Merge branch 'ab/gc-docs'

Update docs around "gc".

* ab/gc-docs:
  gc docs: remove incorrect reference to gc.auto=0
  gc docs: clarify that "gc" doesn't throw away referenced objects
  gc docs: note "gc --aggressive" in "fast-import"
  gc docs: downplay the usefulness of --aggressive
  gc docs: note how --aggressive impacts --window & --depth
  gc docs: fix formatting for "gc.writeCommitGraph"
  gc docs: re-flow the "gc.*" section in "config"
  gc docs: include the "gc.*" section from "config" in "gc"
  gc docs: clean grammar for "gc.bigPackThreshold"
  gc docs: stop noting "repack" flags
  gc docs: modernize the advice for manually running "gc"

Documentation/config/gc.txt

 gc.aggressiveDepth::
 	The depth parameter used in the delta compression
 	algorithm used by 'git gc --aggressive'.  This defaults
-	to 50.
+	to 50, which is the default for the `--depth` option when
+	`--aggressive` isn't in use.
++
+See the documentation for the `--depth` option in
+linkgit:git-repack[1] for more details.
 
 gc.aggressiveWindow::
 	The window size parameter used in the delta compression
 	algorithm used by 'git gc --aggressive'.  This defaults
-	to 250.
+	to 250, which is a much more aggressive window size than
+	the default `--window` of 10.
++
+See the documentation for the `--window` option in
+linkgit:git-repack[1] for more details.
 
 gc.auto::
 	When there are approximately more than this many loose
 	objects in the repository, `git gc --auto` will pack them.
 	Some Porcelain commands use this command to perform a
 	light-weight garbage collection from time to time.  The
-	default value is 6700.  Setting this to 0 disables it.
+	default value is 6700.
++
+Setting this to 0 disables not only automatic packing based on the
+number of loose objects, but any other heuristic `git gc --auto` will
+otherwise use to determine if there's work to do, such as
+`gc.autoPackLimit`.
 
 gc.autoPackLimit::
 	When there are more than this many packs that are not
 	marked with `*.keep` file in the repository, `git gc
 	--auto` consolidates them into one larger pack.  The
 	default value is 50.  Setting this to 0 disables it.
+	Setting `gc.auto` to 0 will also disable this.
++
+See the `gc.bigPackThreshold` configuration variable below. When in
+use, it'll affect how the auto pack limit works.
 
 gc.autoDetach::
 	Make `git gc --auto` return immediately and run in background
@@ -36,11 +53,16 @@ Note that if the number of kept packs is more than gc.autoPackLimit,
 this configuration variable is ignored, all packs except the base pack
 will be repacked. After this the number of packs should go below
 gc.autoPackLimit and gc.bigPackThreshold should be respected again.
++
+If the amount of memory estimated for `git repack` to run smoothly is
+not available and `gc.bigPackThreshold` is not set, the largest pack
+will also be excluded (this is the equivalent of running `git gc` with
+`--keep-base-pack`).
 
 gc.writeCommitGraph::
 	If true, then gc will rewrite the commit-graph file when
-	linkgit:git-gc[1] is run. When using linkgit:git-gc[1]
-	'--auto' the commit-graph will be updated if housekeeping is
+	linkgit:git-gc[1] is run. When using `git gc --auto`
+	the commit-graph will be updated if housekeeping is
 	required. Default is false. See linkgit:git-commit-graph[1]
 	for details.
 
@@ -94,6 +116,12 @@ gc.<pattern>.reflogExpireUnreachable::
 	With "<pattern>" (e.g. "refs/stash")
 	in the middle, the setting applies only to the refs that
 	match the <pattern>.
++
+These types of entries are generally created as a result of using `git
+commit --amend` or `git rebase` and are the commits prior to the amend
+or rebase occurring.  Since these changes are not part of the current
+project most users will want to expire them sooner, which is why the
+default is more aggressive than `gc.reflogExpire`.
 
 gc.rerereResolved::
 	Records of conflicted merge you resolved earlier are

Documentation/git-fast-import.txt

@@ -1398,6 +1398,13 @@ deltas are suboptimal (see above) then also adding the `-f` option
 to force recomputation of all deltas can significantly reduce the
 final packfile size (30-50% smaller can be quite typical).
 
