From 34e3bd10e3891afc965a7fb8fdcaacbdc900b2d5 Mon Sep 17 00:00:00 2001 From: pennae Date: Tue, 21 Dec 2021 13:56:57 +0100 Subject: avoid copies of parser input data MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 {}; 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 {}; 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 --- src/libutil/util.cc | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'src/libutil') diff --git a/src/libutil/util.cc b/src/libutil/util.cc index 43fea1b1e..f15a617b0 100644 --- a/src/libutil/util.cc +++ b/src/libutil/util.cc @@ -669,7 +669,9 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts) string drainFD(int fd, bool block, const size_t reserveSize) { - StringSink sink(reserveSize); + // the parser needs two extra bytes to append terminating characters, other users will + // not care very much about the extra memory. + StringSink sink(reserveSize + 2); drainFD(fd, sink, block); return std::move(*sink.s); } -- cgit v1.2.3