Simon Danisch / Oct 09 2019
Julia for Pythonistas
About me
Outline
- What is Julia
- Performance & FFI
- Writing Libraries
- Noteworthy examples
- Calling Julia from Python
- Caveats
What is Julia
- Python at the speed of C, with elegant syntax for math like Matlab
- Multi purpose
- Multi paradigm (functional, object inheritance)
- Appeared in ~2012
Noteworthy Features
- Best of lisp (macros, code as datastructure, functional programming)
- dispatches functions on multiple arguments
- optional type system
- introspection & reflections
- cross platform
When to use Julia
When performance is key
But I don’t need performance!
- Python performance is good enough for me!
- Imagine learning a new language, at the same time your project runs into performance issues
I'll just call out to C
Rely on mature external libraries for performance-critical code
- Someone has to write those libraries
- some problems are impossible or awkward to express as calls to C-libraries
I'll miss all the great Libraries
Julia talks to all major Languages - mostly without overhead!
PyCall
Passing functions:
using PyCall so = pyimport("scipy.optimize") function mycallback(x) cos(x) - x end so.newton(mycallback, 1) # or with lambda so.newton(x-> cos(x) - x, 1)
0.739085
Inherit from Python classes:
P = pyimport("numpy.polynomial") mutable struct Doubler <: P.Polynomial function __init__(self, x=10) self.x = x end my_method(self, arg1::Number) = arg1 + 20 x2.get(self) = self.x * 2 function x2.set!(self, new_val) self.x = new_val / 2 end end Doubler().x2
20
Write whole functions
py""" def pymap(f, A, B): for i in range(len(A)): A[i] = f(B[i]) """ A = rand(Float32, 10) pymap(f, a, b) = (py"pymap"(f, a, b); a) result = pymap(sin, zeros(Float32, 10), A) result ≈ sin.(A)
true
Calling C
void c_map(void *fptr, float *A, float *B, int len) { float (*f)(float) = (float (*)(float))fptr; for (int i = 0; i< len; ++i) A[i] = (*f)(B[i]); }
cmap.c
C
# Great shell integration: run(`gcc -shared -fPIC -O3 /cmap.c -o /cmap.so`) function cmap(f, A, B) # Get a C function pointer from a Julia function fptr = ($f, Float32, (Float32,)) ccall( (:c_map, "/cmap.so"), # function name, library Nothing, # return type (Ptr{Nothing}, Ptr{Float32}, Ptr{Float32}, Cint), # argument types fptr, A, B, length(A) # arguments ) return A end result = cmap(sin, zeros(Float32, 10), A) result ≈ sin.(A)
true
C++
Cxx can compile arbitrary C++ code and link it with Julia and other C++ libraries
using Cxx cxx""" void cxxmap(void *fptr, float *A, float *B, int len) { float (*f)(float) = (float (*)(float))fptr; for (auto x = 0; x < len; ++x) A[x] = (*f)(B[x]); } """ function cxxmap(f, A, B) fptr = ($f, Float32, (Float32,)).ptr cxxmap(fptr, pointer(A), pointer(B), Cint(length(A))) return A end result = cxxmap(sin, zeros(Float32, 10), A) result ≈ sin.(A)
true
Also: JavaCall, RCall, Matlab
Let's compare the performance:
using BenchmarkTools a, b = zeros(Float32, 10^7), rand(Float32, 10^7) c_b = cmap($sin, $a, $b) cxx_b = cxxmap($sin, $a, $b) python_b = pymap(sin, a, b); julia_b = map!($sin, $a, $b); timings = [python_b, c_b, cxx_b, julia_b]
4-element Array{Float64,1}:
55.9617
0.150005
0.151949
0.0924455
timings[1] ./ timings
4-element Array{Float64,1}:
1.0
373.064
368.293
605.348
Woah, Python is slow
- nobody would write loops, right?
