Normalization-bench

This repo contains some benchmarks for normalizing untyped lambda calculus and doing beta-conversion checking on it. WIP, I plan to add more benchmarks and languages to this and also different implementations in particular languages. PRs are welcome.

My motivation is type checking, where the more sophisticated type systems, such as dependent type theories, require evaluating open lambda terms at type checking time (and comparing them for conversion).

I've been using relatively simple AST interpreters for this purpose, and I wanted to see what kind of difference compiled HOAS (higher-order abstract syntax) makes in performance. I thought that perhaps a modern JIT language would be good, because it would allow us to compile syntax to machine code via HOAS on the fly. In contrast, GHC would be clumsier for compiled HOAS because of the dependency on ghc and slow compilation. Or so I thought.

Setup

Test computer:

• Ubuntu 16.04 LTS
• i7-3770 @ 3.9 GHz
• 8GB RAM

Scala:

• sbt 1.4.9
• openjdk 1.8.0_232
• Runtime options: export SBT_OPTS="-Xss1000M -Xmx4G"
• alternatively: graalvm 21.0.0.2, with the same runtime options

nodejs:

• node v13.6.0
• Runtime options: ulimit -s unlimited, running with node --max-old-space-size=4000 --stack-size=6000000.

F#:

• dotnet 3.0.101 (dotnet core)
• F# 4.0
• Runtime options: ulimit -s unlimited, running with dotnet run -c Release.

GHC:

• ghc 8.8.1
• LLVM 7.1
• Compile options: -O2 -fllvm -rtsopts.
• Runtime options: +RTS -A1G -s.

Coq

• coqc 8.11.1
• coqc -impredicative-set.
• ulimit -s unlimited.
• Runtime options: export OCAMLRUNPARAM="v=0x400,s=100000000,i=100000000".
• timing with Time vernacular command.

OCaml

• ocamlopt 4.10.0+flambda
• Compile options: -O3.
• ulimit -s unlimited.
• Runtime options: export OCAMLRUNPARAM="v=0x400,s=100000000,i=100000000".

smalltt

• smalltt 0.2.0.0
• Runtime options: +RTS -A1G.

luajit

• luajit 2.1.0-beta3

Benchmarks & implementations

Benchmarks are normalization and beta-conversion checking on Church-coded unary numbers and binary trees.

All deep HOAS implementations are virtually the same; they use idiomatic ADTs for terms and HOAS values, or straightforward ADT emulation in the case of javascript.

In GHC, we have a call-by-need and a call-by-value HOAS version. The difference in the source code is just a couple of bangs. We also have an AST interpreted CBV normalizer. The interpreter is fairly efficient, but we don't have any optimization on the object language.

In Coq, we use well-typed impredicative Church encodings, which are slightly more expensive than the untyped ones, because it also abstracts over a type parameter. We use eq_refl for conversion checking. Smalltt is similar to the Coq implementation.

I made a non-trivial amount of effort trying to tune runtime options, but I don't have much experience in tuning things besides GHC, so feel free to correct me.

Results

Times in milliseconds. For Coq & smalltt results are from a single run. For everything else results are averages of 20 runs.

Commentary

F#. Performance here was the most disappointing. I had hopes for F# since it is the nicest language of the JIT pack by a significant margin, and I thought it would be tolerable or even superior to Haskell to implement everything in F#, because of the support for high-performance non-persistent data structures, monomorphization, unpacked structures, etc. Now, there could be some GC option which magically repairs this, but I've tried and have not found a way to make it go faster.

Scala. I had never used Scala before, and I found the language relatively pleasant, and definitely vastly better than Java or even Clojure. That said, performance seems good for trees but degrades sharply from 5M to 10M with natural numbers; perhaps there is some issue with deep stacks.

GraalVM Scala had excellent performance with trees, but there was a serious slowdown with natural numbers, indicating some issue with deep stacks.

nodejs. Somewhat disappointing all around, although better than F#.

GHC CBV interpreter is doing pretty well. It's already at worst half as fast as Scala, and there are a number of optimizations still on the table. I'd first try to add known call optimization.

OCaml HOAS is pretty good, with surprisingly fast tree conversion, although worse than GHC elsewhere. The effect of free memory RTS options was huge here: 2-10x speedup.

Coq. I note that while some benchmarks (e.g. cbv natural numbers) benefited enormously from the free memory for GC, some of them slowed down 10-30%. Overal the free memory RTS setting performed better than defaults. It's worth noting that native_compute performs significantly worse than OCaml HOAS, though the two should be similar; probably native_compute does not use flambda optimization options at all.

smalltt is slower than the barebones GHC interpreter, which is as expected, because smalltt has an evaluator which does significantly more bookkeeping (serving elaboration purposes).

luajit appears to be very poorly optimized for closure workload. I was not able to figure out a way to increase stack sizes, hence the natural number benchmarks don't finish. Moreover, the tree bencharks run out of heap space above 2 million tree sizes, on my 8GB RAM system.

General comments.

• Stack space was an issue with JITs, OCaml and Coq. All of these have very inadequate stack space defaults. The UX of increasing stack space was best with Scala, where I applaud JVM for at least providing a platform-independent stack size option, although sbt specifically has a bug which required me to set options via SBT_OPTS environment variable, instead of program flags. In contrast, F# and nodejs don't seem to have such option, and I had to use ulimit. OCaml and Coq have a stack limit option in OCAMLRUNPARAM, but when I maxed that out I still got overflow errors, so I used ulimit here as well, which worked.
• Most solutions benefit from throwing more memory at GC, some dramatically. GHC solutions & smalltt had 3-6 times speedup from -A1G. Scala and nodejs performance becomes dismal without free RAM; I plan to put these numbers up here as well. I suspect that F# is crippled by the same issues, but I was not able to use options to throw more memory at it.
• Startup time in nodejs is excellent, but F# and Scala are rather sluggish. GC pause variance is way more in the non-GHC solutions, with occasional multi-second pauses in nodejs and Scala.
• There are some stability issues in nodejs and Scala. In the former, I sometimes got runtime exceptions for 10M Nat, presumably related to too deep stacks (it was not stack overflow exception though, but some internal error!). In the latter, with 5M and 10M Nats, sometimes I got a sudden memory hike which resulted in out-of-memory process termination.
• All JIT solutions are CBV, however, CBN is essential for type checking and unfortunately there is a good chance that CBN would be a significant performance hit there. This should be also benchmarked.

TODO

• Add bench figures without free memory options.
• Add figures for AST interpreters.
• More benchmarks
• Agda, Lean 3/4, mlang.

Preliminary analysis & conclusions

Modern JIT platforms don't seem to be good enough at lambda calculus. From the current benchmarking, JVM seems to be the best choice. However, I haven't yet benchmarked call-by-need, which could possibly be a significant performance hit on JVM. At the moment it does not seem worth to write high-performance proof assistant implementation in any of the JIT languages, because plain hand-written interpreters in GHC are much more convenient to maintain and develop and have comparable performance to JIT HOAS. Perhaps compiling directly to CLR/JVM bytecode would be better, but I am skeptical, and I would be hard pressed to implement that myself.

A very high-effort solution is the Lean way: implement runtime system from scratch. I would also like to do this once for a minimal untyped lambda calculus, and just try to make it as fast as possible. I am sure GHC HOAS can be beaten this way, but I am not sure that the effort is worth it.

I am entertaining more seriously the solution to use GHC HOAS in real-world type theory implementations, by primarily using interpretation but constantly doing compilation via GHC in the background, and switching out definitions. Combined with more sophisticated diffing and incrementalization, this could have good UX even in a single module.