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Opened by mrkkrp@trac-mrkkrp
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newtype wrapping of a monadic stack kills performance

I have a project where I in one module (A) I decided to build something like a minimal framework for the other bigger module (B) to use. One of the main points of the framework is that the monadic stacks are hidden behind newtypes and in those monads you can only use the functions that the module A provides.

The module A can be found here:

https://github.com/mrkkrp/mmark/blob/ghc-bug-newtypes/Text/MMark/Parser/Internal.hs

There are two monadic stack wrapped with newtypes: BParser and IParser.

The module B is this:

https://github.com/mrkkrp/mmark/blob/ghc-bug-newtypes/Text/MMark/Parser.hs

But it's not really relevant.

Now if I change newtypes to type synonyms like so:

type BParser a = ParsecT MMarkErr Text (State BlockState) a
type IParser a = StateT InlineState (Parsec MMarkErr Text) a

and do corresponding minor corrections, I get 2x less allocations and almost 2x faster code (before):

Case                                           Allocated  GCs     Max
with file: data/bench-yaml-block.md              119,080    0  11,088
with file: data/bench-thematic-break.md           74,368    0  10,224
with file: data/bench-heading.md                 901,928    0   9,432
with file: data/bench-fenced-code-block.md       145,744    0   9,368
with file: data/bench-indented-code-block.md     124,312    0   9,368
with file: data/bench-unordered-list.md        2,010,496    1  10,784
with file: data/bench-ordered-list.md          2,025,016    1  10,728
with file: data/bench-blockquote.md            1,961,672    1  42,648
with file: data/bench-paragraph.md             2,084,104    1  42,648
with file: data/comprehensive.md              25,899,496   24  44,200

benchmarking with file: data/comprehensive.md
time                 3.590 ms   (3.578 ms .. 3.601 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 3.555 ms   (3.546 ms .. 3.565 ms)
std dev              31.07 μs   (24.63 μs .. 39.90 μs)

After:

Case                                           Allocated  GCs     Max
with file: data/bench-yaml-block.md              116,864    0  11,088
with file: data/bench-thematic-break.md           64,776    0  10,392
with file: data/bench-heading.md                 615,672    0   9,432
with file: data/bench-fenced-code-block.md       144,736    0   9,368
with file: data/bench-indented-code-block.md     123,352    0   9,368
with file: data/bench-unordered-list.md          795,072    0  41,568
with file: data/bench-ordered-list.md            808,808    0  41,512
with file: data/bench-blockquote.md              826,392    0  41,568
with file: data/bench-paragraph.md               881,432    0  41,568
with file: data/comprehensive.md              10,945,104   10  44,440

benchmarking with file: data/comprehensive.md
time                 2.451 ms   (2.448 ms .. 2.456 ms)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 2.432 ms   (2.427 ms .. 2.437 ms)
std dev              15.86 μs   (13.16 μs .. 19.01 μs)

I'm not great at reading non-trivial core, but I gave it a shot and dumped some core. One thing I noticed that core for the module A is the same in both cases. Then the problem is an inter-module problem, probably like when you don't dump definitions into interface files with INLINEABLE and end up without specialization, but here it perhaps has to do with the fact that B has no information about internals of the monadic stacks from A, but I'm not sure.

Here is the beginnings of core dumps (before):

==================== Tidy Core ====================
2017-12-23 04:55:17.967944505 UTC

Result size of Tidy Core
  = {terms: 210,169,
     types: 209,675,
     coercions: 82,492,
     joins: 973/3,753}

After:

==================== Tidy Core ====================
2017-12-23 04:58:46.386265108 UTC

Result size of Tidy Core
  = {terms: 301,256,
     types: 263,104,
     coercions: 28,560,
     joins: 1,726/5,393}

So it looks like newtype wrapping reduces GHC's ability to create join points SPJ talked about recently.

I do understand that you expect a minimal reproducing example but I could not produce it even though I spent several hours in futile attempts. I started by creating a small project, defining a similar stack with a newtype wrapper and using it in a simple parser in another module. Then I benchmarked the parser. There is no difference between newtyped and code and the code that uses just type synonyms. I tried different variations, no difference.

The core output is too big for me to analyze, it's like 25 Mb and 33 Mb, and I have no idea what's going on there.

You're welcome to experiment with the repo, there are benchmarks for memory usage and criterion benchmarks for speed of execution:

https://github.com/mrkkrp/mmark

I have created two branches I won't touch, one with newtypes and another one with type synonyms:

Just checkout one of these and run stack bench.

This is the commit that changes newtypes to type synonyms:

https://github.com/mrkkrp/mmark/commit/759d8d4aa52dd57a393299c63e8c9b70d0d43290

I'm submitting this because my friend convinced me that it's better to let you know (even though I could not create a minimal reproducing example on my own) than to let it go completely unnoticed.

Edited by mrkkrp

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