Re: [PATCH v2 06/13] merge-ort: add data structures for in-memory caching of rename detection

From: Elijah Newren <newren@gmail.com>
To: Derrick Stolee <stolee@gmail.com>
Cc: Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>,
	Git Mailing List <git@vger.kernel.org>,
	Derrick Stolee <dstolee@microsoft.com>,
	Jonathan Tan <jonathantanmy@google.com>,
	Taylor Blau <me@ttaylorr.com>
Subject: Re: [PATCH v2 06/13] merge-ort: add data structures for in-memory caching of rename detection
Date: Mon, 17 May 2021 20:55:07 -0700	[thread overview]
Message-ID: <CABPp-BFOSBVP-9A6BQegpaPRA+iU=ZQCiJYrTEkq0H9b+xRjEQ@mail.gmail.com> (raw)
In-Reply-To: <b9bb5b44-47ce-8198-c546-8f07d03ef863@gmail.com>

On Mon, May 17, 2021 at 6:41 AM Derrick Stolee <stolee@gmail.com> wrote:
>
> On 5/3/21 10:12 PM, Elijah Newren via GitGitGadget wrote:
> > From: Elijah Newren <newren@gmail.com>
> >
> > When there are many renames between the old base of a series of commits
> > and the new base for a series of commits, the sequence of merges
> > employed to transplant those commits (from a cherry-pick or rebase
> > operation) will repeatedly detect the exact same renames.  This is
> > wasted effort.
> >
> > Add data structures which will be used to cache rename detection
> > results, along with the initialization and deallocation of these data
> > structures.  Future commits will populate these caches, detect the
> > appropriate circumstances when they can be used, and employ them to
> > avoid re-detecting the same renames repeatedly.
>
> I appreciate the definitions and boilerplate for these data
> structures being isolated to their own patch.
>
> > @@ -140,6 +140,37 @@ struct rename_info {
> >       int callback_data_nr, callback_data_alloc;
> >       char *callback_data_traverse_path;
> >
> > +     /*
> > +      * cached_pairs: Caching of renames and deletions.
> > +      *
> > +      * These are mappings recording renames and deletions of individual
> > +      * files (not directories).  They are thus a map from an old
> > +      * filename to either NULL (for deletions) or a new filename (for
> > +      * renames).
> > +      */
> > +     struct strmap cached_pairs[3];
> > +
> > +     /*
> > +      * cached_target_names: just the destinations from cached_pairs
> > +      *
> > +      * We sometimes want a fast lookup to determine if a given filename
> > +      * is one of the destinations in cached_pairs.  cached_target_names
> > +      * is thus duplicative information, but it provides a fast lookup.
> > +      */
> > +     struct strset cached_target_names[3];
>
> These two work well together. Very clear.
>
> > +     /*
> > +      * cached_irrelevant: Caching of rename_sources that aren't relevant.
> > +      *
> > +      * cached_pairs records both renames and deletes.  Sometimes we
> > +      * do not know if a path is a rename or a delete because we pass
> > +      * RELEVANT_LOCATION to diffcore_rename_extended() and based on
> > +      * various optimizations it returns without detecting whether that
> > +      * path is actually a rename or a delete.  We need to cache such
> > +      * paths too, but separately from cached_pairs.
> > +      */
> > +     struct strset cached_irrelevant[3];
>
> I'm having a hard time parsing what these "irrelevant" paths will be.
> It seems like diffcore_rename_extended() will report something other
> than "rename" or "delete" for some paths. Could we explicitly mark
> that state as "irrelevant"?

The state is better known as RELEVANT_NO_MORE, yes.

>         /*
>          * cached_irrelevant: Caching of rename_sources that aren't relevant.
>          *
>          * cached_pairs records both renames and deletes.  Sometimes we
>          * do not know if a path is a rename or a delete because we pass
>          * RELEVANT_LOCATION to diffcore_rename_extended() which might
>          * describe a path as "irrelevant" instead of as a "rename" or "delete".
>          *  We need to cache such paths too, but separately from cached_pairs.
>          */
>
> Does this make sense? diffcore_rename_extended() might need an update
> to match this extra, explicit state.

Hmm, let's flesh out the description a bit and try to be more
explicit.  How about:

    /*
     * cached_irrelevant: Caching of rename_sources that aren't relevant.
     *
     * If we try to detect a rename for a source path and succeed, it's
     * part of a rename.  If we try to detect a rename for a source path
     * and fail, then it's a delete.  If we do not try to detect a rename
     * for a path, then we don't know if it's a rename or a delete.  If
     * merge-ort doesn't think the path is relevant, then we just won't
     * cache anything for that path.  But there's a slight problem in
     * that merge-ort can think a path is RELEVANT_LOCATION, but due to
     * commit 9bd342137e ("diffcore-rename: determine which
     * relevant_sources are no longer relevant", 2021-03-13),
     * diffcore-rename can downgrade the path to RELEVANT_NO_MORE.  To
     * avoid excessive calls to diffcore_rename_extended() we still need
     * to cache such paths, though we cannot record them as either
     * renames or deletes.  So we cache them here as a "turned out to be
     * irrelevant *for this commit*" as they are often also irrelevant
     * for subsequent commits, though we will have to do some extra
     * checking to see whether such paths become relevant for rename
     * detection when cherry-picking/rebasing subsequent commits.
     */