[PATCH 0/5] Skip 'cache_tree_update()' when 'prime_cache_tree()' is called immediate after

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

Following up on a discussion [1] around cache tree refreshes in 'git reset',
this series updates callers of 'unpack_trees()' to skip its internal
invocation of 'cache_tree_update()' when 'prime_cache_tree()' is called
immediately after 'unpack_trees()'. 'cache_tree_update()' can be an
expensive operation, and it is redundant when 'prime_cache_tree()' clears
and rebuilds the cache tree from scratch immediately after.

The first patch adds a test directly comparing the execution time of
'prime_cache_tree()' with that of 'cache_tree_update()'. The results show
that on a fully-valid cache tree, they perform the same, but on a
fully-invalid cache tree, 'prime_cache_tree()' is multiple times faster
(although both are so fast that the total execution time of 100 invocations
is needed to compare the results in the default perf repo).

The second patch introduces the 'skip_cache_tree_update' option for
'unpack_trees()', but does not use it yet.

The remaining three patches update callers that make the aforementioned
redundant cache tree updates. The performance impact on these callers ranges
from "negligible" (in 'rebase') to "substantial" (in 'read-tree') - more
details can be found in the commit messages of the patch associated with the
affected code path.

Thanks!

 * Victoria

[1] https://lore.kernel.org/git/xmqqlf30edvf.fsf@gitster.g/ [2]
https://lore.kernel.org/git/f4881b7455b9d33c8a53a91eda7fbdfc5d11382c.1627066238.git.jonathantanmy@google.com/

Victoria Dye (5):
  cache-tree: add perf test comparing update and prime
  unpack-trees: add 'skip_cache_tree_update' option
  reset: use 'skip_cache_tree_update' option
  read-tree: use 'skip_cache_tree_update' option
  rebase: use 'skip_cache_tree_update' option

 Makefile                           |  1 +
 builtin/read-tree.c                |  4 +++
 builtin/reset.c                    |  2 ++
 reset.c                            |  1 +
 sequencer.c                        |  1 +
 t/helper/test-cache-tree.c         | 52 ++++++++++++++++++++++++++++++
 t/helper/test-tool.c               |  1 +
 t/helper/test-tool.h               |  1 +
 t/perf/p0006-read-tree-checkout.sh |  8 +++++
 t/perf/p0090-cache-tree.sh         | 27 ++++++++++++++++
 t/perf/p7102-reset.sh              | 21 ++++++++++++
 t/t1022-read-tree-partial-clone.sh |  2 +-
 unpack-trees.c                     |  3 +-
 unpack-trees.h                     |  3 +-
 14 files changed, 124 insertions(+), 3 deletions(-)
 create mode 100644 t/helper/test-cache-tree.c
 create mode 100755 t/perf/p0090-cache-tree.sh
 create mode 100755 t/perf/p7102-reset.sh


base-commit: 3b08839926fcc7cc48cf4c759737c1a71af430c1
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1411%2Fvdye%2Ffeature%2Fcache-tree-optimization-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1411/vdye/feature/cache-tree-optimization-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1411
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