sync: Always use WORKER_BATCH_SIZE

With 551285fa35, the comment about number
of workers no longer stands - dict is shared among multiprocesses and
real time information is available.

Using 2.7k projects as the baseline, using chunk size of 4 takes close
to 5 minutes. A chunk size of 32 takes this down to 40s - a reduction of
rougly 8 times which matches the increase.

R=gavinmak@google.com

Bug: b/371638995
Change-Id: Ida5fd8f7abc44b3b82c02aa0f7f7ae01dff5eb07
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/438523
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

project.py

@@ -2396,26 +2396,25 @@ class Project:
         try:
             # if revision (sha or tag) is not present then following function
             # throws an error.
-            self.bare_git.rev_list(
-                "-1",
-                "--missing=allow-any",
-                "%s^0" % self.revisionExpr,
-                "--",
-                log_as_error=False,
-            )
+            revs = [f"{self.revisionExpr}^0"]
+            upstream_rev = None
             if self.upstream:
-                rev = self.GetRemote().ToLocal(self.upstream)
+                upstream_rev = self.GetRemote().ToLocal(self.upstream)
+                revs.append(upstream_rev)
+
             self.bare_git.rev_list(
                 "-1",
                 "--missing=allow-any",
-                    "%s^0" % rev,
+                *revs,
                 "--",
                 log_as_error=False,
             )
+
+            if self.upstream:
                 self.bare_git.merge_base(
                     "--is-ancestor",
                     self.revisionExpr,
-                    rev,
+                    upstream_rev,
                     log_as_error=False,
                 )
             return True

subcmds/sync.py

@@ -131,6 +131,11 @@ def _SafeCheckoutOrder(checkouts: List[Project]) -> List[List[Project]]:
     return res
 
 
+def _chunksize(projects: int, jobs: int) -> int:
+    """Calculate chunk size for the given number of projects and jobs."""
+    return min(max(1, projects // jobs), WORKER_BATCH_SIZE)
+
+
 class _FetchOneResult(NamedTuple):
     """_FetchOne return value.
 
@@ -819,7 +824,6 @@ later is required to fix a server side protocol bug.
     def _Fetch(self, projects, opt, err_event, ssh_proxy, errors):
         ret = True
 
-        jobs = opt.jobs_network
         fetched = set()
         remote_fetched = set()
         pm = Progress(
@@ -849,6 +853,8 @@ later is required to fix a server side protocol bug.
             objdir_project_map.setdefault(project.objdir, []).append(project)
         projects_list = list(objdir_project_map.values())
 
+        jobs = min(opt.jobs_network, len(projects_list))
+
         def _ProcessResults(results_sets):
             ret = True
             for results in results_sets:
@@ -888,35 +894,22 @@ later is required to fix a server side protocol bug.
         Sync.ssh_proxy = None
 
         # NB: Multiprocessing is heavy, so don't spin it up for one job.
-        if len(projects_list) == 1 or jobs == 1:
+        if jobs == 1:
             self._FetchInitChild(ssh_proxy)
             if not _ProcessResults(
                 self._FetchProjectList(opt, x) for x in projects_list
             ):
                 ret = False
         else:
-            # Favor throughput over responsiveness when quiet.  It seems that
-            # imap() will yield results in batches relative to chunksize, so
-            # even as the children finish a sync, we won't see the result until
-            # one child finishes ~chunksize jobs.  When using a large --jobs
-            # with large chunksize, this can be jarring as there will be a large
-            # initial delay where repo looks like it isn't doing anything and
-            # sits at 0%, but then suddenly completes a lot of jobs all at once.
-            # Since this code is more network bound, we can accept a bit more
-            # CPU overhead with a smaller chunksize so that the user sees more
-            # immediate & continuous feedback.
-            if opt.quiet:
-                chunksize = WORKER_BATCH_SIZE
-            else:
+            if not opt.quiet:
                 pm.update(inc=0, msg="warming up")
-                chunksize = 4
             with multiprocessing.Pool(
                 jobs, initializer=self._FetchInitChild, initargs=(ssh_proxy,)
             ) as pool:
                 results = pool.imap_unordered(
                     functools.partial(self._FetchProjectList, opt),
                     projects_list,
-                    chunksize=chunksize,
+                    chunksize=_chunksize(len(projects_list), jobs),
                 )
                 if not _ProcessResults(results):
                     ret = False

tests/test_subcmds_sync.py

@@ -355,6 +355,30 @@ class SafeCheckoutOrder(unittest.TestCase):
         )
 
 
+class Chunksize(unittest.TestCase):
+    """Tests for _chunksize."""
+
+    def test_single_project(self):
+        """Single project."""
+        self.assertEqual(sync._chunksize(1, 1), 1)
+
+    def test_low_project_count(self):
+        """Multiple projects, low number of projects to sync."""
+        self.assertEqual(sync._chunksize(10, 1), 10)
+        self.assertEqual(sync._chunksize(10, 2), 5)
+        self.assertEqual(sync._chunksize(10, 4), 2)
+        self.assertEqual(sync._chunksize(10, 8), 1)
+        self.assertEqual(sync._chunksize(10, 16), 1)
+
+    def test_high_project_count(self):
+        """Multiple projects, high number of projects to sync."""
+        self.assertEqual(sync._chunksize(2800, 1), 32)
+        self.assertEqual(sync._chunksize(2800, 16), 32)
+        self.assertEqual(sync._chunksize(2800, 32), 32)
+        self.assertEqual(sync._chunksize(2800, 64), 32)
+        self.assertEqual(sync._chunksize(2800, 128), 21)
+
+
 class GetPreciousObjectsState(unittest.TestCase):
     """Tests for _GetPreciousObjectsState."""