Skip 'cache_tree_update()' when 'prime_cache_tree()' is called immediate after

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 partially- or fully-invalid cache tree (the more likely case in commands with the aforementioned redundancy), 'prime_cache_tree()' is faster.

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.

Changes since V2
================

* Cleaned up option handling & provided more informative error messages in 'test-tool cache-tree'. The changes don't affect any behavior in the added tests & 'test-tool cache-tree' won't be used outside of development, but the improvements here will help future readers avoid propagating error-prone implementations.
* Note that the suggestion to change the "unknown subcommand" error to a 'usage()' error was not taken, as it would be somewhat cumbersome to use a formatted string with it. This is in line with other custom subcommand parsing in Git, such as in 'fsmonitor--daemon.c'.

Changes since V1
================

* Rewrote 'p0090' to more accurately and reliably test 'prime_cache_tree()' vs. 'cache_tree_update()'.
* Moved iterative cache tree update out of C and into the shell tests (to avoid potential runtime optimizations)
* Added a "control" test to document how much of the execution time is startup overhead
* Added tests demonstrating performance in partially-invalid cache trees.
* Fixed the use of 'prime_cache_tree()' in 'test-tool cache-tree', changing it from using the tree at HEAD to the current cache tree.

Thanks!

* Victoria

[1] https://lore.kernel.org/git/[email protected]/ [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         | 64 ++++++++++++++++++++++++++++++
 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         | 36 +++++++++++++++++
 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, 145 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-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1411/vdye/feature/cache-tree-optimization-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1411

Range-diff vs v2:

1: 833519d87c8 ! 1: 2b48a684156 cache-tree: add perf test comparing update and prime

     @@ Commit message
          partially invalid (e.g., 'git reset --hard'), 'prime_cache_tree()' will
          likely perform better than 'cache_tree_update()' in typical cases.
      
     +    Helped-by: SZEDER Gábor <[email protected]>
          Signed-off-by: Victoria Dye <[email protected]>
      
       ## Makefile ##
     @@ t/helper/test-cache-tree.c (new)
      +
      +	setup_git_directory();
      +
     -+	parse_options(argc, argv, NULL, options, test_cache_tree_usage, 0);
     ++	argc = parse_options(argc, argv, NULL, options, test_cache_tree_usage, 0);
      +
      +	if (read_cache() < 0)
     -+		die("unable to read index file");
     ++		die(_("unable to read index file"));
      +
      +	oidcpy(&oid, &the_index.cache_tree->oid);
      +	tree = parse_tree_indirect(&oid);
     @@ t/helper/test-cache-tree.c (new)
      +			cache_tree_invalidate_path(&the_index, the_index.cache[i * interval]->name);
      +	}
      +
     -+	if (!argc)
     -+		die("Must specify subcommand");
     ++	if (argc != 1)
     ++		usage_with_options(test_cache_tree_usage, options);
      +	else if (!strcmp(argv[0], "prime"))
      +		prime_cache_tree(the_repository, &the_index, tree);
      +	else if (!strcmp(argv[0], "update"))
      +		cache_tree_update(&the_index, WRITE_TREE_SILENT | WRITE_TREE_REPAIR);
      +	/* use "control" subcommand to specify no-op */
      +	else if (!!strcmp(argv[0], "control"))
     -+		die("Unknown command %s", argv[0]);
     ++		die(_("Unhandled subcommand '%s'"), argv[0]);
      +
      +	return 0;
      +}
 2:  b015a4f531c = 2:  0e03614f0fd unpack-trees: add 'skip_cache_tree_update' option
 3:  4f6039971b8 = 3:  386f18ca36a reset: use 'skip_cache_tree_update' option
 4:  5a646bc47c9 = 4:  ea5c82ce992 read-tree: use 'skip_cache_tree_update' option
 5:  fffe2fc17ed = 5:  100c01e936c rebase: use 'skip_cache_tree_update' option

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