On 1/6/2021 2:15 PM, Elijah Newren wrote:

On Sun, Jan 3, 2021 at 7:02 PM Derrick Stolee [off-list ref] wrote:

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On 12/31/2020 3:03 PM, Elijah Newren wrote:
Others use sparse-checkout to remove a few large files unless they
need them. I'm less interested in this case, myself.

Both perspectives get better with partial clone because the download
size shrinks significantly. While partial clone has a sparse-checkout
style filter, it is hard to compute on the server side. Further, it
is not very forgiving of someone wanting to change their sparse
definition after cloning. Tree misses are really expensive, and I find
that the extra network transfer of the full tree set is a price that is
worth paying.

Out of curiosity, is that because the promisor handling doesn't do
nice batching of trees to download, as is done for blobs, or is there
a more fundamental reason they are really expensive?  (I'm just
wondering if we are risking changing design in some areas based on
suboptimal implementation of other things.  I don't actually have
experience with partial clones yet, though, so I'm basically just
querying about random but interesting things without any experience to
back it up.)

GitHub doesn't support pathspec filters for partial clone because it
is too expensive to calculate that initial packfile (cannot use
reachability bitmaps). Even outside of that initial cost, we have
problems.

The biggest problem is that we ask for the tree as a one-off request.
There are two ways to approach this:

1. Ask for all trees that are reachable from that tree so we can
   complete the tree walk (current behavior). This downloads trees we
   already have, most of the time.

2. Ask for only that tree and no extra objects. This causes the request
   count to increase significantly, especially during a 'git pull' or
   'git checkout' that spans a large distance.

In either case, commands like "git log -- README.md" are really bad in
a treeless clone (--filter=tree:0).

For the sparse-checkout case, we still need the trees outside of our
sparse cone in order to construct an index, even if we never actually
check out those files. (Maybe not forever, though...)

And maybe the solution would be to ask the server for your missing
trees in the entire history when you change sparse-checkout definition,
but what does that request look like?

 client> I have these commits with trees according to this pathspec.
 client> I want these commits with trees according to a new pathspec.
 server> *flips table*

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I think there are three possible situations:

1. sparse-checkout should not affect the behavior at all.

   An example for this is "git commit". We want the root tree to contain
   all of the subtrees and blobs that are out of the sparse-checkout
   definition. The underlying object model should never change.

2. sparse-checkout should change the default, but users can opt-out.

   The examples I think of here are 'git grep' and 'git rm', as we have
   discussed recently. Having a default of "you already chose to be in
   a sparse-checkout, so we think this behavior is better for you"
   should continue to be pursued.

3. Users can opt-in to a sparse-checkout version of a behavior.

   The example in this case is "git diff". Perhaps we would want to see
   a diff scoped only to our sparse definition, but that should not be
   the default. It is too risky to change the output here without an
   explicit choice by the user.

I'm curious why you put grep and diff in different categories.  A
plain "git diff" without revisions will give the same output whether
or not it restricts to the sparsity paths (because the other paths are
unchanged), so restricting is purely an optimization question.  Making
"git diff REVISION" restrict to the sparsity paths would be a
behavioral change as you note, but "git grep [REVISION]" would also
require a behavioral change to limit to the sparsity paths.  If it's
too risky to change the output for git diff with revisions, why is it
not also too risky to do that with git grep with revisions?

I generally think of 'grep' as being "search for something I care about"
which is easier to justify scoping to sparse-checkouts.

'diff' is something that I usually think of as "compare two git objects"
and it is operating on immutable data.

The practical difference comes into play with a blobless partial clone:
'diff' will download blobs that need a content comparison, so the cost
is relative to the number of changed paths in that region and relative
to the requested output. 'grep' will download every blob reachable from
the root tree. We've seen too many cases of users trying 'git grep' to
search the Windows codebase and complaining that it takes too long
(because they are downloading 3 million blobs one at a time).

Also, I think you are missing a really important category:

4. sparse-checkout changes the behavior of commands and there is no
opt-out or configurability provided.

