Subject: Re: [PATCH v1] dir.c: don't flag the index as dirty for changes to the untracked cache

From: Brandon Williams <[email-masked]>
Date: Mon, 5 Feb 2018 13:58:05 -0800

On 02/05, Ben Peart wrote:
> The untracked cache saves its current state in the UNTR index extension.
> Currently, _any_ change to that state causes the index to be flagged as dirty
> and written out to disk.  Unfortunately, the cost to write out the index can
> exceed the savings gained by using the untracked cache.  Since it is a cache
> that can be updated from the current state of the working directory, there is
> no functional requirement that the index be written out for every change to the
> untracked cache.
> 
> Update the untracked cache logic so that it no longer forces the index to be
> written to disk except in the case where the extension is being turned on or
> off.  When some other git command requires the index to be written to disk, the
> untracked cache will take advantage of that to save it's updated state as well.
> This results in a performance win when looked at over common sequences of git
> commands (ie such as a status followed by add, commit, etc).
> 
> After this patch, all the logic to track statistics for the untracked cache
> could be removed as it is only used by debug tracing used to debug the untracked
> cache.

So we don't need to update it every time because its just a cache
and if its inaccurate between status calls that's ok?  So only
operations like add and commit will actually write out the untracked
cache (as a part of writing out the index).  Sounds ok.

What benefit is there to using the untracked cache then?  Sounds like
you should just turn it off instead?
(I'm sure this is a naive question :D )

> 
> Signed-off-by: Ben Peart <[email-masked]>
> ---
> 
> Notes:
>     Base Ref: master
>     Web-Diff: https://github.com/benpeart/git/commit/20c2e8d787
>     Checkout: git fetch https://github.com/benpeart/git untracked-cache-v1 && git checkout 20c2e8d787
> 
>  dir.c                             | 3 ++-
>  t/t7063-status-untracked-cache.sh | 3 +++
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/dir.c b/dir.c
> index 7c4b45e30e..da93374f0c 100644
> --- a/dir.c
> +++ b/dir.c
> @@ -2297,7 +2297,8 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
>  				 dir->untracked->gitignore_invalidated,
>  				 dir->untracked->dir_invalidated,
>  				 dir->untracked->dir_opened);
> -		if (dir->untracked == istate->untracked &&
> +		if (getenv("GIT_TEST_UNTRACKED_CACHE") &&
> +			dir->untracked == istate->untracked &&
>  		    (dir->untracked->dir_opened ||
>  		     dir->untracked->gitignore_invalidated ||
>  		     dir->untracked->dir_invalidated))
> diff --git a/t/t7063-status-untracked-cache.sh b/t/t7063-status-untracked-cache.sh
> index e5fb892f95..6ef520e823 100755
> --- a/t/t7063-status-untracked-cache.sh
> +++ b/t/t7063-status-untracked-cache.sh
> @@ -14,6 +14,9 @@ test_description='test untracked cache'
>  # See <[email-masked]> if you want to know
>  # more.
>  
> +GIT_TEST_UNTRACKED_CACHE=true
> +export GIT_TEST_UNTRACKED_CACHE
> +
>  sync_mtime () {
>  	find . -type d -ls >/dev/null
>  }
> 
> base-commit: 5be1f00a9a701532232f57958efab4be8c959a29
> -- 
> 2.15.0.windows.1
> 

-- 
Brandon Williams