add cli arg to limit download parallelism

news/12923.feature.rst

@@ -1 +1 @@
-Download concrete dists for metadata-only resolves in parallel using worker threads.
+Download concrete dists for metadata-only resolves in parallel using worker threads. Add ``--batch-download-parallelism`` CLI flag to limit parallelism.

src/pip/_internal/cli/cmdoptions.py

@@ -231,6 +231,24 @@ class PipOption(Option):
     help="Specify whether the progress bar should be used [on, off, raw] (default: on)",
 )
 
+
+batch_download_parallelism: Callable[..., Option] = partial(
+    Option,
+    "--batch-download-parallelism",
+    dest="batch_download_parallelism",
+    type="int",
+    default=10,
+    help=(
+        "Maximum parallelism employed for batch downloading of metadata-only dists"
+        " (default %default parallel requests)."
+        " Note that more than 10 downloads may overflow the requests connection pool,"
+        " which may affect performance."
+        " Note also that commands such as 'install --dry-run' should avoid downloads"
+        " entirely, and so will not be affected by this option."
+    ),
+)
+
+
 log: Callable[..., Option] = partial(
     PipOption,
     "--log",

src/pip/_internal/cli/req_command.py

@@ -100,6 +100,7 @@ def make_requirement_preparer(
         use_user_site: bool,
         download_dir: Optional[str] = None,
         verbosity: int = 0,
+        batch_download_parallelism: Optional[int] = None,
     ) -> RequirementPreparer:
         """
         Create a RequirementPreparer instance for the given parameters.
@@ -141,6 +142,7 @@ def make_requirement_preparer(
             use_user_site=use_user_site,
             lazy_wheel=lazy_wheel,
             verbosity=verbosity,
+            batch_download_parallelism=batch_download_parallelism,
             legacy_resolver=legacy_resolver,
         )
 

src/pip/_internal/commands/download.py

@@ -47,6 +47,7 @@ def add_options(self) -> None:
         self.cmd_opts.add_option(cmdoptions.pre())
         self.cmd_opts.add_option(cmdoptions.require_hashes())
         self.cmd_opts.add_option(cmdoptions.progress_bar())
+        self.cmd_opts.add_option(cmdoptions.batch_download_parallelism())
         self.cmd_opts.add_option(cmdoptions.no_build_isolation())
         self.cmd_opts.add_option(cmdoptions.use_pep517())
         self.cmd_opts.add_option(cmdoptions.no_use_pep517())
@@ -116,6 +117,7 @@ def run(self, options: Values, args: List[str]) -> int:
             download_dir=options.download_dir,
             use_user_site=False,
             verbosity=self.verbosity,
+            batch_download_parallelism=options.batch_download_parallelism,
         )
 
         resolver = self.make_resolver(

src/pip/_internal/commands/install.py

@@ -237,6 +237,7 @@ def add_options(self) -> None:
         self.cmd_opts.add_option(cmdoptions.prefer_binary())
         self.cmd_opts.add_option(cmdoptions.require_hashes())
         self.cmd_opts.add_option(cmdoptions.progress_bar())
+        self.cmd_opts.add_option(cmdoptions.batch_download_parallelism())
         self.cmd_opts.add_option(cmdoptions.root_user_action())
 
         index_opts = cmdoptions.make_option_group(
@@ -359,6 +360,7 @@ def run(self, options: Values, args: List[str]) -> int:
                 finder=finder,
                 use_user_site=options.use_user_site,
                 verbosity=self.verbosity,
+                batch_download_parallelism=options.batch_download_parallelism,
             )
             resolver = self.make_resolver(
                 preparer=preparer,

src/pip/_internal/commands/wheel.py

@@ -67,6 +67,7 @@ def add_options(self) -> None:
         self.cmd_opts.add_option(cmdoptions.ignore_requires_python())
         self.cmd_opts.add_option(cmdoptions.no_deps())
         self.cmd_opts.add_option(cmdoptions.progress_bar())
+        self.cmd_opts.add_option(cmdoptions.batch_download_parallelism())
 
         self.cmd_opts.add_option(
             "--no-verify",
@@ -131,6 +132,7 @@ def run(self, options: Values, args: List[str]) -> int:
             download_dir=options.wheel_dir,
             use_user_site=False,
             verbosity=self.verbosity,
+            batch_download_parallelism=options.batch_download_parallelism,
         )
 
         resolver = self.make_resolver(

src/pip/_internal/network/download.py

@@ -13,7 +13,7 @@
 from pip._vendor.requests.models import Response
 
 from pip._internal.cli.progress_bars import get_download_progress_renderer
-from pip._internal.exceptions import NetworkConnectionError
+from pip._internal.exceptions import CommandError, NetworkConnectionError
 from pip._internal.models.index import PyPI
 from pip._internal.models.link import Link
 from pip._internal.network.cache import is_from_cache
@@ -226,11 +226,20 @@ def __init__(
         self,
         session: PipSession,
         progress_bar: str,
+        max_parallelism: Optional[int] = None,
     ) -> None:
         self._session = session
         # FIXME: support progress bar with parallel downloads!
         logger.info("Ignoring progress bar %s for parallel downloads", progress_bar)
 
+        if max_parallelism is None:
+            max_parallelism = 1
+        if max_parallelism < 1:
+            raise CommandError(
+                f"invalid batch download parallelism {max_parallelism}: must be >=1"
+            )
+        self._max_parallelism: int = max_parallelism
+
     def __call__(
         self, links: Iterable[Link], location: Path
     ) -> Iterable[Tuple[Link, Tuple[Path, Optional[str]]]]:
@@ -254,7 +263,7 @@ def __call__(
         q: "Queue[Union[Tuple[Link, Path, Optional[str]], BaseException]]" = Queue()
         event = Event()
         # Limit downloads to 10 at a time so we can reuse our connection pool.
-        semaphore = Semaphore(value=10)
+        semaphore = Semaphore(value=self._max_parallelism)
 
         # Distribute request i/o across equivalent threads.
         # NB: event-based/async is likely a better model than thread-per-request, but

src/pip/_internal/operations/prepare.py

@@ -230,6 +230,7 @@ def __init__(
         use_user_site: bool,
         lazy_wheel: bool,
         verbosity: int,
+        batch_download_parallelism: Optional[int],
         legacy_resolver: bool,
     ) -> None:
         super().__init__()
@@ -239,7 +240,9 @@ def __init__(
         self.build_tracker = build_tracker
         self._session = session
         self._download = Downloader(session, progress_bar)
-        self._batch_download = BatchDownloader(session, progress_bar)
+        self._batch_download = BatchDownloader(
+            session, progress_bar, max_parallelism=batch_download_parallelism
+        )
         self.finder = finder
 
         # Where still-packed archives should be written to. If None, they are

tests/unit/test_req.py

@@ -106,6 +106,7 @@ def _basic_resolver(
                 use_user_site=False,
                 lazy_wheel=False,
                 verbosity=0,
+                batch_download_parallelism=None,
                 legacy_resolver=True,
             )
             yield Resolver(