The most obvious examples are switch and checkout -- their modified
behavior is really the /point/ of sparse-checkouts and if you want to
"opt out" then just don't use sparse-checkouts.  `reset --hard` can go
in the same bucket; it's modified in the same way.  However, some
commands are modified in a different way, but also have no opt-out --
for example, merge, rebase, cherry-pick, revert, and stash, all "try"
to avoid writing files to the working tree that match the sparsify
specifications, but will vivify files which have conflicts (and maybe
a few additional files based on implementation shortcomings).  Another
command that behaves differently than any of these, and is also
non-configurable in this change, is git-add.  It'll ignore any tracked
files with the SKIP_WORKTREE bit set, even if the file is present.
That's really helpful thing for "git add -A [GLOB_OR_DIRECTORY]" to
do, as we don't want sparsity to accidentally be treated as a
directive to remove files from the repository.

True. Except for these, the opt-in/out is "git sparse-checkout init"
and "git sparse-checkout disable". If I want "git checkout" to behave
differently, then I modify my sparse-checkout definition or disable
it altogether.

Perhaps instead we should think of this category as the "core
functionality of sparse-checkout."

I think more commands should fall under this fourth category as well,
including rm.

The biggest issue with 'rm' is that users may want to use it to
delete paths outside of their sparse-checkout according to a
pathspec. This is especially true since it is the current
behavior, so if we change it by default we might discover more
complaints than the current requests for a way to limit to the
sparse-checkout definition.

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  * mt/grep-sparse-checkout: figure out shorter flag names.  Default to
      --no-restrict-to-sparse, for now.  Then merge it for git-2.31.

I don't want to derail your high-level conversation too much, but by the
end of January I hope to send an RFC to create a "sparse index" which allows
the index to store entries corresponding to a directory with the skip-
worktree bit on. The biggest benefit is that commands like 'git status' and
'git add' will actually change their performance based on the size of the
sparse-checkout definition and not the total number of paths at HEAD.

This is _awesome_; I think it'll be huge.  It'll cause even more
commands behavior to change, of course, but in a good way.  And I
don't consider this derailing at all but extending the discussion
complete with extra investigation work.  :-)

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The other thing that happens once we have that idea is that these behaviors
in 'git grep' or 'git rm' actually become _easier_ to implement because we
don't even have an immediate reference to the blobs outside of the sparse
cone (assuming cone mode).

The tricky part (that I'm continuing to work on, hence no RFC today) is
enabling the part where a user can opt-in to the old behavior. This requires
parsing trees to expand the index as necessary. A simple approach is to
create an in-memory index that is the full expansion at HEAD, when necessary.
It will be better to do expansions in a targeted way.

I'm not sure if you're just thinking of the old mt/rm-sparse-checkout
and commenting on it, or if you're actively disagreeing with my
proposal for rm.

I remember the discussion around how making 'rm' sparse-aware was more
complicated than "only look at entries without CE_SKIP_WORKTREE" but
it might be easier with a sparse-index. So my intention here was to
see if we should _delay_ our investigation here until I can at least
get a prototype ready for inspection.

I'm also saying that perhaps we could redirect this discussion around
how to opt-in/out of these changes. Much like your "category 4" above
being "behavior expected when in a sparse-checkout," what if this
behavior of restricting to the sparse set was expected when using a
sparse-index instead of based on config options or run-time arguments?

What if we had something like "git update-index --[no-]sparse" to
toggle between the two states?

That's my intention with bringing up my half-baked idea before I have
code to show for it.

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(Your merge-ort algorithm is critical to the success here, since that doesn't
use the index as a data structure. I expect to make merge-ort the default for
users with a sparse index. Your algorithm will be done first.)

Well, at 50 added/changed lines per patch, I've only got ~50 more
patches to go for ort after the ones I submitted Monday (mostly
optimization related).  If I submit 10 patches per week (starting next
week since I already sent a big patchset this week), then maybe
mid-to-late February.  That's a more aggressive pace than we've
managed so far, but maybe it gets easier towards the end?  Anyway,
hopefully that helps you with timing predictions.

