A Curious Case of a Memory that Just Didn't Go Away Quickly

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The idea for the talk came from a series of performance investigations in which we tried to nail a bug that was causing unusually long pauses on our production machines. What was even more interesting was that the slowdown seemed to have quadratic characteristics, but all the GCs algorithms are certainly linear? It turns out that there are corner cases where this is not true. In fact, we have proposed a (accepted) bug report to GHC outlining this particular problem: https://gitlab.haskell.org/ghc/ghc/-/issues/19897

While these findings are interesting on its own, they would make a rather hermetic presentation. So instead of presenting the dry facts, the talk would turn towards topics that I believe to be valuable independent of a particular case: how to profile a misbehaving program and collect conclusive results pointing to the real cause; overview of existing GHC memory management tactics and data structures; general structures that can be used in memory management (trees, skip-lists) and their characteristics; how to hack on GHC to implement a new idea; how to use a hacked GHC.

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