Subject: Re: [PATCH] packfile.c: speed up loading lots of packfiles.

From: SZEDER Gábor <[email-masked]>
Date: Mon, 2 Dec 2019 18:40:35 +0100

On Wed, Nov 27, 2019 at 04:24:53PM -0600, Colin Stolley wrote:
> When loading packfiles on start-up, we traverse the internal packfile
> list once per file to avoid reloading packfiles that have already
> been loaded. This check runs in quadratic time, so for poorly
> maintained repos with a large number of packfiles, it can be pretty
> slow.
> 
> Add a hashmap containing the packfile names as we load them so that
> the average runtime cost of checking for already-loaded packs becomes
> constant.
> 
> Add a perf test to p5303 to show speed-up.
> 
> The existing p5303 test runtimes are dominated by other factors and do
> not show an appreciable speed-up. The new test in p5303 clearly exposes
> a speed-up in bad cases. In this test we create 10,000 packfiles and
> measure the start-up time of git rev-parse, which does little else
> besides load in the packs.
> 
> Here are the numbers for the new p5303 test:
> 
> Test                         HEAD^             HEAD
> ---------------------------------------------------------------------
> 5303.12: load 10,000 packs   1.03(0.92+0.10)   0.12(0.02+0.09) -88.3%
> 
> Thanks-to: Jeff King <[email-masked]>
> Signed-off-by: Colin Stolley <[email-masked]>
> ---

This patch break test 'gc --keep-largest-pack' in 't6500-gc.sh' when
run with GIT_TEST_MULTI_PACK_INDEX=1, because there is a duplicate
entry in '.git/objects/info/packs':

  expecting success of 6500.7 'gc --keep-largest-pack':
          test_create_repo keep-pack &&
          (
                  cd keep-pack &&
                  test_commit one &&
                  test_commit two &&
                  test_commit three &&
                  git gc &&
                  ( cd .git/objects/pack && ls *.pack ) >pack-list &&
                  test_line_count = 1 pack-list &&
                  BASE_PACK=.git/objects/pack/pack-*.pack &&
                  test_commit four &&
                  git repack -d &&
                  test_commit five &&
                  git repack -d &&
                  ( cd .git/objects/pack && ls *.pack ) >pack-list &&
                  test_line_count = 3 pack-list &&
                  git gc --keep-largest-pack &&
                  ( cd .git/objects/pack && ls *.pack ) >pack-list &&
                  test_line_count = 2 pack-list &&
                  awk "/^P /{print \$2}" <.git/objects/info/packs >pack-info &&
                  test_line_count = 2 pack-info &&
                  test_path_is_file $BASE_PACK &&
                  git fsck
          )
  
  + test_create_repo keep-pack
  Initialized empty Git repository in /home/szeder/src/git/t/trash directory.t6500-gc/keep-pack/.git/
  + cd keep-pack
  + test_commit one
  [master (root-commit) d79ce16] one
   Author: A U Thor <[email-masked]>
   1 file changed, 1 insertion(+)
   create mode 100644 one.t
  + test_commit two
  [master 139b20d] two
   Author: A U Thor <[email-masked]>
   1 file changed, 1 insertion(+)
   create mode 100644 two.t
  + test_commit three
  [master 7c7cd71] three
   Author: A U Thor <[email-masked]>
   1 file changed, 1 insertion(+)
   create mode 100644 three.t
  + git gc
  Computing commit graph generation numbers:  33% (1/3)^MComputing commit graph generation numbers:  66% (2/3)^MComputing commit graph generation numbers: 100% (3/3)^MComputing commit graph generation numbers: 100% (3/3), done.
  + cd .git/objects/pack
  + ls pack-a4b37b9b5458e8116b1c1840185b39fb5e6b8726.pack
  + test_line_count = 1 pack-list
  + BASE_PACK=.git/objects/pack/pack-*.pack
  + test_commit four
  [master fd8d77e] four
   Author: A U Thor <[email-masked]>
   1 file changed, 1 insertion(+)
   create mode 100644 four.t
  + git repack -d
  + test_commit five
  [master a383792] five
   Author: A U Thor <[email-masked]>
   1 file changed, 1 insertion(+)
   create mode 100644 five.t
  + git repack -d
  + cd .git/objects/pack
  + ls pack-057d7f493a7c26d58090f4777ff66d4c226c4408.pack pack-54feec766fc7d2d204b03879d96f4595d7e48c37.pack pack-a4b37b9b5458e8116b1c1840185b39fb5e6b8726.pack
  + test_line_count = 3 pack-list
  + git gc --keep-largest-pack
  Computing commit graph generation numbers:  20% (1/5)^MComputing commit graph generation numbers:  40% (2/5)^MComputing commit graph generation numbers:  60% (3/5)^MComputing commit graph generation numbers:  80% (4/5)^MComputing commit graph generation numbers: 100% (5/5)^MComputing commit graph generation numbers: 100% (5/5), done.
  + cd .git/objects/pack
  + ls pack-390dbbb8e27c014b080c08dfc482d4982d4c6644.pack pack-a4b37b9b5458e8116b1c1840185b39fb5e6b8726.pack
  + test_line_count = 2 pack-list
  + awk /^P /{print $2}
  + test_line_count = 2 pack-info
  test_line_count: line count for pack-info != 2
  pack-a4b37b9b5458e8116b1c1840185b39fb5e6b8726.pack
  pack-a4b37b9b5458e8116b1c1840185b39fb5e6b8726.pack
  pack-390dbbb8e27c014b080c08dfc482d4982d4c6644.pack
  error: last command exited with $?=1
  not ok 7 - gc --keep-largest-pack