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improve parser performance a bit #5812

Merged
merged 4 commits into from
Jan 17, 2022
Merged

improve parser performance a bit #5812

merged 4 commits into from
Jan 17, 2022

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pennae
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@pennae pennae commented Dec 21, 2021

the current parser is not as efficient as it could be. this is an attempt at optimizing it at least a little, and we get around 4% improvement on nixos system builds out of it. pure parsing (such as loading the massive hackage registry) benefits much more, around 13%.

edolstra
edolstra reviewed Dec 21, 2021
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edolstra
edolstra reviewed Dec 21, 2021
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@abathur abathur mentioned this pull request Dec 21, 2021
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@pennae pennae requested a review from edolstra January 3, 2022 17:00
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pennae commented Jan 12, 2022

@edolstra @thufschmitt may we ask this PR be checked? :) pretty much everything else we've not opened a PR for yet depends on this in one way or other.

@pennae pennae mentioned this pull request Jan 12, 2022
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thufschmitt
thufschmitt approved these changes Jan 13, 2022
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Just one small comment, but LGTM otherwise

(note that I’m not really familiar with the parser code, so I might have missed something)

src/libexpr/parser.y Show resolved Hide resolved
@pennae
don't strdup tokens in the lexer
61a9d16 
every stringy token the lexer returns is turned into a Symbol and not
used further, so we don't have to strdup. using a string_view is
sufficient, but due to limitations of the current parser we have to use
a POD type that holds the same information.

gives ~2% on system build, 6% on search, 8% on parsing alone

 # before

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     610.6 ms ±   2.4 ms    [User: 602.5 ms, System: 7.8 ms]
  Range (min … max):   606.6 ms … 617.3 ms    50 runs

Benchmark 2: nix eval -f hackage-packages.nix
  Time (mean ± σ):     430.1 ms ±   1.4 ms    [User: 393.1 ms, System: 36.7 ms]
  Range (min … max):   428.2 ms … 434.2 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      3.032 s ±  0.005 s    [User: 2.808 s, System: 0.223 s]
  Range (min … max):    3.023 s …  3.041 s    50 runs

 # after

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     574.7 ms ±   2.8 ms    [User: 566.3 ms, System: 8.0 ms]
  Range (min … max):   569.2 ms … 580.7 ms    50 runs

Benchmark 2: nix eval -f hackage-packages.nix
  Time (mean ± σ):     394.4 ms ±   0.8 ms    [User: 361.8 ms, System: 32.3 ms]
  Range (min … max):   392.7 ms … 395.7 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.976 s ±  0.005 s    [User: 2.757 s, System: 0.218 s]
  Range (min … max):    2.966 s …  2.990 s    50 runs
@pennae
avoid allocations in SymbolTable::create
eee0bce 
speeds up parsing by ~3%, system builds by a bit more than 1%

 # before

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     574.7 ms ±   2.8 ms    [User: 566.3 ms, System: 8.0 ms]
  Range (min … max):   569.2 ms … 580.7 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     394.4 ms ±   0.8 ms    [User: 361.8 ms, System: 32.3 ms]
  Range (min … max):   392.7 ms … 395.7 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.976 s ±  0.005 s    [User: 2.757 s, System: 0.218 s]
  Range (min … max):    2.966 s …  2.990 s    50 runs

 # after

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     572.4 ms ±   2.3 ms    [User: 563.4 ms, System: 8.6 ms]
  Range (min … max):   566.9 ms … 579.1 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     381.7 ms ±   1.0 ms    [User: 348.3 ms, System: 33.1 ms]
  Range (min … max):   380.2 ms … 387.7 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.936 s ±  0.005 s    [User: 2.715 s, System: 0.221 s]
  Range (min … max):    2.923 s …  2.946 s    50 runs
@pennae
avoid copies of parser input data
34e3bd1 
when given a string yacc will copy the entire input to a newly allocated
location so that it can add a second terminating NUL byte. since the
parser is a very internal thing to EvalState we can ensure that having
two terminating NUL bytes is always possible without copying, and have
the parser itself merely check that the expected NULs are present.

 # before

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     572.4 ms ±   2.3 ms    [User: 563.4 ms, System: 8.6 ms]
  Range (min … max):   566.9 ms … 579.1 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     381.7 ms ±   1.0 ms    [User: 348.3 ms, System: 33.1 ms]
  Range (min … max):   380.2 ms … 387.7 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.936 s ±  0.005 s    [User: 2.715 s, System: 0.221 s]
  Range (min … max):    2.923 s …  2.946 s    50 runs

 # after

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     571.7 ms ±   2.4 ms    [User: 563.3 ms, System: 8.0 ms]
  Range (min … max):   566.7 ms … 579.7 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     376.6 ms ±   1.0 ms    [User: 345.8 ms, System: 30.5 ms]
  Range (min … max):   374.5 ms … 379.1 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.922 s ±  0.006 s    [User: 2.707 s, System: 0.215 s]
  Range (min … max):    2.906 s …  2.934 s    50 runs
@pennae
optimize unescapeStr
72f4209 
mainly to avoid an allocation and a copy of a string that can be
modified in place (ever since EvalState holds on to the buffer, not the
generated parser itself).

 # before

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     571.7 ms ±   2.4 ms    [User: 563.3 ms, System: 8.0 ms]
  Range (min … max):   566.7 ms … 579.7 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     376.6 ms ±   1.0 ms    [User: 345.8 ms, System: 30.5 ms]
  Range (min … max):   374.5 ms … 379.1 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.922 s ±  0.006 s    [User: 2.707 s, System: 0.215 s]
  Range (min … max):    2.906 s …  2.934 s    50 runs

 # after

Benchmark 1: nix search --offline nixpkgs hello
  Time (mean ± σ):     570.4 ms ±   2.8 ms    [User: 561.3 ms, System: 8.6 ms]
  Range (min … max):   564.6 ms … 578.1 ms    50 runs

Benchmark 2: nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
  Time (mean ± σ):     375.4 ms ±   1.3 ms    [User: 343.2 ms, System: 31.7 ms]
  Range (min … max):   373.4 ms … 378.2 ms    50 runs

Benchmark 3: nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
  Time (mean ± σ):      2.925 s ±  0.006 s    [User: 2.704 s, System: 0.219 s]
  Range (min … max):    2.910 s …  2.942 s    50 runs
@edolstra
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I still don't understand why the null terminators are necessary. That seems very hacky and infects other parts of the code (like drainFD).

@pennae
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pennae commented Jan 17, 2022

the generated parser demands them, no idea why (though we'd guess it's to do with lookahead in the absence of a separate tokenizer). we can either add an extra terminator ourselves or have the generated parser do it at the cost of an extra alloc+copy. and while we do agree that it's kinda terrible and it does infect anything it touches, adding that extra byte feels well worth it for avoiding potentially hundreds of megabytes of string copies that only happen to add that one extra byte 🙁

@edolstra edolstra merged commit fc2443a into NixOS:master Jan 17, 2022
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Thanks for the explanation!

@pennae pennae deleted the small-perf-improvements branch January 19, 2022 16:13
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This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-24/17230/1

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@thufschmitt thufschmitt thufschmitt approved these changes

@edolstra edolstra Awaiting requested review from edolstra edolstra is a code owner

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