On my end, this does make the ort work look like there's finally some
light at the end of the tunnel; I just hope it's not an oncoming
train. :-)

While I expect to have an RFC ready at the end of the month, I expect
I will be working on sparse-index for the entire 2021 calendar year
before it will be fully ready to use by end-users. I expect my RFC to
have fast "git status" and "git add" times, but other commands will
have a guard that expands a sparse-index into a "full" index before
proceeding. This protection will keep behavior consistent but will
cause performance problems. Iteratively removing these guards and
implementing "sparse-aware" versions of each index operation will take
time and care.

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My point in bringing this up is that perhaps we should pause concrete work on
updating other builtins until we have a clearer idea of what a sparse index
could look like and how the implementation would change based on having one
or not. I hope that my RFC will be illuminating in this regard.

Are you suggesting to pause any work on those pieces of the proposal
that might be affected by your sparse index, or pause any work at all
on sparse-checkouts?  For example, I think
en/stash-apply-sparse-checkout that's been sitting in seen is good to
merge down to master now.  I suspect mt/rm-sparse-checkout WITH my
suggested changes (no configurability -- similar to git-add) and a
better warning/error message for git-add are some examples of cleanups
that could be done before your sparse index, but if you're worried
about conflicting I certainly don't want to derail your project.  (I
agree that anything with configurability and touching on "behavior A"
or "sparse parallelax", like mt/grep-sparse-checkout would be better
if we waited on.  I do feel pretty bad for how much we've made Matheus
wait on that series, but waiting does still seem best.)

I don't want to hold up valuable work. It's just tricky to navigate
parallel efforts in the same space. I'm asking for a little more time
to get my stuff together to see if it would influence your work.

But it is unreasonable for me to "squat" on the feature and keep others
from making valuable improvements.

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    * shorter flag names than --[no-]restrict-to-sparse.  --dense and
      --sparse?  --[no-]restrict?

--full-workdir?

Hmm.  "workdir" sounds like an abbreviation of "working directory",
which is the place where the files are checked out.  And the working
directory is sparse in a sparse-checkout.  So isn't this misleading?
Or did you intend for this option to be the name for requesting a
sparser set?  (If so, isn't "full" in its name a bit weird?)

Also, what would the inverse name of --full-workdir be?  I was looking
to add options for both restricting the command to the sparser set and
for expanding to the full set of files.  Though I guess as you note
below, you perhaps might be in favor of only one of these without
configuration options to adjust defaults.

Right. Perhaps --full-tree or --sparse-tree would be better? I was
trying to link the adjectives "full" and "sparse" to a noun that they
modify.

--dense already exists in rev-list to describe a form of history
simplification.

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--> Commands that would change for behavior A
  * bisect
    * Only consider commits touching paths matching sparsity patterns
  * diff
    * When given revisions, only show subset of files matching sparsity
      patterns.  If pathspecs are given, intersect them with sparsity
      patterns.
  * log
    * Only consider commits touching at least one path matching sparsity
      patterns.  If pathspecs are given, paths must match both the
      pathspecs and the sparsity patterns in order to be considered
      relevant and be shown.
  * gitk
    * See log
  * shortlog
    * See log
  * grep
    * See mt/grep-sparse-checkout; it's been discussed in detail..and is
      implemented.  (Other than that we don't want behavior A to be the
      default when so many commands do not support it yet.)

  * show-branch
    * See log
  * whatchanged
    * See log
  * show (at least for commits)
    * See diff

  * blame
    * With -C or -C -C, only detect lines moved/copied from files that match
      the sparsity paths.
  * annotate
    * See blame.

this "behavior A" idea is the one I'm most skeptical about. Creating a
way to opt-in to a sparse definition might be nice. It might be nice to
run "git log --simplify-sparse" to see the simplified history when only
caring about commits that changed according to the current sparse-checkout
definitions. Expand that more when asking for diffs as part of that log,
and the way we specify the option becomes tricky.

