Subject: Re: [PATCH v2 5/5] Accelerate clear_skip_worktree_from_present_files() by caching

From: Victoria Dye <[email-masked]>
Date: Fri, 14 Jan 2022 20:39:48 -0500

Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <[email-masked]>
> 
> Trying to clear the skip-worktree bit from files that are present does
> present some computational overhead, for sparse-checkouts.  (We do not
> do the bit clearing in non-sparse-checkouts.)  Optimize it as follows:
> 
> Rather than lstat()'ing every SKIP_WORKTREE path, take advantage of the
> fact that entire directories will often be missing, especially for cone
> mode and even more so ever since commit 55dfcf9591 ("sparse-checkout:
> clear tracked sparse dirs", 2021-09-08).  If we have already determined
> that the parent directory of a file (or other previous ancestor) does
> not exist, then the file cannot exist either so we do not need to
> lstat() it separately.
> 
> Timings for p2000 included below, reformatted to fit in normal commit
> message line lengths, which compare three things:
>   * Timings before this series
>   * Timings of the unoptimized version of
>     clear_skip_worktree_from_present_files() from a few commits ago
>   * Timings after the optimization in this commit
> 
> (NOTE: t/perf/ appears to have timing resolution only down to 0.01 s,
> which presents significant measurement error when timings only differ by
> 0.01s.  I don't trust any such timings below, and yet all the optimized
> results differ by at most 0.01s.)
> 
> Test        Before Series    Unoptimized              Optimized
> -----------------------------------------------------------------------------
> *git status*
> full-v3     0.15(0.10+0.06)  0.32(0.16+0.17) +113.3%  0.16(0.10+0.07) +6.7%
> full-v4     0.15(0.11+0.05)  0.32(0.17+0.16) +113.3%  0.16(0.11+0.05) +6.7%
> sparse-v3   0.04(0.03+0.04)  0.04(0.02+0.05) +0.0%    0.04(0.02+0.05) +0.0%
> sparse-v4   0.04(0.03+0.04)  0.04(0.02+0.05) +0.0%    0.04(0.03+0.05) +0.0%
> 
> *git add -A*
> full-v3     0.40(0.30+0.07)  0.56(0.36+0.17) +40.0%   0.39(0.30+0.07) -2.5%
> full-v4     0.37(0.28+0.07)  0.54(0.37+0.16) +45.9%   0.38(0.29+0.07) +2.7%
> sparse-v3   0.06(0.04+0.05)  0.08(0.05+0.05) +33.3%   0.06(0.05+0.04) +0.0%
> sparse-v4   0.05(0.03+0.05)  0.05(0.04+0.04) +0.0%    0.06(0.04+0.05) +20.0%
> 
> *git add .*
> full-v3     0.40(0.31+0.07)  0.57(0.37+0.17) +42.5%   0.41(0.30+0.08) +2.5%
> full-v4     0.38(0.30+0.06)  0.55(0.37+0.16) +44.7%   0.38(0.30+0.06) +0.0%
> sparse-v3   0.06(0.04+0.05)  0.06(0.05+0.04) +0.0%    0.06(0.03+0.05) +0.0%
> sparse-v4   0.06(0.05+0.05)  0.06(0.04+0.05) +0.0%    0.06(0.04+0.06) +0.0%
> 
> *git commit -a -m A*
> full-v3     0.41(0.32+0.06)  0.58(0.39+0.17) +41.5%   0.42(0.32+0.07) +2.4%
> full-v4     0.39(0.30+0.07)  0.56(0.38+0.17) +43.6%   0.40(0.31+0.07) +2.6%
> sparse-v3   0.04(0.03+0.04)  0.04(0.03+0.04) +0.0%    0.04(0.03+0.04) +0.0%
> sparse-v4   0.04(0.03+0.05)  0.04(0.03+0.05) +0.0%    0.04(0.03+0.04) +0.0%
> 
> *git checkout -f -*
> full-v3     0.56(0.46+0.07)  0.73(0.55+0.16) +30.4%   0.57(0.47+0.08) +1.8%
> full-v4     0.54(0.45+0.07)  0.71(0.53+0.17) +31.5%   0.55(0.45+0.07) +1.9%
> sparse-v3   0.06(0.04+0.04)  0.06(0.04+0.05) +0.0%    0.06(0.04+0.05) +0.0%
> sparse-v4   0.05(0.05+0.04)  0.05(0.04+0.05) +0.0%    0.06(0.04+0.05) +20.0%
> 
> *git reset*
