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From: "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>
To: git@vger.kernel.org
Cc: Johannes.Schindelin@gmx.de, sandals@crustytoothpaste.net,
	steadmon@google.com, jrnieder@gmail.com, peff@peff.net,
	congdanhqx@gmail.com, phillip.wood123@gmail.com,
	emilyshaffer@google.com, sluongng@gmail.com,
	jonathantanmy@google.com,
	Derrick Stolee <derrickstolee@github.com>,
	Derrick Stolee <dstolee@microsoft.com>
Subject: [PATCH v3 09/20] maintenance: add prefetch task
Date: Thu, 30 Jul 2020 22:24:15 +0000	[thread overview]
Message-ID: <9e38ade15c3a3eeec58dc262f1835d344c4899c0.1596147867.git.gitgitgadget@gmail.com> (raw)
In-Reply-To: <pull.671.v3.git.1596147867.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

When working with very large repositories, an incremental 'git fetch'
command can download a large amount of data. If there are many other
users pushing to a common repo, then this data can rival the initial
pack-file size of a 'git clone' of a medium-size repo.

Users may want to keep the data on their local repos as close as
possible to the data on the remote repos by fetching periodically in
the background. This can break up a large daily fetch into several
smaller hourly fetches.

The task is called "prefetch" because it is work done in advance
of a foreground fetch to make that 'git fetch' command much faster.

However, if we simply ran 'git fetch <remote>' in the background,
then the user running a foregroudn 'git fetch <remote>' would lose
some important feedback when a new branch appears or an existing
branch updates. This is especially true if a remote branch is
force-updated and this isn't noticed by the user because it occurred
in the background. Further, the functionality of 'git push
--force-with-lease' becomes suspect.

When running 'git fetch <remote> <options>' in the background, use
the following options for careful updating:

1. --no-tags prevents getting a new tag when a user wants to see
   the new tags appear in their foreground fetches.

2. --refmap= removes the configured refspec which usually updates
   refs/remotes/<remote>/* with the refs advertised by the remote.
   While this looks confusing, this was documented and tested by
   b40a50264ac (fetch: document and test --refmap="", 2020-01-21),
   including this sentence in the documentation:

	Providing an empty `<refspec>` to the `--refmap` option
	causes Git to ignore the configured refspecs and rely
	entirely on the refspecs supplied as command-line arguments.

3. By adding a new refspec "+refs/heads/*:refs/prefetch/<remote>/*"
   we can ensure that we actually load the new values somewhere in
   our refspace while not updating refs/heads or refs/remotes. By
   storing these refs here, the commit-graph job will update the
   commit-graph with the commits from these hidden refs.

4. --prune will delete the refs/prefetch/<remote> refs that no
   longer appear on the remote.

5. --no-write-fetch-head prevents updating FETCH_HEAD.

We've been using this step as a critical background job in Scalar
[1] (and VFS for Git). This solved a pain point that was showing up
in user reports: fetching was a pain! Users do not like waiting to
download the data that was created while they were away from their
machines. After implementing background fetch, the foreground fetch
commands sped up significantly because they mostly just update refs
and download a small amount of new data. The effect is especially
dramatic when paried with --no-show-forced-udpates (through
fetch.showForcedUpdates=false).

[1] https://github.com/microsoft/scalar/blob/master/Scalar.Common/Maintenance/FetchStep.cs

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
---
 Documentation/git-maintenance.txt | 15 +++++++++
 builtin/gc.c                      | 52 +++++++++++++++++++++++++++++++
 t/t7900-maintenance.sh            | 24 ++++++++++++++
 3 files changed, 91 insertions(+)

diff --git a/Documentation/git-maintenance.txt b/Documentation/git-maintenance.txt
index 9204762e21..d134192fa8 100644
--- a/Documentation/git-maintenance.txt
+++ b/Documentation/git-maintenance.txt
@@ -53,6 +53,21 @@ since it will not expire `.graph` files that were in the previous
 `commit-graph-chain` file. They will be deleted by a later run based on
 the expiration delay.
 