--simplify-sparse is a really long name to need to specify at every
invocation.  Also, if we have --[no]-restrict or --sparse/--dense
options at the git level (rather than the subcommand level), then I
think we don't want extra ones like this at the subcommand level.

Also, if the option appears at the global git level, doesn't that
remove the trickiness of revision traversal vs. diff outputting in
commands like log?  It just automatically applies to both.  (The only
trickiness would be if you wanted to somehow apply sparsity patterns
to just revision traversal or just diff outputting but not to both,
but that's already tricky in log with explicit pathspecs and we've
traditionally had files restrict both.)

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But I also want to avoid doing this as a default or even behind a config
setting. We already get enough complains about "missing commits" when
someone does a bad merge so "git log -- file" simplifies away a commit
that exists in the full history. Imagine someone saying "on my machine,
'git log' shows the commit, but my colleague can't see it!" I would really
like to avoid adding to that confusion if possible.

That's a good point.  A really good point.  Maybe we do only want to
allow explicit requests for this behavior -- and thus need very short
option name for it.

And even though I mentioned earlier that "having a sparse-index might
be a good way to opt-in," I would still say that simplifying commit
history in 'git log' or reducing diff output would still require a
short command-line option.

Here's a not-even-half-baked idea for thought: What if we allowed a
configuration option to control this, BUT whenever a command like
diff/grep/log restricts output based on the sparsity paths due solely
to the configuration option, it prints a small reminder on stderr at
the beginning of the output (e.g. "Note: output limited to sparsity
paths, as per sparse.restrictCmds setting")?

I'm not thrilled with this idea, but perhaps the warning can be
toggled by an advice.* config option.

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--> Commands whose behavior I'm still uncertain of:
  * worktree add
    * for behavior A (marrying sparse-checkout with partial clone), we
      should almost certainly copy sparsity paths from the previous
      worktree (we either have to do that or have some kind of
      specify-at-clone-time default set of sparsity paths)
    * for behavior B, we may also want to copy sparsity paths from the
      previous worktree (much like a new command line shell will copy
      $PWD from the previous one), but it's less clear.  Should it?

I think 'git worktree add' should at minimum continue using a sparse-
checkout if the current working directory has one. Worktrees are a
great way to scale the creation of multiple working directories for
the same repository without re-cloning all of the history. In a partial
clone case, it's really important that we don't explode the workdir in
the new worktree (or even download all those blobs).

Okay, sounds like you agree with me for the partial clone case -- it's
necessary.

But what about the non-partial clone case?  I think it should adopt
the sparsity in that case too, but Junio has objected in the past.
I'm pretty sure Junio wasn't thinking about the partial clone case,
where I think it seems obvious and compelling.  But I'm not sure how
best to convince him in the non-partial clone case (or maybe I already
did; he didn't respond further after his initial objection).

We might want to consider certain behavior to be on by default when
enough other optional features are enabled. A philosophy such as "We
see you are using partial clone and sparse-checkout, so we restricted
the search in 'git grep' for your own good" might be useful here.

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Thanks for starting the discussion. Perhaps more will pick it up as
they return from the holiday break.

Thanks for jumping in and pushing it much further with sparse indices
(or is it sparse indexes?)  I'm excited.

Another way to push this discussion further would be to create a
forward-looking documentation file in Documentation/technical.
We could use such a documentation as a place to organize thoughts
and plans, especially things like:

* How sparse-checkout works and why users need it.
* How it works (and doesn't work) with partial clone.
* Plans for modifying behavior in sparse scenarios:
  - Current behavior that is wrong or suspicious.
  - Commands that could have different default behavior.
  - Commands that could have different opt-in behavior.
  (This section would include a description of the planned flag
   that modifies behavior to limit to the sparse set.)

I would add a section about sparse-index into such a document, if it
existed.

As things get implemented, these items could be moved out of the
technical documentation and into Documentation/git-sparse-checkout.txt
so we have a central place for users to discover how sparse-checkout
can change their behavior.

Thanks,
-Stolee