Now, can we go faster?
using BenchmarkTools function map_threaded!(f, a, b) Threads. for i in eachindex(a, b) a[i] = f(b[i]) end return a end
map_threaded! (generic function with 1 method)
threaded_b = map_threaded!($sin, $a, $b)
0.0398836
using CuArrays, CUDAnative acu, bcu = cu(a), cu(b) # transfer to GPU memory # calls to cuda are async cuda_b = CuArrays. (map!($(CUDAnative.sin), $acu, $bcu))
0.000746504
push!(timings, threaded_b, cuda_b);
timings[2] ./ timings
6-element Array{Float64,1}:
0.0026805
1.0
0.98721
1.62264
3.76108
200.944
Take away
- State of the art performance
- simple to get there
- great interop
- Still interactive & fun
Building Libraries in Julia
New motto: come for the performance, stay for the syntax!
- it's nice to have an optional type system & multiple dispatch
- Julia libraries are very easy to extend
0.2s
Julia 1.2 (Julia)
module ColorBase abstract type AbstractColor end struct RGB{T} <: AbstractColor r::T g::T b::T end Base.Tuple(rgb::RGB) = (rgb.r, rgb.g, rgb.b) function Base.:(+)(a::T, b::T) where T <: AbstractColor tuple = (Tuple(a) .+ Tuple(b)) return T(tuple...) end export AbstractColor end module ColorExtensions using ..ColorBase # Syntax for using module in same namespace struct XYZSpace{T} <: AbstractColor x::T y::T z::T end Base.Tuple(hsl::XYZSpace) = (hsl.x, hsl.y, hsl.z) export XYZSpace end;
using .ColorExtensions, .ColorBase xyz = XYZSpace(0.1, 0.33, 0.5) + XYZSpace(0.1, 0.2, 0.3)
XYZSpace{Float64}(0.2, 0.53, 0.8)
Same performance as built-in types
colors = [XYZSpace(rand(3)...) for i in 1:10^7] sum($colors);
builtin = rand(3 * 10^7) sum($builtin);
Julia is the Language with most code reuse (Is it?!)
Catchy Examples
Data Science
using CSV, DataFrames, Plots, StatsPlots theme(:wong) data = CSV.read(emissions.csv) data.co2 = data.co2 ./ 10^9; co2max = by(data, :country, :co2 => maximum) sort!(co2max, :co2_maximum, rev = true) bad_countries = co2max.country[1:10] the_worst = filter(data) do row row.country in bad_countries end the_worst plot( :Year, :co2, group = :country, legend = :topleft, linewidth = 4, title = "Total Accumulative Emission" )
LINQ like query with Query.jl
using FileIO, Query, VegaLite, Dates data |> (co2 = _.co2, year = Date(_.Year)) |> ({_.country}) |> (maximum(_.co2)) |> (5) |> (_, __) |> # This is essentially ungroup DataFrame |> ( :line, x=:year, y=:co2, color=:country, title="Emissions", width = 500, height = 300 )
Flux
- Features comparable to Tensorflow
- Mostly as fast
- Julia GPU acceleration
- Written 100% in Julia, just a ~2000 LOCs
- Works with plain Julia Arrays, types and functions
From the Flux model zoo:
using Flux using Flux: crossentropy, normalise, onecold, onehotbatch, params using Statistics: mean labels = Flux.Data.Iris.labels() features = Flux.Data.Iris.features() normed_features = normalise(features, dims=2) klasses = sort(unique(labels)) onehot_labels = onehotbatch(labels, klasses) train_indices = [1:3:150 ; 2:3:150] X_train = normed_features[:, train_indices] y_train = onehot_labels[:, train_indices] X_test = normed_features[:, 3:3:150] y_test = onehot_labels[:, 3:3:150] model = Chain( Dense(4, 3), # A normal Julia type, predefined in Flux softmax # A normal julia function ) loss(x, y) = crossentropy(model(x), y) accuracy(x, y) = mean(onecold(model(x)) .== onecold(y)) data_iterator = Iterators.repeated((X_train, y_train), 110) Flux.train!(loss, params(model), data_iterator, Descent(0.5))
accuracy_score = accuracy(X_test, y_test) accuracy_score > 0.8
true
Cassette - Hook into the Compiler!