> full-v3     0.34(0.26+0.05)  0.51(0.34+0.15) +50.0%   0.34(0.26+0.06) +0.0%
> full-v4     0.32(0.24+0.06)  0.49(0.32+0.15) +53.1%   0.33(0.25+0.06) +3.1%
> sparse-v3   0.04(0.03+0.04)  0.04(0.03+0.04) +0.0%    0.04(0.03+0.04) +0.0%
> sparse-v4   0.03(0.03+0.04)  0.03(0.02+0.04) +0.0%    0.03(0.03+0.04) +0.0%
> 
> *git reset --hard*
> full-v3     0.57(0.46+0.07)  0.90(0.61+0.25) +57.9%   0.57(0.45+0.08) +0.0%
> full-v4     0.54(0.46+0.05)  0.88(0.59+0.26) +63.0%   0.55(0.45+0.07) +1.9%
> sparse-v3   0.07(0.03+0.03)  0.07(0.04+0.03) +0.0%    0.07(0.03+0.03) +0.0%
> sparse-v4   0.06(0.03+0.03)  0.06(0.04+0.02) +0.0%    0.06(0.03+0.03) +0.0%
> 
> *git reset -- does-not-exist*
> full-v3     0.35(0.27+0.06)  0.52(0.32+0.17) +48.6%   0.35(0.27+0.06) +0.0%
> full-v4     0.33(0.26+0.05)  0.50(0.33+0.15) +51.5%   0.33(0.26+0.06) +0.0%
> sparse-v3   0.04(0.03+0.04)  0.04(0.03+0.04) +0.0%    0.04(0.03+0.04) +0.0%
> sparse-v4   0.04(0.02+0.04)  0.03(0.02+0.04) -25.0%   0.03(0.02+0.04) -25.0%
> 
> *git diff*
> full-v3     0.07(0.04+0.04)  0.24(0.11+0.14) +242.9%  0.07(0.04+0.04) +0.0%
> full-v4     0.07(0.03+0.05)  0.24(0.13+0.12) +242.9%  0.08(0.04+0.05) +14.3%
> sparse-v3   0.02(0.01+0.04)  0.02(0.01+0.04) +0.0%    0.02(0.01+0.05) +0.0%
> sparse-v4   0.02(0.02+0.03)  0.02(0.01+0.04) +0.0%    0.02(0.01+0.04) +0.0%
> 
> *git diff --cached*
> full-v3     0.05(0.03+0.02)  0.22(0.12+0.09) +340.0%  0.05(0.03+0.01) +0.0%
> full-v4     0.05(0.03+0.01)  0.23(0.12+0.11) +360.0%  0.05(0.03+0.02) +0.0%
> sparse-v3   0.01(0.00+0.00)  0.01(0.00+0.00) +0.0%    0.01(0.00+0.00) +0.0%
> sparse-v4   0.01(0.00+0.00)  0.01(0.00+0.00) +0.0%    0.01(0.00+0.00) +0.0%
> 
> *git blame f2/f4/a*
> full-v3     0.18(0.13+0.05)  0.52(0.29+0.23) +188.9%  0.19(0.15+0.04) +5.6%
> full-v4     0.19(0.15+0.04)  0.52(0.28+0.23) +173.7%  0.19(0.14+0.04) +0.0%
> sparse-v3   0.10(0.08+0.02)  0.10(0.09+0.01) +0.0%    0.10(0.09+0.01) +0.0%
> sparse-v4   0.10(0.08+0.02)  0.10(0.08+0.02) +0.0%    0.10(0.08+0.02) +0.0%
> 
> *git blame f2/f4/f3/a*
> full-v3     0.45(0.36+0.08)  0.78(0.51+0.27) +73.3%   0.45(0.37+0.08) +0.0%
> full-v4     0.45(0.37+0.08)  0.78(0.51+0.26) +73.3%   0.45(0.37+0.08) +0.0%
> sparse-v3   0.36(0.32+0.04)  0.36(0.31+0.05) +0.0%    0.36(0.31+0.04) +0.0%
> sparse-v4   0.36(0.31+0.05)  0.36(0.31+0.05) +0.0%    0.36(0.31+0.04) +0.0%
> 
> *git checkout-index -f --all*
> full-v3     0.07(0.02+0.05)  0.24(0.12+0.12) +242.9%  0.08(0.04+0.04) +14.3%
> full-v4     0.07(0.03+0.04)  0.24(0.11+0.13) +242.9%  0.08(0.03+0.04) +14.3%
> sparse-v3   0.04(0.01+0.03)  0.04(0.00+0.03) +0.0%    0.04(0.01+0.03) +0.0%
> sparse-v4   0.04(0.01+0.02)  0.04(0.01+0.03) +0.0%    0.04(0.01+0.02) +0.0%
> 
> *git update-index --add --remove f2/f4/a*
> full-v3     0.29(0.23+0.02)  0.46(0.30+0.12) +58.6%   0.30(0.24+0.02) +3.4%
> full-v4     0.27(0.22+0.02)  0.45(0.29+0.12) +66.7%   0.28(0.22+0.03) +3.7%
> sparse-v3   0.02(0.02+0.00)  0.02(0.01+0.00) +0.0%    0.02(0.01+0.00) +0.0%
> sparse-v4   0.02(0.02+0.00)  0.02(0.02+0.00) +0.0%    0.02(0.02+0.00) +0.0%
> 
> So, with the optimization, the extra work appears to be essentially 0
> for sparse-checkouts that are also using sparse-indexes (even before my
> optimization), and the extra work appears to be just marginally more
> than 0 for sparse-checkouts that are using full indexes.
> 

