0.0699 μs ±309.43% 0.0600 μs 0.160 μs
Struct9-first-opt 12.73 M 0.0786 μs ±328.39% 0.0700 μs 0.130 μs
Struct9-last 11.66 M 0.0857 μs ±414.23% 0.0800 μs 0.21 μs
Struct9-last-opt 10.74 M 0.0931 μs ±322.63% 0.0800 μs 0.21 μs
Comparison:
Record1 18.38 M
Record9-first 15.52 M - 1.18x slower
Record9-last 15.45 M - 1.19x slower
Struct1 15.13 M - 1.21x slower
Struct1-opt 14.48 M - 1.27x slower
Struct9-first 14.31 M - 1.28x slower
Struct9-first-opt 12.73 M - 1.44x slower
Struct9-last 11.66 M - 1.58x slower
Struct9-last-opt 10.74 M - 1.71x slower
Memory usage statistics:
Name Memory usage
Record1 96 B
Record9-first 160 B - 1.67x memory usage
Record9-last 160 B - 1.67x memory usage
Struct1 112 B - 1.17x memory usage
Struct1-opt 112 B - 1.17x memory usage
Struct9-first 176 B - 1.83x memory usage
Struct9-first-opt 176 B - 1.83x memory usage
Struct9-last 176 B - 1.83x memory usage
Struct9-last-opt 176 B - 1.83x memory usage
**All measurements for memory usage were the same**
So Records are faster in general (in all tested cases here actually) than Structs, but only marginally so, so much so that only the most performance sensitive code would really care, so in general most people shouldn’t care. 
I’m surprised that putting the struct type in the update syntax doesn’t make it faster actually as it could infer some existing structure, but I guess that all it would be doing is adding an extra runtime check or so (hence the module-optional variants are slower in the end).
EDIT: Personal Opinion time: Personally I’d prefer records were ubiquitous and used struct syntax (first class records in other words). Records in every language I’ve seen are statically sized, there is no point in them being maps, especially if they ‘own’ their module definition as structs do now then all the proper accessors for Access and extra data would all be accessible as they are for structs as well and as such by using those generated macro’s then you could generate getting/setting code that would be even more efficient than how structs work now. HOWEVER, Elixir is extremely poorly typed and doesn’t know what the type of a given thing would be, and Erlang works around that by requiring using the record name at all uses of a record variable, Elixir tries to be a little more succinct, and that succinctness is at odds with efficiency, and so the first-class syntax uses the slightly less efficient version in order for ease of use and relegates the more efficient version to a side set of macro’s since you require the names anyway. If Elixir had a decent typing system then you’d be able to have both efficiency and succinctness, but maybe that’s for an Elixir 2.0 or something. ^.^
EDIT: Hmm, a possible workaround for the first-class syntax access would be just dispatching based on the ‘module’ in the type-tag of the record, it would be a ‘remote call’ on the BEAM but might be good… I should test…
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