Polysemy: Mea Culpa


Alexis King gave an utterly fantastic talk today on the deep inner workings of Haskell programs’ performance profiles. It’s really very excellent and you should go watch it if you haven’t already. I’ve been extremely burned out on Polysemy and effect-system-related topics lately, but it seems like as good a time as any to discuss what’s going on with the library. Why do Alexis’ benchmarks clearly show something other than my claim that Polysemy was “zero cost?” In short:

I screwed up.

The core Haskell that’s being run in Alexis’ benchmark probably looks like this, though at one point I did indeed get the countdown benchmark to completely optimize away. My claim to being zero-cost was based on this result, which was possible, but required patching GHC, enabling -flate-specialise -O2 -fstatic-argument-transformation -fmax-simplifier-iterations=10 as well as a GHC patch cajoling the optimizer into running extra hard.

My patch to GHC just barely missed the 8.8 deadline, which meant it wouldn’t be publicly available until GHC 8.10, roughly a year away. And until then, Polysemy had no chance of being fast.

The result of all this: fast code, relying on a house of cards of optimization, only on a compiler that didn’t exist yet. It worked, but was a huge hassle to test, and because of that, I didn’t do it very often, nor did I make it easy for others to verify my claims.

My mindset has always been that the “free monads are too slow” argument is overblown and irrelevant to 99% of programs, and my original goal with Polysemy was to show that there was nothing fundamentally wrong with the approach; that if we tried hard enough, we really could pull amazing performance out of free monads.

It’s been about a year now, so my recollection is hazy, but I think I must have somehow conflated “fast programs are possible in Polysemy” with “Polysemy is zero-cost.” There was absolutely no deception intended, but it appears I deceived myself, and the community because of that. I’m sorry.

Sometime near the end of 2019, Lexi showed me her research into why the effect system benchmarks were extremely misleading (as mentioned in her talk.) Her research made it very evident that all effect systems were “cheating” in the benchmark shootout, and I attributed Polysemy’s pre-super-optimized terrible benchmark numbers to “not cheating as much.” If the optimizer was what was making other effect systems fast, but only in single-module programs, presumably they would also perform badly in real-world, multiple-module programs, and would see the same performance characteristics as Polysemy. I didn’t confirm this experimentally.

Plus, I figured, if performance truly is a problem, and not the overactive fear I thought it was, surely someone would have filed a bug complaining that Polysemy wasn’t as fast as it claimed. To date, nobody has filed that bug, and I continue to believe it’s an overblown issue — though that isn’t to say we shouldn’t fix it if we can. Lexi’s package eff seems to be working towards that solution, and I applaud her for all of the work she’s been putting into this problem.

So that’s more or less the story. But there are a few loose ends; such as why Lexi and I are seeing different benchmarking results. I realize this doesn’t actually matter, and I agree with her that Polysemy is in fact slow. That being said, I feel like enough of my reputation is on the line that I’d like to put towards some more evidence that I didn’t fabricate the whole thing. Also, the investigation will unearth some more systematic problems.

First and foremost, the last time I looked at the source of Lexi’s benchmarks, I noted that they don’t use polysemy-plugin, which the documentation states is necessary for the good performance. I don’t remember where these benchmarks actually are, but it doesn’t matter, because even if she had enabled the plugin, Polysemy would still not optimize away.

Why not? Polysemy’s performance was extremely reliant on unfolding of its recursive bind operation. As described here, you could trick GHC into unfolding a recursive call once by explicitly giving a loop-breaker. In essence, it required transforming the following recursive call:

factorial :: Int -> Int
factorial 0 = 1
factorial n = n * factorial (n - 1)
{-# INLINE factorial #-}

Into this:

factorial :: Int -> Int
factorial 0 = 1
factorial n = n * factorial' (n - 1)
{-# INLINE factorial #-}

factorial' :: Int -> Int
factorial' = factorial
{-# NOINLINE factorial' #-}

For whatever reason, this trick exposes enough of Polysemy’s bind so that the simplifier could inline away the expensive bits. But this was tedious! Every recursive call needed an explicit loop-breaker, and missing one would silently jeopardize your performance! Doing this by hand seemed antithetical to Polysemy’s other goal of no-boilerplate, and so at some point we factored out this logic into a GHC plugin, and then removed our hand-written loop-breakers.. The initial implementation of that plugin is described in this blog post.