+prefetch::
+	The `prefetch` task updates the object directory with the latest
+	objects from all registered remotes. For each remote, a `git fetch`
+	command is run. The refmap is custom to avoid updating local or remote
+	branches (those in `refs/heads` or `refs/remotes`). Instead, the
+	remote refs are stored in `refs/prefetch/<remote>/`. Also, tags are
+	not updated.
++
+This is done to avoid disrupting the remote-tracking branches. The end users
+expect these refs to stay unmoved unless they initiate a fetch.  With prefetch
+task, however, the objects necessary to complete a later real fetch would
+already be obtained, so the real fetch would go faster.  In the ideal case,
+it will just become an update to bunch of remote-tracking branches without
+any object transfer.
+
 gc::
 	Cleanup unnecessary files and optimize the local repository. "GC"
 	stands for "garbage collection," but this task performs many
diff --git a/builtin/gc.c b/builtin/gc.c
index b57bc7b0ff..1f20428286 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -28,6 +28,7 @@
 #include "blob.h"
 #include "tree.h"
 #include "promisor-remote.h"
+#include "remote.h"
 
 #define FAILED_RUN "failed to run %s"
 
@@ -769,6 +770,52 @@ static int maintenance_task_commit_graph(void)
 	return 1;
 }
 
+static int fetch_remote(const char *remote)
+{
+	struct child_process child = CHILD_PROCESS_INIT;
+
+	child.git_cmd = 1;
+	strvec_pushl(&child.args, "fetch", remote, "--prune", "--no-tags",
+		     "--no-write-fetch-head", "--refmap=", NULL);
+
+	strvec_pushf(&child.args, "+refs/heads/*:refs/prefetch/%s/*", remote);
+
+	if (opts.quiet)
+		strvec_push(&child.args, "--quiet");
+
+	return !!run_command(&child);
+}
+
+static int fill_each_remote(struct remote *remote, void *cbdata)
+{
+	struct string_list *remotes = (struct string_list *)cbdata;
+
+	string_list_append(remotes, remote->name);
+	return 0;
+}
+
+static int maintenance_task_prefetch(void)
+{
+	int result = 0;
+	struct string_list_item *item;
+	struct string_list remotes = STRING_LIST_INIT_DUP;
+
+	if (for_each_remote(fill_each_remote, &remotes)) {
+		error(_("failed to fill remotes"));
+		result = 1;
+		goto cleanup;
+	}
+
+	for (item = remotes.items;
+	     item && item < remotes.items + remotes.nr;
+	     item++)
+		result |= fetch_remote(item->string);
+
+cleanup:
+	string_list_clear(&remotes, 0);
+	return result;
+}
+
 static int maintenance_task_gc(void)
 {
 	struct child_process child = CHILD_PROCESS_INIT;
@@ -796,6 +843,7 @@ struct maintenance_task {
 };
 
 enum maintenance_task_label {
+	TASK_PREFETCH,
 	TASK_GC,
 	TASK_COMMIT_GRAPH,
 
@@ -804,6 +852,10 @@ enum maintenance_task_label {
 };
 
 static struct maintenance_task tasks[] = {
+	[TASK_PREFETCH] = {
+		"prefetch",
+		maintenance_task_prefetch,
+	},
 	[TASK_GC] = {
 		"gc",
 		maintenance_task_gc,
diff --git a/t/t7900-maintenance.sh b/t/t7900-maintenance.sh
index 3cdccb24df..5294396a24 100755
--- a/t/t7900-maintenance.sh
+++ b/t/t7900-maintenance.sh
@@ -43,4 +43,28 @@ test_expect_success 'run --task duplicate' '
 	test_i18ngrep "cannot be selected multiple times" err
 '
 