+Instead of running `git repack` you can also run `git gc
+--aggressive`, which will also optimize other things after an import
+(e.g. pack loose refs). As noted in the "AGGRESSIVE" section in
+linkgit:git-gc[1] the `--aggressive` option will find new deltas with
+the `-f` option to linkgit:git-repack[1]. For the reasons elaborated
+on above using `--aggressive` after a fast-import is one of the few
+cases where it's known to be worthwhile.
 
 MEMORY UTILIZATION
 ------------------

Documentation/git-gc.txt

@@ -20,17 +20,16 @@ created from prior invocations of 'git add', packing refs, pruning
 reflog, rerere metadata or stale working trees. May also update ancillary
 indexes such as the commit-graph.
 
-Users are encouraged to run this task on a regular basis within
-each repository to maintain good disk space utilization and good
-operating performance.
+When common porcelain operations that create objects are run, they
+will check whether the repository has grown substantially since the
+last maintenance, and if so run `git gc` automatically. See `gc.auto`
+below for how to disable this behavior.
 
-Some git commands may automatically run 'git gc'; see the `--auto` flag
-below for details. If you know what you're doing and all you want is to
-disable this behavior permanently without further considerations, just do:
-
-----------------------
-$ git config --global gc.auto 0
-----------------------
+Running `git gc` manually should only be needed when adding objects to
+a repository without regularly running such porcelain commands, to do
+a one-off repository optimization, or e.g. to clean up a suboptimal
+mass-import. See the "PACKFILE OPTIMIZATION" section in
+linkgit:git-fast-import[1] for more details on the import case.
 
 OPTIONS
 -------
@@ -40,35 +39,17 @@ OPTIONS
 	space utilization and performance.  This option will cause
 	'git gc' to more aggressively optimize the repository at the expense
 	of taking much more time.  The effects of this optimization are
-	persistent, so this option only needs to be used occasionally; every
-	few hundred changesets or so.
+	mostly persistent. See the "AGGRESSIVE" section below for details.
 
 --auto::
 	With this option, 'git gc' checks whether any housekeeping is
 	required; if not, it exits without performing any work.
-	Some git commands run `git gc --auto` after performing
-	operations that could create many loose objects. Housekeeping
-	is required if there are too many loose objects or too many
-	packs in the repository.
-+
-If the number of loose objects exceeds the value of the `gc.auto`
-configuration variable, then all loose objects are combined into a
-single pack using `git repack -d -l`.  Setting the value of `gc.auto`
-to 0 disables automatic packing of loose objects.
 +
-If the number of packs exceeds the value of `gc.autoPackLimit`,
-then existing packs (except those marked with a `.keep` file
-or over `gc.bigPackThreshold` limit)
-are consolidated into a single pack by using the `-A` option of
-'git repack'.
-If the amount of memory is estimated not enough for `git repack` to
-run smoothly and `gc.bigPackThreshold` is not set, the largest
-pack will also be excluded (this is the equivalent of running `git gc`
-with `--keep-base-pack`).
-Setting `gc.autoPackLimit` to 0 disables automatic consolidation of
-packs.
+See the `gc.auto` option in the "CONFIGURATION" section below for how
+this heuristic works.
 +
-If houskeeping is required due to many loose objects or packs, all
+Once housekeeping is triggered by exceeding the limits of
+configuration options such as `gc.auto` and `gc.autoPackLimit`, all
 other housekeeping tasks (e.g. rerere, working trees, reflog...) will
 be performed as well.
 
@@ -96,69 +77,39 @@ be performed as well.
 	`.keep` files are consolidated into a single pack. When this
 	option is used, `gc.bigPackThreshold` is ignored.
 
