t/perf: add tests for pack bitmaps

This adds a few basic perf tests for the pack bitmap code to
show off its improvements. The tests are:

  1. How long does it take to do a repack (it gets slower
     with bitmaps, since we have to do extra work)?

  2. How long does it take to do a clone (it gets faster
     with bitmaps)?

  3. How does a small fetch perform when we've just
     repacked?

  4. How does a clone perform when we haven't repacked since
     a week of pushes?

Here are results against linux.git:

Test                      origin/master       this tree
-----------------------------------------------------------------------
5310.2: repack to disk    33.64(32.64+2.04)   67.67(66.75+1.84) +101.2%
5310.3: simulated clone   30.49(29.47+2.05)   1.20(1.10+0.10) -96.1%
5310.4: simulated fetch   3.49(6.79+0.06)     5.57(22.35+0.07) +59.6%
5310.6: partial bitmap    36.70(43.87+1.81)   8.18(21.92+0.73) -77.7%

You can see that we do take longer to repack, but we do way
better for further clones. A small fetch performs a bit
worse, as we spend way more time on delta compression (note
the heavy user CPU time, as we have 8 threads) due to the
lack of name hashes for the bitmapped objects.

The final test shows how the bitmaps degrade over time
between packs. There's still a significant speedup over the
non-bitmap case, but we don't do quite as well (we have to
spend time accessing the "new" objects the old fashioned
way, including delta compression).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p5310-pack-bitmaps.sh

@@ -0,0 +1,56 @@
+#!/bin/sh
+
+test_description='Tests pack performance using bitmaps'
+. ./perf-lib.sh
+
+test_perf_large_repo
+
+# note that we do everything through config,
+# since we want to be able to compare bitmap-aware
+# git versus non-bitmap git
+test_expect_success 'setup bitmap config' '
+	git config pack.writebitmaps true
+'
+
+test_perf 'repack to disk' '
+	git repack -ad
+'
+
+test_perf 'simulated clone' '
+	git pack-objects --stdout --all </dev/null >/dev/null
+'
+
+test_perf 'simulated fetch' '
+	have=$(git rev-list HEAD~100 -1) &&
+	{
+		echo HEAD &&
+		echo ^$have
+	} | git pack-objects --revs --stdout >/dev/null
+'
+
+test_expect_success 'create partial bitmap state' '
+	# pick a commit to represent the repo tip in the past
+	cutoff=$(git rev-list HEAD~100 -1) &&
+	orig_tip=$(git rev-parse HEAD) &&
+
+	# now kill off all of the refs and pretend we had
+	# just the one tip
+	rm -rf .git/logs .git/refs/* .git/packed-refs
+	git update-ref HEAD $cutoff
+
+	# and then repack, which will leave us with a nice
+	# big bitmap pack of the "old" history, and all of
+	# the new history will be loose, as if it had been pushed
+	# up incrementally and exploded via unpack-objects
+	git repack -Ad
+
+	# and now restore our original tip, as if the pushes
+	# had happened
+	git update-ref HEAD $orig_tip
+'
+
+test_perf 'partial bitmap' '
+	git pack-objects --stdout --all </dev/null >/dev/null
+'
+
+test_done