Visualization of Julia profiling data




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This package contains tools for visualizing profiling data collected with Julia's built-in sampling profiler. It can be helpful for getting a big-picture overview of the major bottlenecks in your code, and optionally highlights lines that trigger garbage collection as potential candidates for optimization.

This type of plot is known as a flame graph.


Within Julia, use the package manager:


Usage and visual interpretation

To demonstrate ProfileView, first we have to collect some profiling data. Here's a simple test function for demonstration:

function profile_test(n)
    for i = 1:n
        A = randn(100,100,20)
        m = maximum(A)
        Afft = fft(A)
        Am = mapslices(sum, A, 2)
        B = A[:,:,5]
        Bsort = mapslices(sort, B, 1)
        b = rand(100)
        C = B.*b

profile_test(1)  # run once to trigger compilation
Profile.clear()  # in case we have any previous profiling data
@profile profile_test(10)

Now that we have profiling data, visualizing it is a simple matter:

julia> using ProfileView

julia> ProfileView.view()

If you're following along, you should see something like this:


This plot is a visual representation of the call graph of the code that you just profiled. The "root" of the tree is at the bottom; if you move your mouse over the long horizontal magenta bar at the bottom, you'll see it's an anonymous function in REPL.jl; the orangish one above that is eval_user_input in the same function. As is explained elsewhere, these are what run your code in the REPL. If you move your mouse upwards, you'll eventually get to the function(s) you ran with @profile.

While the vertical axis therefore represents nesting depth, the horizontal axis represents the amount of time (more precisely, the number of backtraces) spent at each line. One sees on the 4th line from the bottom, there are several differently-colored bars, each corresponding to a different line of profile_test. The fact that they are all positioned on top of the lower magenta bar means that all of these lines are called by the same "parent" function. Within a block of code, they are sorted in order of increasing line number, to make it easier for you to compare to the source code.

From this visual representation, we can very quickly learn several things about this function:

  • The most deeply-nested line corresponds to mapslices(sort, B, 1), resulting in the tall "stack" of bars on the right edge. However, this call does not take much time, because these bars are narrow horizontally.

  • In contrast, the two most time-consuming operations are the calls to fft and mapslices(sum, A, 2). (This is more time-consuming than the mapslices(sort,...) simply because it has to process more data.)

  • In this plot, red is a special color: it is reserved for function calls that trigger garbage collection, a time-consuming process that sometimes serves as a bottleneck or as a warning of type-instability. Here one could see one example of garbage-collection occuring inside fft, and several inside mapslices.

Further discussion of the proper interpretation of the red bars can be found below.

GUI features

Gtk Interface

  • Right-clicking on a bar will cause information about this line to be printed in the REPL. This can be a convenient way to "mark" lines for later investigation.

  • Click-drag lets you zoom in on a specific region of the image. Double-click to restore the full view. You can pan by scrolling the mouse (scroll=vertical, SHIFT-scroll=horizontal), and change zoom level with CTRL-scroll. You can also use your keyboard (arrow keys, plus SHIFT and CTRL modifiers).

  • To use the Gtk interface in IJulia, set PROFILEVIEW_USEGTK = true in the Main module before using ProfileView.

  • The toolbar at the top contains two icons to load and save profile data, respectively. Clicking the save icon will prompt you for a filename. Launching ProfileView.view(nothing) opens a blank window; you can populate it with saved data by clicking on the "open" icon.

IJulia (SVG) Interface

  • Double-clicking on a bar will zoom in the graph around the bar.

  • Double-clicking on the background will zoom out to show the entire graph.

  • Click-drag anywhere on the graph will allow for panning.

Command-line options

The view command has the following syntax:

function view(data = Profile.fetch(); lidict = nothing, C = false, colorgc = true, fontsize = 12, combine = true)

Here is the meaning of the different arguments:

  • The first is the vector containing backtraces. You can use data1 = copy(Profile.fetch()); Profile.clear() to store and examine results from multiple profile runs simultaneously.

  • lidict is a dictionary containing "line information." See the section on saving profile data below.

  • C is a flag controlling whether lines corresponding to C and Fortran code are displayed. (Internally, ProfileView uses the information from C backtraces to learn about garbage-collection and to disambiguate the call graph).

  • colorgc, when true, causes lines triggering garbage-collection to be displayed in red.

  • fontsize controls the size of the font displayed as a tooltip.

  • combine is explained elsewhere.

Saving profile data manually

If you're using the Gtk backend, the easiest approach is to click on the "Save as" icon.

From the REPL, you can save profile data for later viewing and analysis using the JLD file format. The main trick is that the backtrace data, on its own, is only valid within a particular julia session. To become portable, you have to save "line information" that looks up the particular line number in the source code corresponding to a particular machine instruction. Here's an example:

li, lidict = Profile.retrieve()
using JLD
@save "/tmp/foo.jlprof" li lidict

Now open a new julia session, and try the following:

using HDF5, JLD, ProfileView
@load "/tmp/profdata.jld"
ProfileView.view(li, lidict=lidict)

Saving ProfileView visualizations

You can share your profiling results with others either as an SVG file or as an IJulia notebook. Simply use



ProfileView.svgwrite("profile_results.svg", bt, lidict)

if you've retrieved stored data.

Alternatively, run ProfileView inside an IJulia notebook and then save the notebook.

Important points about garbage-collection

You should expect the width and placement of the red bars to vary from one profile to the next, because garbage-collection is an infrequent event and therefore highly variable.

Lines triggering garbage-collection are often fruitful targets for optimization. However, note that red bars indicate garbage-collection, not allocation, and in reality it's the latter that is usually the true source of trouble. It's quite possible to have a large allocation that procedes without garbage collection (and therefore doesn't take much time), only to be followed by a tiny allocation that triggers garbage collection but which cannot itself be profitably optimized. Your best way to avoid being fooled is to collect quite a lot of profiling data (i.e., run your code for longer), so that all operations allocating memory will likely trigger a garbage-collection at some point.