using Cassette Cassette. Ctx # this prehook implements simple trace logging for overdubbed functions Cassette.prehook(::Ctx, f, args...) = println(f, args) Cassette.overdub(Ctx(), *, 1, 2.0)
2.0
Cassette.overdub(::Ctx, ::typeof(convert), ::Type{Float64}, x::Float64) = x * 3.0
Cassette.overdub(Ctx(), *, 1, 2.0)
6.0
Interactive Graphics & Mimis
260.9s
Julia 1.2 (Julia)
using Mimi, MimiDICE2013 using AbstractPlotting, WGLMakie, Markdown using Observables, Hyperscript, JSServe using JSServe: with_session, Slider using JSServe.DOM using AbstractPlotting: scatter, scatter! set_theme!(markersize = 5,font = "Dejavu Sans", resolution = (500, 400)) baseline = MimiDICE2013.get_model() run(baseline) interacted = MimiDICE2013.get_model() with_session() do session time_tatm = getdataframe(baseline, :climatedynamics, :TATM) time = time_tatm.time control_rate_s = JSServe.Slider(LinRange(0, 1, 100)) control_rate = map(control_rate_s) do r round(r, digits = 3) end year = JSServe.Slider(1:length(time)) data = map(control_rate) do control set_param!(interacted, :emissions, :MIU, fill(control, 60)) run(interacted) return something.(interacted[:climatedynamics, :TATM], 0.0) end scene = scatter(time, something.(time_tatm.TATM, 0.0)) scatter!(time, data, color = :red) scene[Axis].names.axisnames = ("Year", "Temperature Increase") scene[Axis].names.textsize = (8, 8) dmg_estimated = map(year, control_rate) do year_idx, _ round(interacted[:damages, :DAMAGES][year_idx], digits = 4) end selected_year = map(i-> time[i], year) b = DOM.font("● baseline", color = :black) ec = DOM.font("● with emission control", color = :red) return md""" # Explore Climate Set amount of emission control: $(control_rate_s) $(control_rate) # Temperature Increase $b | $ec $(scene) # Estimated damage in year $(selected_year) $(year) $(dmg_estimated) trillion USD per year. """ end
Cool, I'm intrigued, but can't leave Python
- You can also call Julia from Python
- could be used instead of C/Cython
132.0s
Python
# Internals of the Python package: import julia # julia package from julia.api import Julia jl = Julia(compiled_modules=False) from julia import Base from julia import Main from julia import AbstractPlotting from julia import WGLMakie from julia import JSServe class Plot: def __init__(self): # Setup Julia Main.eval(""" url = strip(ENV["NEXTJOURNAL_RUNTIME_SERVICE_URL"]) ENV["JULIA_WEBIO_BASEURL"] = url ENV["WEBIO_SERVER_HOST_URL"] = "0.0.0.0" ENV["WEBIO_HTTP_PORT"] = 9998 surl = replace(url, "http" => "ws") * "/webio_websocket/" ENV["WEBIO_WEBSOCKT_URL"] = surl JSServe.__init__() """) self.func = None self.plot = None def __getattr__(self, symbol): # Return Julia function if Main.eval(f"isdefined(AbstractPlotting, :{symbol})"): self.func = Main.eval(f"getfield(AbstractPlotting, :{symbol})") return self else: return None def __call__(self, *args, **kwargs): if (self.func != None): self.plot = self.func(*args, **kwargs) return self def _repr_webio_(self): if (self.plot != None): return Base.repr( "application/vnd.webio.application+html", self.plot ).decode("utf-8") else: return "" M = Plot()
import numpy as np X = np.arange(-2, 2, 0.25) Y = np.arange(-2, 2, 0.25) X, Y = np.meshgrid(X, Y) R = np.sqrt(X**2 + Y**2) Z = np.sin(R) M.surface(X, Y, Z, resolution = (500, 400), colormap = "magma")
What's bad
Compile Times
- can be compiled ahead of time though
- With Atom / Revise, one can compile diffs
Deploying Julia
It's a work in progress...
- Prototypes already exist and people are deploying Julia
- Filesize big, awkward to build
So when should I use Julia?
- Now! If you need performance and plan to write your own libraries
- In ~1-2 Years if you want a smooth deploy
- In ~3-5 Years if you want a 100% smooth experience