git grep with leading inverted bracket expression

If the first atom of a regex is a bracket expression with an inverted range, git grep is very slow.

$ time git grep 'struct_size' >/dev/null

real 0m0.368s user 0m0.563s sys 0m0.453s

$ time git grep '[^t]truct_size' >/dev/null

real 0m31.529s user 1m54.909s sys 0m0.805s

If the bracket expression is moved to even the second position in the string, it runs much faster:

$ time git grep 's[^p]ruct_size' >/dev/null

real 0m3.989s user 0m13.939s sys 0m0.403s

It's pretty bad with even a '.' as the first character:

$ time git grep '.truct_size' >/dev/null

real 0m14.514s user 0m52.624s sys 0m0.598s

$ git --version git version 2.17.1

Setting LANG=C improves matters by a factor of 3-4 (depending if you count real or user time):

$ time git grep '[^t]truct_size' >/dev/null real 0m10.035s user 0m28.795s sys 0m0.537s

(this is using something pretty close to Linus' current HEAD of the linux repository, an i7-7500, 16GB memory).

On Thu, Jun 07 2018, Matthew Wilcox wrote:

I have some WIP patches to fix all of this, which I'll hopefully submit before 2.19 is out the door.

What you've discovered here is how shitty your libc regex engine is, because unless you provide -P and compile with a reasonably up-to-date libpcre (preferably v2) with JIT that's what you'll get.

The reason stuff like 'struct_size' is so much faster is because there we don't use any regex engine at all, but rather an optimized fixed-string searcher.

With our own benchmarks modified per your E-Mail:
    
    diff --git a/t/perf/p7820-grep-engines.sh b/t/perf/p7820-grep-engines.sh
    index 8b09c5bf32..fe4c5681da 100755
    --- a/t/perf/p7820-grep-engines.sh
    +++ b/t/perf/p7820-grep-engines.sh
    @@ -28,11 +28,10 @@ then
     fi
    
     for pattern in \
    -       'how.to' \
    -       '^how to' \
    -       '[how] to' \
    -       '\(e.t[^ ]*\|v.ry\) rare' \
    -       'm\(ú\|u\)lt.b\(æ\|y\)te'
    +       'struct size' \
    +       '[^t]truct_size' \
    +       's[^p]ruct_size' \
    +       '.truct_size'
     do
            for engine in basic extended perl
            do

I get these results against linux.git:

    $ GIT_PERF_LARGE_REPO=~/g/linux ./run p7820-grep-engines.sh
    [...]
    Test                                      this tree
    ----------------------------------------------------------
    7820.1: basic grep 'struct size'          0.23(0.52+0.76)
    7820.2: extended grep 'struct size'       0.22(0.60+0.61)
    7820.3: perl grep 'struct size'           0.22(0.56+0.65)
    7820.5: basic grep '[^t]truct_size'       4.29(29.43+0.51)
    7820.6: extended grep '[^t]truct_size'    4.27(29.59+0.36)
    7820.7: perl grep '[^t]truct_size'        0.21(0.40+0.69)
    7820.9: basic grep 's[^p]ruct_size'       0.49(2.22+0.49)
    7820.10: extended grep 's[^p]ruct_size'   0.43(2.24+0.48)
    7820.11: perl grep 's[^p]ruct_size'       0.21(0.38+0.71)
    7820.13: basic grep '.truct_size'         4.42(31.29+0.44)
    7820.14: extended grep '.truct_size'      4.50(31.18+0.46)
    7820.15: perl grep '.truct_size'          0.21(0.35+0.75)

So you need to just use an up-to-date libpcre2 & -P and performance won't suck.

My WIP patches will make us use PCRE for all grep modes, using an API it has to convert basic & extended regexp syntax to its own syntax, so we'll be able to do that transparently.

On Thu, Jun 07, 2018 at 09:09:25PM +0200, Ævar Arnfjörð Bjarmason wrote:

I'm using Debian's build, and it is linked against a recent libpcre2:
$ ldd /usr/lib/git-core/git
	libpcre2-8.so.0 => /usr/lib/x86_64-linux-gnu/libpcre2-8.so.0 (0x00007f59ad5f2000)
$ dpkg --status libpcre2-8-0
Version: 10.31-3

But I wasn't using -P.  If I do, then I see the performance numbers you do:

$ time git grep -P '[^t]truct_size' >/dev/null real 0m0.354s user 0m0.340s sys 0m0.639s $ time git grep -P 'struct_size' >/dev/null real 0m0.336s user 0m0.552s sys 0m0.457s $ time git grep 'struct_size' >/dev/null real 0m0.335s user 0m0.535s sys 0m0.474s

I don't tend to use terribly advanced regexps, so I'll just set grep.patternType to 'perl' and then it'll automatically be fast for me without your patches ;-)

That's clearly the right answer.  Thanks!

On Thu, Jun 07 2018, Matthew Wilcox wrote:

Yeah that's recent enough & will get you all the benefits.

Indeed, if you're happy with that that'll do it.

Yeah, unfortunately git-grep's default is "basic" regexp which has a really atrocious syntax that's different enough from extended & Perl's that we probably couldn't just switch it over.

That won't be needed with my patches, but maybe I'll follow-up with something to s/basic/extended/g by default, because on side effect of having the pattern converter is that we could have a warning whenever the user has a pattern that would be different under extended/perl, so we can see how common that is.