Merge branch 'jk/index-pack-w-more-threads'

Long ago, we decided to use 3 threads by default when running the
index-pack task in parallel, which has been adjusted a bit upwards.

* jk/index-pack-w-more-threads:
  index-pack: adjust default threading cap
  p5302: count up to online-cpus for thread tests
  p5302: disable thread-count parameter tests by default

builtin/index-pack.c

@@ -1798,9 +1798,22 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 
 	if (HAVE_THREADS && !nr_threads) {
 		nr_threads = online_cpus();
-		/* An experiment showed that more threads does not mean faster */
-		if (nr_threads > 3)
-			nr_threads = 3;
+		/*
+		 * Experiments show that going above 20 threads doesn't help,
+		 * no matter how many cores you have. Below that, we tend to
+		 * max at half the number of online_cpus(), presumably because
+		 * half of those are hyperthreads rather than full cores. We'll
+		 * never reduce the level below "3", though, to match a
+		 * historical value that nobody complained about.
+		 */
+		if (nr_threads < 4)
+			; /* too few cores to consider capping */
+		else if (nr_threads < 6)
+			nr_threads = 3; /* historic cap */
+		else if (nr_threads < 40)
+			nr_threads /= 2;
+		else
+			nr_threads = 20; /* hard cap */
 	}
 
 	curr_pack = open_pack_file(pack_name);

t/perf/README

@@ -84,6 +84,15 @@ You can set the following variables (also in your config.mak):
 	probably be about linux.git size for optimal results.
 	Both default to the git.git you are running from.
 
+    GIT_PERF_EXTRA
+	Boolean to enable additional tests. Most test scripts are
+	written to detect regressions between two versions of Git, and
+	the output will compare timings for individual tests between
+	those versions. Some scripts have additional tests which are not
+	run by default, that show patterns within a single version of
+	Git (e.g., performance of index-pack as the number of threads
+	changes). These can be enabled with GIT_PERF_EXTRA.
+
 You can also pass the options taken by ordinary git tests; the most
 useful one is:
 

t/perf/p5302-pack-index.sh

@@ -13,35 +13,36 @@ test_expect_success 'repack' '
 	export PACK
 '
 
-test_perf 'index-pack 0 threads' '
+# Rather than counting up and doubling each time, count down from the endpoint,
+# halving each time. That ensures that our final test uses as many threads as
+# CPUs, even if it isn't a power of 2.
+test_expect_success 'set up thread-counting tests' '
+	t=$(test-tool online-cpus) &&
+	threads= &&
+	while test $t -gt 0
+	do
+		threads="$t $threads"
+		t=$((t / 2))
+	done
+'
+
+test_perf PERF_EXTRA 'index-pack 0 threads' '
 	rm -rf repo.git &&
 	git init --bare repo.git &&
 	GIT_DIR=repo.git git index-pack --threads=1 --stdin < $PACK
 '
 
-test_perf 'index-pack 1 thread ' '
-	rm -rf repo.git &&
-	git init --bare repo.git &&
-	GIT_DIR=repo.git GIT_FORCE_THREADS=1 git index-pack --threads=1 --stdin < $PACK
-'
-
-test_perf 'index-pack 2 threads' '
-	rm -rf repo.git &&
-	git init --bare repo.git &&
-	GIT_DIR=repo.git git index-pack --threads=2 --stdin < $PACK
-'
-
-test_perf 'index-pack 4 threads' '
-	rm -rf repo.git &&
-	git init --bare repo.git &&
-	GIT_DIR=repo.git git index-pack --threads=4 --stdin < $PACK
-'
-
-test_perf 'index-pack 8 threads' '
-	rm -rf repo.git &&
-	git init --bare repo.git &&
-	GIT_DIR=repo.git git index-pack --threads=8 --stdin < $PACK
-'
+for t in $threads
+do
+	THREADS=$t
+	export THREADS
+	test_perf PERF_EXTRA "index-pack $t threads" '
+		rm -rf repo.git &&
+		git init --bare repo.git &&
+		GIT_DIR=repo.git GIT_FORCE_THREADS=1 \
+		git index-pack --threads=$THREADS --stdin <$PACK
+	'
+done
 
 test_perf 'index-pack default number of threads' '
 	rm -rf repo.git &&

t/perf/perf-lib.sh

@@ -245,3 +245,5 @@ test_at_end_hook_ () {
 test_export () {
 	export "$@"
 }
+
+test_lazy_prereq PERF_EXTRA 'test_bool_env GIT_PERF_EXTRA false'