Subject: Re: [PATCH] index: add trace2 region for clear skip worktree

From: Junio C Hamano <[email-masked]>
Date: Tue, 25 Oct 2022 20:16:45 -0700

"Anh Le via GitGitGadget" <[email-masked]> writes:

> From: Anh Le <[email-masked]>
>
> In a large repository using sparse checkout, checking
> whether a file marked with skip worktree is present
> on disk and its skip worktree bit should be cleared
> can take a considerable amount of time. Add a trace2
> region to surface this information.
>
> Signed-off-by: Anh Le <[email-masked]>
> ---
>     index: add trace2 region for clear skip worktree
>     
>     In large repository using sparse checkout, checking whether a file
>     marked with skip worktree is present on disk and its skip worktree bit
>     should be cleared can take a considerable amount of time. Add a trace2
>     region to surface this information.

It is easy to see that the change is no-op from functionality's
standpoint.  The condition under which ce->ce_flags loses the
CE_SKIP_WORKTREE bit is the same as before, and the only change is
that in the iteration a couple of variables are incremented, which
may (or may not) have performance impact, but shouldn't break
correctness.

I am not sure about the value of these counters, honestly.

If we hit a sparse dir just once, we fully flatten the index and go
back to the restart state, and it would be a bug if we still see a
sparsified directory in the in-core index after that point, so
restart_count would be at most one, wouldn't it?  Why do we even
need to count with intmax_t?

Similarly, path_count is bounded by istate->cache_nr.  As you know
the approximate size of your project, I am not sure what extra
information you want to get by counting the paths with the skip bit
set before the first sparsified directory in the in-core index
twice, and the other paths with the skip bit set just once, and
adding these numbers together.  Also again, I am not quite sure what
the point is to count the paths in intmax_t.

> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1368%2FHaizzz%2Fmaster-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1368/Haizzz/master-v1
> Pull-Request: https://github.com/git/git/pull/1368
>
>  sparse-index.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/sparse-index.c b/sparse-index.c
> index e4a54ce1943..d11049c8aeb 100644
> --- a/sparse-index.c
> +++ b/sparse-index.c
> @@ -493,24 +493,33 @@ void clear_skip_worktree_from_present_files(struct index_state *istate)
>  	int dir_found = 1;
>  
>  	int i;
> +	intmax_t path_count = 0;
> +	intmax_t restart_count = 0;
>  
>  	if (!core_apply_sparse_checkout ||
>  	    sparse_expect_files_outside_of_patterns)
>  		return;
>  
> +	trace2_region_enter("index", "clear_skip_worktree_from_present_files", istate->repo);
>  restart:
>  	for (i = 0; i < istate->cache_nr; i++) {
>  		struct cache_entry *ce = istate->cache[i];
>  
> -		if (ce_skip_worktree(ce) &&
> -		    path_found(ce->name, &last_dirname, &dir_len, &dir_found)) {
> -			if (S_ISSPARSEDIR(ce->ce_mode)) {
> -				ensure_full_index(istate);
> -				goto restart;
> +		if (ce_skip_worktree(ce)) {
> +			path_count++;
> +			if (path_found(ce->name, &last_dirname, &dir_len, &dir_found)) {
> +				if (S_ISSPARSEDIR(ce->ce_mode)) {
> +					ensure_full_index(istate);
> +					restart_count++;
> +					goto restart;
> +				}
> +				ce->ce_flags &= ~CE_SKIP_WORKTREE;
>  			}
> -			ce->ce_flags &= ~CE_SKIP_WORKTREE;
>  		}
>  	}
> +	trace2_data_intmax("index", istate->repo, "clear_skip_worktree_from_present_files/path_count", path_count);
> +	trace2_data_intmax("index", istate->repo, "clear_skip_worktree_from_present_files/restart_count", restart_count);
> +	trace2_region_leave("index", "clear_skip_worktree_from_present_files", istate->repo);
>  }
>  
>  /*
>
> base-commit: 1fc3c0ad407008c2f71dd9ae1241d8b75f8ef886