+test_expect_success 'run --task=prefetch with no remotes' '
+	git maintenance run --task=prefetch 2>err &&
+	test_must_be_empty err
+'
+
+test_expect_success 'prefetch multiple remotes' '
+	git clone . clone1 &&
+	git clone . clone2 &&
+	git remote add remote1 "file://$(pwd)/clone1" &&
+	git remote add remote2 "file://$(pwd)/clone2" &&
+	git -C clone1 switch -c one &&
+	git -C clone2 switch -c two &&
+	test_commit -C clone1 one &&
+	test_commit -C clone2 two &&
+	GIT_TRACE2_EVENT="$(pwd)/run-prefetch.txt" git maintenance run --task=prefetch &&
+	grep ",\"fetch\",\"remote1\"" run-prefetch.txt &&
+	grep ",\"fetch\",\"remote2\"" run-prefetch.txt &&
+	test_path_is_missing .git/refs/remotes &&
+	test_cmp clone1/.git/refs/heads/one .git/refs/prefetch/remote1/one &&
+	test_cmp clone2/.git/refs/heads/two .git/refs/prefetch/remote2/two &&
+	git log prefetch/remote1/one &&
+	git log prefetch/remote2/two
+'
+
 test_done
-- 
gitgitgadget


  parent reply	other threads:[~2020-07-30 22:24 UTC|newest]

