Visualization Libraries for ghc-debug

This page serves as a conclusion to my Google Summer of Code project this past summer.

I had great fun this summer working on ghc-debug. Thank you so much to the community at #ghc on IRC for being so welcoming to me, and to Matthew Pickering for mentoring me.

Google Summer of Code Project Link

https://summerofcode.withgoogle.com/projects/#6368742172262400

Completed Work

• Added support for exporting the resulting graph from the TypePointsFrom analysis to the GML graph visualization format: ghc-debug merge request #3

• Fixed a Hadrian (GHC build system) bug that prevented a GHC with ghc-debug support being built on Darwin: GHC merge request #6082

• Added convenience functions for converting a generalized census data type to Vega-Lite visualizations: ghc-debug merge request #4

  • Converts a single census into a bar chart
  • Converts a series of censuses into a stacked area chart

• Support for outputing root traces as flamegraphs: ghc-debug merge request #4, under FlameGraph.hs

• Attempted converting existing memory leak detection methods from BLeak: ghc-debug merge request #5

  • Created functions to calculate total transitive closure size
  • Uses total number of closures transitively pointed to, to determine leaks
  • Characterizes leaks caused by lazy evaluation as long lists of the same kind of thunks

Examples and Pretty Pictures!

The following visualizations are all from analyzing the following classic Haskell space leak, using my newly created library functions.

The value of the lazy MVar is not evaluated until it has to be printed, causing a leak in the form of a linked list of thunks.

main :: IO ()
main = withGhcDebug $ do
    mvar <- newMVar (1 :: Int)
    replicateM_ 50 $ modifyMVar_ mvar (\x -> return (x+1))
    putStrLn "Hit Enter to continue...\n"
    _ <- getLine

    replicateM_ 100 $ modifyMVar_ mvar (\x -> return (x+1))
    putStrLn "Hit Enter to continue..."
    _ <- getLine

    replicateM_ 150 $ modifyMVar_ mvar (\x -> return (x+1))
    putStrLn "Hit Enter to continue..."
    _ <- getLine

    -- Prevent GC by using the MVar
    val <- takeMVar mvar
    putStrLn . show $ val

Flame Graph

Heap Census

Click to open in new page

Heap Census over Time (Stacked Area)

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TODOs: How to Further Improve ghc-debug

• Adapt more of existing analysis functions to output to the generalized census type

• Continue experimenting with BLeak, to find the characterization that describes a memory leak the best. Current linked list characterization is not sufficient.