JuliaPlots/Plots.jl

[FR] Improve performance of plotting heatmaps

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#4,520 opened on Nov 15, 2022

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help wantedperformance

Description

Reported on Slack by Jesse Chan, repeated calls to plot! to produce overlying heatmaps was very slow.

const mat = randn(512, 512)

function f(x, n)
    plot()
    for _ in 1:n
        heatmap!(x)
    end
end

f(mat, 100)

Allocation profiler shows the large majority of allocations occurring in heatmap_edges: image

And the large majority of computation time in expand_extrema! (which calls heatmap_edges) and update_clims (my old nemesis): image

Contributor guide

Tech stack
None
Domain
performance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic Julia knowledgeFamiliarity with Plots.jl internalsProfiling experience
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
Investigate the functions `heatmap edges` and `expand extrema!` within the Plots.jl source. The allocation profiler indicates that most allocations occur in `heatmap edges`. Focus on reducing allocations in that function, possibly by pre allocating arrays or using in place operations. Check the discussion in the issue for additional context.