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From: Derrick Stolee <stolee@gmail.com>
To: Junio C Hamano <gitster@pobox.com>,
	Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, 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: Re: [PATCH v2 10/18] maintenance: add incremental-repack task
Date: Fri, 24 Jul 2020 11:03:26 -0400	[thread overview]
Message-ID: <54bfc8a9-e3bf-4199-9c58-8ed6ad50fb36@gmail.com> (raw)
In-Reply-To: <xmqqeep1q4d0.fsf@gitster.c.googlers.com>

On 7/23/2020 6:00 PM, Junio C Hamano wrote:
> "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> 1. 'git multi-pack-index write' creates a multi-pack-index file if
>>    one did not exist, and otherwise will update the multi-pack-index
>>    with any new pack-files that appeared since the last write. This
>>    is particularly relevant with the background fetch job.
>>
>>    When the multi-pack-index sees two copies of the same object, it
>>    stores the offset data into the newer pack-file. This means that
>>    some old pack-files could become "unreferenced" which I will use
>>    to mean "a pack-file that is in the pack-file list of the
>>    multi-pack-index but none of the objects in the multi-pack-index
>>    reference a location inside that pack-file."
> 
> An obvious alternative is to favor the copy in the older pack,
> right?  Is the expectation that over time, most of the objects that
> are relevant would reappear in newer packs, so that eventually by
> favoring the copies in the newer packs, we can retire and remove the
> old pack, keeping only the newer ones?
> 
> But would that assumption hold?  The old packs hold objects that are
> necessary for the older parts of the history, so unless you are
> cauterizing away the old history, these objects in the older packs
> are likely to stay with us longer than those used by the newer parts
> of the history, some of which may not even have been pushed out yet
> and can be rebased away?

If we created a new pack-file containing an already-packed object,
then shouldn't we assume that the new pack-file does a _better_ job
of compressing that object? Or at least, doesn't make it worse?

For example, if we use 'git multi-pack-index repack --batch-size=0',
then this creates a new pack-file containing every previously-packed
object. This new pack-file should have better delta compression than
the previous setup across multiple pack-files. We want this new
pack-file to be used, not the old ones.

This "pick the newest pack" strategy is also what allows us to safely
use the 'expire' option to drop old pack-files. If we always keep the
old copies, then when we try to switch the new pack-files, we cannot
delete the old packs safely because a concurrent Git process could
be trying to reference it. One race is as follows:

 * Process A opens the multi-pack-index referring to old pack P. It
   doesn't open the pack-file as it hasn't tried to parse objects yet.

 * Process B is 'git multi-pack-index expire'. It sees that pack P can
   be dropped because all objects appear in newer pack-files. It deletes
   P.

 * Process A tries to read from P, and this fails. A must reprepare its
   representation of the pack-files.

This is the less disruptive race since A can recover with a small cost
to its performance. The worse race (on Windows) is this:

 * Process A loads the multi-pack-index and tries to parse an object by
   loading "old" pack P.

 * Process B tries to delete P. However, on Windows the handle to P by
   A prevents the deletion.

At this point, there could be two resolutions. The first is to have
the 'expire' fail because we can't delete A. This means we might never
delete A in a busy repository. The second is that the 'expire' command
continues and drops A from the list in the multi-pack-index. However,
now all Git processes add A to the packed_git list because it isn't
referenced by the multi-pack-index.

In summary: the decision to pick the newer copies is a fundamental
part of how the write->expire->repack loop was designed.

>> 2. 'git multi-pack-index expire' deletes any unreferenced pack-files
>>    and updaes the multi-pack-index to drop those pack-files from the
>>    list. This is safe to do as concurrent Git processes will see the
>>    multi-pack-index and not open those packs when looking for object
>>    contents. (Similar to the 'loose-objects' job, there are some Git
>>    commands that open pack-files regardless of the multi-pack-index,
>>    but they are rarely used. Further, a user that self-selects to
>>    use background operations would likely refrain from using those
>>    commands.)
> 
> OK.
> 
>> 3. 'git multi-pack-index repack --bacth-size=<size>' collects a set
>>    of pack-files that are listed in the multi-pack-index and creates
>>    a new pack-file containing the objects whose offsets are listed
>>    by the multi-pack-index to be in those objects. The set of pack-
>>    files is selected greedily by sorting the pack-files by modified
>>    time and adding a pack-file to the set if its "expected size" is
>>    smaller than the batch size until the total expected size of the
>>    selected pack-files is at least the batch size. The "expected
>>    size" is calculated by taking the size of the pack-file divided
>>    by the number of objects in the pack-file and multiplied by the
>>    number of objects from the multi-pack-index with offset in that
>>    pack-file. The expected size approximats how much data from that
> 
> approximates.
> 
>>    pack-file will contribute to the resulting pack-file size. The
>>    intention is that the resulting pack-file will be close in size
>>    to the provided batch size.
> 
>> +static int maintenance_task_incremental_repack(void)
>> +{
>> +	if (multi_pack_index_write()) {
>> +		error(_("failed to write multi-pack-index"));
>> +		return 1;
>> +	}
>> +
>> +	if (multi_pack_index_verify()) {
>> +		warning(_("multi-pack-index verify failed after initial write"));
>> +		return rewrite_multi_pack_index();
>> +	}
>> +
>> +	if (multi_pack_index_expire()) {
>> +		error(_("multi-pack-index expire failed"));
>> +		return 1;
>> +	}
>> +
>> +	if (multi_pack_index_verify()) {
>> +		warning(_("multi-pack-index verify failed after expire"));
>> +		return rewrite_multi_pack_index();
>> +	}
>> +	if (multi_pack_index_repack()) {
>> +		error(_("multi-pack-index repack failed"));
>> +		return 1;
>> +	}
> 
> Hmph, I wonder if these warning should come from each helper
> functions that are static to this function anyway.
> 
> It also makes it easier to reason about this function by eliminating
> the need for having a different pattern only for the verify helper.
> Instead, verify could call rewrite internally when it notices a
> breakage.  I.e.
> 
> 	if (multi_pack_index_write())
> 		return 1;
> 	if (multi_pack_index_verify("after initial write"))
> 		return 1;
> 	if (multi_pack_index_exire())
> 		return 1;
> 	...

This is a cleaner model. I'll work on that.

> Also, it feels odd, compared to our internal API convention, that
> positive non-zero is used as an error here.
...
> Exactly the same comment as 08/18 about natural/inherent ordering
> applies here as well.

I'll leave these to be resolved in the earlier messages.

Thanks,
-Stolee


  reply	other threads:[~2020-07-24 15:03 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 [this message]
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     ` [PATCH v3 09/20] maintenance: add prefetch task Derrick Stolee via GitGitGadget
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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