Thank you for including these results! In the cone-mode case, I'm happy to
see that the optimization effectively brings the cost of correcting
SKIP_WORKTREE down to 0. As for the unoptimized version (which, if I'm
reading this correctly, still approximates some worst-case non-cone cases),
I think correcting SKIP_WORKTREE is probably still worth the performance
cost - in the relatively massive test repo used in p2000, it's still only
~0.2-0.3s across the board.

> Signed-off-by: Elijah Newren <[email-masked]>
> ---
>  sparse-index.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 54 insertions(+), 2 deletions(-)
> 
> diff --git a/sparse-index.c b/sparse-index.c
> index b82648b10ee..eed170cd8f7 100644
> --- a/sparse-index.c
> +++ b/sparse-index.c
> @@ -341,18 +341,70 @@ void ensure_correct_sparsity(struct index_state *istate)
>  		ensure_full_index(istate);
>  }
>  
> +static int path_found(const char *path, const char **dirname, size_t *dir_len,
> +		      int *dir_found)
> +{
> +	struct stat st;
> +	char *newdir;
> +	char *tmp;
> +
> +	/*
> +	 * If dirname corresponds to a directory that doesn't exist, and this
> +	 * path starts with dirname, then path can't exist.
> +	 */
> +	if (!*dir_found && !memcmp(path, *dirname, *dir_len))
> +		return 0;
> +

I really like how this version of the optimization works, and ultimately
find it simpler than the hashmap approach from the RFC. Looks good!

> +	/*
> +	 * If path itself exists, return 1.
> +	 */
> +	if (!lstat(path, &st))
> +		return 1;
> +
> +	/*
> +	 * Otherwise, path does not exist so we'll return 0...but we'll first
> +	 * determine some info about its parent directory so we can avoid
> +	 * lstat calls for future cache entries.
> +	 */
> +	newdir = strrchr(path, '/');
> +	if (!newdir)
> +		return 0; /* Didn't find a parent dir; just return 0 now. */
> +
> +	/*
> +	 * If path starts with directory (which we already lstat'ed and found),
> +	 * then no need to lstat parent directory again.
> +	 */
> +	if (*dir_found && *dirname && memcmp(path, *dirname, *dir_len))
> +		return 0;
> +
> +	/* Free previous dirname, and cache path's dirname */
> +	*dirname = path;
> +	*dir_len = newdir - path + 1;
> +
> +	tmp = xstrndup(path, *dir_len);
> +	*dir_found = !lstat(tmp, &st);
> +	free(tmp);
> +
> +	return 0;
> +}
> +
>  void clear_skip_worktree_from_present_files(struct index_state *istate)
>  {
> +	const char *last_dirname = NULL;
> +	size_t dir_len = 0;
> +	int dir_found = 1;
> +
>  	int i;
> +
>  	if (!core_apply_sparse_checkout)
>  		return;
>  
>  restart:
>  	for (i = 0; i < istate->cache_nr; i++) {
>  		struct cache_entry *ce = istate->cache[i];
> -		struct stat st;
>  
> -		if (ce_skip_worktree(ce) && !lstat(ce->name, &st)) {
> +		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;