Thread overview: 164+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-07 14:21 [PATCH 00/21] Maintenance builtin, allowing 'gc --auto' customization Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 01/21] gc: use the_repository less often Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 02/21] gc: use repository in too_many_loose_objects() Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 03/21] gc: use repo config Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 04/21] gc: drop the_repository in log location Derrick Stolee via GitGitGadget
2020-07-09  2:22   ` Jonathan Tan
2020-07-09 11:13     ` Derrick Stolee
2020-07-07 14:21 ` [PATCH 05/21] maintenance: create basic maintenance runner Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 06/21] maintenance: add --quiet option Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 07/21] maintenance: replace run_auto_gc() Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 08/21] maintenance: initialize task array and hashmap Derrick Stolee via GitGitGadget
2020-07-09  2:25   ` Jonathan Tan
2020-07-09 13:15     ` Derrick Stolee
2020-07-09 13:51       ` Junio C Hamano
2020-07-07 14:21 ` [PATCH 09/21] maintenance: add commit-graph task Derrick Stolee via GitGitGadget
2020-07-09  2:29   ` Jonathan Tan
2020-07-09 11:14     ` Derrick Stolee
2020-07-09 22:52       ` Jeff King
2020-07-09 23:41         ` Derrick Stolee
2020-07-07 14:21 ` [PATCH 10/21] maintenance: add --task option Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 11/21] maintenance: take a lock on the objects directory Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 12/21] maintenance: add fetch task Derrick Stolee via GitGitGadget
2020-07-09  2:35   ` Jonathan Tan
2020-07-07 14:21 ` [PATCH 13/21] maintenance: add loose-objects task Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 14/21] maintenance: add pack-files task Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 15/21] maintenance: auto-size pack-files batch Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 16/21] maintenance: create maintenance.<task>.enabled config Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 17/21] maintenance: use pointers to check --auto Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 18/21] maintenance: add auto condition for commit-graph task Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 19/21] maintenance: create auto condition for loose-objects Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 20/21] maintenance: add pack-files auto condition Derrick Stolee via GitGitGadget
2020-07-07 14:21 ` [PATCH 21/21] midx: use start_delayed_progress() Derrick Stolee via GitGitGadget
2020-07-08 23:57 ` [PATCH 00/21] Maintenance builtin, allowing 'gc --auto' customization Emily Shaffer
2020-07-09 11:21   ` Derrick Stolee
2020-07-09 12:43     ` Derrick Stolee
2020-07-09 23:16       ` Jeff King
2020-07-09 23:45         ` Derrick Stolee
2020-07-10 18:46           ` Emily Shaffer
2020-07-10 19:30             ` Son Luong Ngoc
2020-07-09 14:05     ` Junio C Hamano
2020-07-09 15:54       ` Derrick Stolee
2020-07-09 16:26         ` Junio C Hamano
2020-07-09 16:56           ` Derrick Stolee
2020-07-23 17:56 ` [PATCH v2 00/18] " Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 01/18] maintenance: create basic maintenance runner Derrick Stolee via GitGitGadget
2020-07-25  1:26     ` Taylor Blau
2020-07-25  1:47     ` Đoàn Trần Công Danh
2020-07-29 22:19     ` Jonathan Nieder
2020-07-30 13:12       ` Derrick Stolee
2020-07-31  0:30         ` Jonathan Nieder
2020-08-03 17:37           ` Derrick Stolee
2020-08-03 17:46             ` Jonathan Nieder
2020-08-03 22:46               ` Taylor Blau
2020-08-03 23:01                 ` Jonathan Nieder
2020-08-03 23:08                   ` Taylor Blau
2020-08-03 23:17                     ` Jonathan Nieder
2020-08-04  0:07                       ` Junio C Hamano
2020-08-04 13:32                       ` Derrick Stolee
2020-08-04 14:42                         ` Jonathan Nieder
2020-08-04 16:32                           ` Derrick Stolee
2020-08-04 17:02                             ` Jonathan Nieder
2020-08-04 17:51                               ` Derrick Stolee
2020-08-05 15:02               ` Derrick Stolee
2020-07-31 16:40         ` Jonathan Nieder
2020-08-03 23:52         ` Jonathan Nieder
2020-07-23 17:56   ` [PATCH v2 02/18] maintenance: add --quiet option Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 03/18] maintenance: replace run_auto_gc() Derrick Stolee via GitGitGadget
2020-07-23 20:21     ` Junio C Hamano
2020-07-25  1:33       ` Taylor Blau
2020-07-30 13:29       ` Derrick Stolee
2020-07-30 13:31         ` Derrick Stolee
2020-07-30 19:00           ` Eric Sunshine
2020-07-30 20:21             ` Derrick Stolee
2020-07-23 17:56   ` [PATCH v2 04/18] maintenance: initialize task array Derrick Stolee via GitGitGadget
2020-07-23 19:57     ` Junio C Hamano
2020-07-24 12:23       ` Derrick Stolee
2020-07-24 12:51         ` Derrick Stolee
2020-07-24 19:39           ` Junio C Hamano
2020-07-25  1:46           ` Taylor Blau
2020-07-29 22:19     ` Emily Shaffer
2020-07-23 17:56   ` [PATCH v2 05/18] maintenance: add commit-graph task Derrick Stolee via GitGitGadget
2020-07-23 20:22     ` Junio C Hamano
2020-07-24 13:09       ` Derrick Stolee
2020-07-24 19:47         ` Junio C Hamano
2020-07-25  1:52           ` Taylor Blau
2020-07-30 13:59             ` Derrick Stolee
2020-07-29  0:22     ` Jeff King
2020-07-23 17:56   ` [PATCH v2 06/18] maintenance: add --task option Derrick Stolee via GitGitGadget