In retrospect, this explicit breaking-of-loops doesn’t seem to be required in the benchmark — only in Polysemy — but that escaped my attention at the time and believing that user-code required this optimization was the main motivation in turning it into a GHC plugin. Anyway…

As it turns out, this plugin didn’t actually work! It was successfully rewriting the core into the explicitly loop-broken version, but for whatever reason, the simplifier wasn’t picking up where we left off. To this day I don’t know why it doesn’t work, but it doesn’t. Instead we proposed to implement this plugin as a renamer pass, but that presents serious implementation problems. Since there was no way in hell Polysemy could possibly be fast before GHC 8.10 (to be released roughly a year in the future) motivation to find a solution to this problem was understandably low, and it fell by the wayside. It has never been fixed, and remains disabled and half-worked around in Polysemy to this day.

Hopefully this is the only reason why Polysemy doesn’t show the excellent (though, admittedly unrepresentative) countdown benchmark results I claimed it did. I’m not invested enough to check for myself, but if you’re interested, I suspect you’ll see excellent core produced by my single-file repro if you compile it on GHC 8.10 under -O2 with the polysemy-plugin and the above flags enabled. If so, I suspect rolling back #8bbd9dc would get the real Polysemy library also doing well on the benchmark. But again, the benchmark performance is meaningless!

Enough history for today. Before ending this post, I’d like to do a tiny STAMP on what went wrong, in the hope that we can all learn something. The goal here is not to pass the buck, but to get a sense of just how much went wrong, how, and why.

By my analysis, the following issues all contributed to Polysemy’s colossal failure:

  • Haskell’s performance is not well understood
    • The effect system benchmarks were meaningless, and if anyone knew that, it was not common knowledge.
    • MTL is widely believed to be more performant than it is.
    • Existing effect systems’ performance is tied largely to GHC’s optimizer firing.
    • Because of lack of understanding, I was tackling bad-performance symptoms rather than causes.
  • Polysemy’s performance was unreliable
    • Required several interlocking pieces to work: a patched compiler, a core plugin, explicit loop-breakers, obscure GHC options.
    • Because the performance was hard to test, we didn’t notice when these pieces didn’t work.
      • Upon noticing the loop-breaking plugin didn’t work, it was unclear how to fix it.
        • Because of requiring a patched GHC, it was not a priority to fix.
          • Not being a priority meant it wasn’t motivating, and so it didn’t get done.
    • Debugging the simplifier is hard work. I was looking at thousands of lines of generated core by eye. Tooling exists, but it is more helpful for navigating core than diffing it.
  • Polysemy’s performance was too hard to test.
    • I missed the GHC deadline
      • My patch lingered for weeks in a finished state
        • Only reviewable by one person, who was on vacation.
        • Stuck doing drive-by improvements that were only suggestions, and not blockers to being merged. This was not made clear to me.
        • The simplifier is really hairy. It’s under-documented, and the function I was touching was over 150 lines of code.
    • I use Stack for my development, Stack doesn’t easily support custom-built GHCs. Therefore I couldn’t use my usual tools to test.
    • I don’t know how to use cabal
      • The documentation is notoriously lacking. As best I can tell, there are no “quick start” tutorials, and the relevant parts of the user manual are mentioned only under a heading that mentions “Nix”.
    • Because of the above two points, I only tested on the single module, and never on the library itself.
  • I had too much ego in the project.
    • I wanted to believe I had accomplished something “impossible.”
    • I had invested several engineering-months of my time working on this problem.
    • I had invested a large chunk of my reputation into free monads.

This post is long enough without diving into those points in more detail, but I’m happy to expand on individual points. Let me know in the comments if you’re interested.

All in all, this is has been the embarrassing affair. But then again, if you haven’t failed in recent memory, you’re not trying hard enough. I’ll strive to do better in the future.