Merge branch 'jk/fewer-pack-rescan'

Internaly we use 0{40} as a placeholder object name to signal the
codepath that there is no such object (e.g. the fast-forward check
while "git fetch" stores a new remote-tracking ref says "we know
there is no 'old' thing pointed at by the ref, as we are creating
it anew" by passing 0{40} for the 'old' side), and expect that a
codepath to locate an in-core object to return NULL as a sign that
the object does not exist.  A look-up for an object that does not
exist however is quite costly with a repository with large number
of packfiles.  This access pattern has been optimized.

* jk/fewer-pack-rescan:
  sha1_file: fast-path null sha1 as a missing object
  everything_local: use "quick" object existence check
  p5551: add a script to test fetch pack-dir rescans
  t/perf/lib-pack: use fast-import checkpoint to create packs
  p5550: factor out nonsense-pack creation

3 files changed, 82 insertions, 23 deletions

diff --git a/t/perf/lib-pack.sh b/t/perf/lib-pack.sh
new file mode 100644
index 0000000..d3865db
--- /dev/null
+++ b/t/perf/lib-pack.sh
@@ -0,0 +1,25 @@
+# Helpers for dealing with large numbers of packs.
+
+# create $1 nonsense packs, each with a single blob
+create_packs () {
+	perl -le '
+		my ($n) = @ARGV;
+		for (1..$n) {
+			print "blob";
+			print "data <<EOF";
+			print "$_";
+			print "EOF";
+			print "checkpoint"
+		}
+	' "$@" |
+	git fast-import
+}
+
+# create a large number of packs, disabling any gc which might
+# cause us to repack them
+setup_many_packs () {
+	git config gc.auto 0 &&
+	git config gc.autopacklimit 0 &&
+	git config fastimport.unpacklimit 0 &&
+	create_packs 500
+}
diff --git a/t/perf/p5550-fetch-tags.sh b/t/perf/p5550-fetch-tags.sh
index a5dc39f..d0e0e01 100755
--- a/t/perf/p5550-fetch-tags.sh
+++ b/t/perf/p5550-fetch-tags.sh
@@ -20,6 +20,7 @@ start to show a noticeable performance problem on my machine, but without
 taking too long to set up and run the tests.
 '
 . ./perf-lib.sh
+. "$TEST_DIRECTORY/perf/lib-pack.sh"
 
 # make a long nonsense history on branch $1, consisting of $2 commits, each
 # with a unique file pointing to the blob at $2.
@@ -44,26 +45,6 @@ create_tags () {
 	git update-ref --stdin
 }
 
-# create $1 nonsense packs, each with a single blob
-create_packs () {
-	perl -le '
-		my ($n) = @ARGV;
-		for (1..$n) {
-			print "blob";
-			print "data <<EOF";
-			print "$_";
-			print "EOF";
-		}
-	' "$@" |
-	git fast-import &&
-
-	git cat-file --batch-all-objects --batch-check='%(objectname)' |
-	while read sha1
-	do
-		echo $sha1 | git pack-objects .git/objects/pack/pack
-	done
-}
-
 test_expect_success 'create parent and child' '
 	git init parent &&
 	git -C parent commit --allow-empty -m base &&
@@ -84,9 +65,7 @@ test_expect_success 'populate parent tags' '
 test_expect_success 'create child packs' '
 	(
 		cd child &&
-		git config gc.auto 0 &&
-		git config gc.autopacklimit 0 &&
-		create_packs 500
+		setup_many_packs
 	)
 '
 
diff --git a/t/perf/p5551-fetch-rescan.sh b/t/perf/p5551-fetch-rescan.sh
new file mode 100755
index 0000000..b99dc23
--- /dev/null
+++ b/t/perf/p5551-fetch-rescan.sh
@@ -0,0 +1,55 @@
+#!/bin/sh
+
+test_description='fetch performance with many packs
+
+It is common for fetch to consider objects that we might not have, and it is an
+easy mistake for the code to use a function like `parse_object` that might
+give the correct _answer_ on such an object, but do so slowly (due to
+re-scanning the pack directory for lookup failures).
+
+The resulting performance drop can be hard to notice in a real repository, but
+becomes quite large in a repository with a large number of packs. So this
+test creates a more pathological case, since any mistakes would produce a more
+noticeable slowdown.
+'
+. ./perf-lib.sh
+. "$TEST_DIRECTORY"/perf/lib-pack.sh
+
+test_expect_success 'create parent and child' '
+	git init parent &&
+	git clone parent child
+'
+
+
+test_expect_success 'create refs in the parent' '
+	(
+		cd parent &&
+		git commit --allow-empty -m foo &&
+		head=$(git rev-parse HEAD) &&
+		test_seq 1000 |
+		sed "s,.*,update refs/heads/& $head," |
+		$MODERN_GIT update-ref --stdin
+	)
+'
+
+test_expect_success 'create many packs in the child' '
+	(
+		cd child &&
+		setup_many_packs
+	)
+'
+
+test_perf 'fetch' '
+	# start at the same state for each iteration
+	obj=$($MODERN_GIT -C parent rev-parse HEAD) &&
+	(
+		cd child &&
+		$MODERN_GIT for-each-ref --format="delete %(refname)" refs/remotes |
+		$MODERN_GIT update-ref --stdin &&
+		rm -vf .git/objects/$(echo $obj | sed "s|^..|&/|") &&
+
+		git fetch
+	)
+'
+
+test_done