2020-07-23 20:21     ` Junio C Hamano
2020-07-23 22:18       ` Junio C Hamano
2020-07-24 13:36       ` Derrick Stolee
2020-07-24 19:50         ` Junio C Hamano
2020-07-23 17:56   ` [PATCH v2 07/18] maintenance: take a lock on the objects directory Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 08/18] maintenance: add prefetch task Derrick Stolee via GitGitGadget
2020-07-23 20:53     ` Junio C Hamano
2020-07-24 14:25       ` Derrick Stolee
2020-07-24 20:47         ` Junio C Hamano
2020-07-25  1:37     ` Đoàn Trần Công Danh
2020-07-25  1:48       ` Junio C Hamano
2020-07-27 14:07         ` Derrick Stolee
2020-07-27 16:13           ` Junio C Hamano
2020-07-27 18:27             ` Derrick Stolee
2020-07-28 16:37               ` [PATCH v2] fetch: optionally allow disabling FETCH_HEAD update Junio C Hamano
2020-07-29  9:12                 ` Phillip Wood
2020-07-29  9:17                   ` Phillip Wood
2020-07-30 15:17                 ` Derrick Stolee
2020-07-23 17:56   ` [PATCH v2 09/18] maintenance: add loose-objects task Derrick Stolee via GitGitGadget
2020-07-23 20:59     ` Junio C Hamano
2020-07-24 14:50       ` Derrick Stolee
2020-07-24 19:57         ` Junio C Hamano
2020-07-29 22:21     ` Emily Shaffer
2020-07-30 15:38       ` Derrick Stolee
2020-07-23 17:56   ` [PATCH v2 10/18] maintenance: add incremental-repack task Derrick Stolee via GitGitGadget
2020-07-23 22:00     ` Junio C Hamano
2020-07-24 15:03       ` Derrick Stolee
2020-07-29 22:22     ` Emily Shaffer
2020-07-23 17:56   ` [PATCH v2 11/18] maintenance: auto-size incremental-repack batch Derrick Stolee via GitGitGadget
2020-07-23 22:15     ` Junio C Hamano
2020-07-23 23:09       ` Eric Sunshine
2020-07-23 23:24         ` Junio C Hamano
2020-07-24 16:09           ` Derrick Stolee
2020-07-24 19:51       ` Derrick Stolee
2020-07-24 20:17         ` Junio C Hamano
2020-07-29 22:23     ` Emily Shaffer
2020-07-30 16:57       ` Derrick Stolee
2020-07-30 19:02         ` Derrick Stolee
2020-07-30 19:24           ` Chris Torek
2020-08-05 12:37           ` Đoàn Trần Công Danh
2020-08-06 13:54             ` Derrick Stolee
2020-07-23 17:56   ` [PATCH v2 12/18] maintenance: create maintenance.<task>.enabled config Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 13/18] maintenance: use pointers to check --auto Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 14/18] maintenance: add auto condition for commit-graph task Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 15/18] maintenance: create auto condition for loose-objects Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 16/18] maintenance: add incremental-repack auto condition Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 17/18] midx: use start_delayed_progress() Derrick Stolee via GitGitGadget
2020-07-23 17:56   ` [PATCH v2 18/18] maintenance: add trace2 regions for task execution Derrick Stolee via GitGitGadget
2020-07-29 22:03   ` [PATCH v2 00/18] Maintenance builtin, allowing 'gc --auto' customization Emily Shaffer
2020-07-30 22:24   ` [PATCH v3 00/20] " Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 01/20] maintenance: create basic maintenance runner Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 02/20] maintenance: add --quiet option Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 03/20] maintenance: replace run_auto_gc() Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 04/20] maintenance: initialize task array Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 05/20] maintenance: add commit-graph task Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 06/20] maintenance: add --task option Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 07/20] maintenance: take a lock on the objects directory Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 08/20] fetch: optionally allow disabling FETCH_HEAD update Junio C Hamano via GitGitGadget
2020-07-30 22:24     ` Derrick Stolee via GitGitGadget [this message]
2020-07-30 22:24     ` [PATCH v3 10/20] maintenance: add loose-objects task Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 11/20] midx: enable core.multiPackIndex by default Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 12/20] maintenance: add incremental-repack task Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 13/20] maintenance: auto-size incremental-repack batch Derrick Stolee via GitGitGadget
2020-07-30 23:36       ` Chris Torek
2020-08-03 17:43         ` Derrick Stolee
2020-07-30 22:24     ` [PATCH v3 14/20] maintenance: create maintenance.<task>.enabled config Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 15/20] maintenance: use pointers to check --auto Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 16/20] maintenance: add auto condition for commit-graph task Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 17/20] maintenance: create auto condition for loose-objects Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 18/20] maintenance: add incremental-repack auto condition Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 19/20] midx: use start_delayed_progress() Derrick Stolee via GitGitGadget
2020-07-30 22:24     ` [PATCH v3 20/20] maintenance: add trace2 regions for task execution Derrick Stolee via GitGitGadget
2020-07-30 23:06     ` [PATCH v3 00/20] Maintenance builtin, allowing 'gc --auto' customization Junio C Hamano
2020-07-30 23:31     ` Junio C Hamano
2020-07-31  2:58       ` Junio C Hamano
2020-08-06 17:58     ` Derrick Stolee

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