`subdatasets` slow for no apparent reason

[mih]

Scenario: dataset with two non-installed subdatasets and 40k top-level subdirectories:

% datalad -f disabled subdatasets 
datalad -f disabled subdatasets  91.01s user 11.94s system 100% cpu 1:42.54 total
% time git submodule
-dfa6d975ea8888ed33bf714c674371e1ab54e834 code/pipeline
-0c7f0b45140dde1d7291b15725e2016231a670b0 inputs/ukb
git submodule  0.02s user 0.02s system 105% cpu 0.035 total
% cat .gitmodules
[submodule "code/pipeline"]
        path = code/pipeline
        url = http://containers.ds.inm7.de/alias/cat
        datalad-id = 99300fb0-ea39-4fd2-b36d-2afc1e9ab7b2
[submodule "inputs/ukb"]
        path = inputs/ukb
        url = http://ukb.ds.inm7.de/alias/bids
        datalad-id = 14a5e5b6-d30d-11ea-8836-b4969157768c
% git submodule
-dfa6d975ea8888ed33bf714c674371e1ab54e834 code/pipeline
-0c7f0b45140dde1d7291b15725e2016231a670b0 inputs/ukb

So 35ms (git) vs 1m42s (datalad). 🤦

And the reason seems to be in GitRepo.get_submodules_():

datalad/datalad/support/gitrepo.py

Lines 2357 to 2368 in 6b7a2f7

	modinfo = self._parse_gitmodules()
	for path, props in self.get_content_info(
	paths=paths,
	ref=None,
	untracked='no').items():
	if props.get('type', None) != 'dataset':
	# make sure this method never talks about non-dataset
	# content
	continue
	props["path"] = path
	props.update(modinfo.get(path, {}))
	yield props

It calls get_content_info() to get everything about the dataset, only to discard anything that is not a type='dataset'. In this case of 2 subdatasets and 80k directories this is highly inefficient.

It seems to make sense to limit the call to get_content_info() to the paths obtain from _parse_gitmodules() already (or even a dedicated git submodule call).

get_submodules_() currently yields records like this:

 {'gitmodule_datalad-id': '14a5e5b6-d30d-11ea-8836-b4969157768c',
  'gitmodule_name': 'inputs/ukb',
  'gitmodule_url': 'http://ukb.ds.inm7.de/alias/bids',
  'gitshasum': '0c7f0b45140dde1d7291b15725e2016231a670b0',
  'path': PosixPath('/home/mih/ukb-gource/ukb-derivatives/inputs/ukb'),
  'type': 'dataset'}

All properties prefixed with gitmodule_ and the path are instantaneously provided by _parse_gitmodules(). The type='dataset' is pretty much implied and the gitshasum could come from get_content_info(), parameterized with the "intersection" of the reported submodule paths and the paths argument of get_submodules_().

However, #6941 points out that get_submodules_() and get_content_info() are actually circular dependencies. So it would probably be better to implement the gitshasum query with a plain ls-files --stage call, instead of going through the full complexity of get_content_info().

[mih]

The following patch would turn 1min42s runtime into 270ms, and it would also remove the circular dependency, as well as the implied performance penalty #6941

diff --git a/datalad/support/gitrepo.py b/datalad/support/gitrepo.py
index 1159af3f8..e6a4bfc29 100644
--- a/datalad/support/gitrepo.py
+++ b/datalad/support/gitrepo.py
@@ -2355,17 +2358,37 @@ class GitRepo(CoreGitRepo):
             return
 
         modinfo = self._parse_gitmodules()
-        for path, props in self.get_content_info(
-                paths=paths,
-                ref=None,
-                untracked='no').items():
-            if props.get('type', None) != 'dataset':
+        if paths:
+            # ease comparison
+            paths = [self.pathobj / p for p in paths]
+            # contrain the report by the given paths
+            modinfo = {
+                # modpath is absolute
+                modpath: modprobs
+                for modpath, modprobs in modinfo.items()
+                # is_relative_to() also match equal paths
+                if any(modpath.is_relative_to(p) for p in paths)
+            }
+        for r in self.call_git_items_(
+            ['ls-files', '--stage', '-z'],
+            sep='\0',
+            files=[str(p.relative_to(self.pathobj)) for p in modinfo.keys()],
+            read_only=True,
+            keep_ends=True,
+        ):
+            if not r.startswith('160000'):
                 # make sure this method never talks about non-dataset
                 # content
                 continue
-            props["path"] = path
-            props.update(modinfo.get(path, {}))
-            yield props
+            props, rpath = r.split('\t')
+            # remove the expected line separator from the path
+            path = self.pathobj / PurePosixPath(rpath[:-1])
+            yield dict(
+                path=path,
+                type='dataset',
+                gitshasum=props.split(' ')[1],
+                **modinfo.get(path, {})
+            )
 
     def get_submodules(self, sorted_=True, paths=None):
         """Return list of submodules.

In my particular testcase this yields a speed-up of factor 350+