+AGGRESSIVE
+----------
+
+When the `--aggressive` option is supplied, linkgit:git-repack[1] will
+be invoked with the `-f` flag, which in turn will pass
+`--no-reuse-delta` to linkgit:git-pack-objects[1]. This will throw
+away any existing deltas and re-compute them, at the expense of
+spending much more time on the repacking.
+
+The effects of this are mostly persistent, e.g. when packs and loose
+objects are coalesced into one another pack the existing deltas in
+that pack might get re-used, but there are also various cases where we
+might pick a sub-optimal delta from a newer pack instead.
+
+Furthermore, supplying `--aggressive` will tweak the `--depth` and
+`--window` options passed to linkgit:git-repack[1]. See the
+`gc.aggressiveDepth` and `gc.aggressiveWindow` settings below. By
+using a larger window size we're more likely to find more optimal
+deltas.
+
+It's probably not worth it to use this option on a given repository
+without running tailored performance benchmarks on it. It takes a lot
+more time, and the resulting space/delta optimization may or may not
+be worth it. Not using this at all is the right trade-off for most
+users and their repositories.
+
 CONFIGURATION
 -------------
 
-The optional configuration variable `gc.reflogExpire` can be
-set to indicate how long historical entries within each branch's
-reflog should remain available in this repository.  The setting is
-expressed as a length of time, for example '90 days' or '3 months'.
-It defaults to '90 days'.
-
-The optional configuration variable `gc.reflogExpireUnreachable`
-can be set to indicate how long historical reflog entries which
-are not part of the current branch should remain available in
-this repository.  These types of entries are generally created as
-a result of using `git commit --amend` or `git rebase` and are the
-commits prior to the amend or rebase occurring.  Since these changes
-are not part of the current project most users will want to expire
-them sooner.  This option defaults to '30 days'.
-
-The above two configuration variables can be given to a pattern.  For
-example, this sets non-default expiry values only to remote-tracking
-branches:
-
-------------
-[gc "refs/remotes/*"]
-	reflogExpire = never
-	reflogExpireUnreachable = 3 days
-------------
-
-The optional configuration variable `gc.rerereResolved` indicates
-how long records of conflicted merge you resolved earlier are
-kept.  This defaults to 60 days.
-
-The optional configuration variable `gc.rerereUnresolved` indicates
-how long records of conflicted merge you have not resolved are
-kept.  This defaults to 15 days.
-
-The optional configuration variable `gc.packRefs` determines if
-'git gc' runs 'git pack-refs'. This can be set to "notbare" to enable
-it within all non-bare repos or it can be set to a boolean value.
-This defaults to true.
-
-The optional configuration variable `gc.writeCommitGraph` determines if
-'git gc' should run 'git commit-graph write'. This can be set to a
-boolean value. This defaults to false.
-
-The optional configuration variable `gc.aggressiveWindow` controls how
-much time is spent optimizing the delta compression of the objects in
-the repository when the --aggressive option is specified.  The larger
-the value, the more time is spent optimizing the delta compression.  See
-the documentation for the --window option in linkgit:git-repack[1] for
-more details.  This defaults to 250.
-
-Similarly, the optional configuration variable `gc.aggressiveDepth`
-controls --depth option in linkgit:git-repack[1]. This defaults to 50.
-
-The optional configuration variable `gc.pruneExpire` controls how old
-the unreferenced loose objects have to be before they are pruned.  The
-default is "2 weeks ago".
-
-Optional configuration variable `gc.worktreePruneExpire` controls how
-old a stale working tree should be before `git worktree prune` deletes
-it. Default is "3 months ago".
+The below documentation is the same as what's found in
+linkgit:git-config[1]:
 
+include::config/gc.txt[]
 
 NOTES
 -----
@@ -168,8 +119,8 @@ anywhere in your repository. In
 particular, it will keep not only objects referenced by your current set
 of branches and tags, but also objects referenced by the index,
 remote-tracking branches, refs saved by 'git filter-branch' in
-refs/original/, or reflogs (which may reference commits in branches
-that were later amended or rewound).
+refs/original/, reflogs (which may reference commits in branches
+that were later amended or rewound), and anything else in the refs/* namespace.
 If you are expecting some objects to be deleted and they aren't, check
 all of those locations and decide whether it makes sense in your case to
 remove those references.
@@ -190,8 +141,7 @@ mitigate this problem:
 
 However, these features fall short of a complete solution, so users who
 run commands concurrently have to live with some risk of corruption (which
-seems to be low in practice) unless they turn off automatic garbage
-collection with 'git config gc.auto 0'.
+seems to be low in practice).
 
 HOOKS
 -----