parallel-checkout: default to four workers on Windows

Documentation/config/checkout.adoc

@@ -24,11 +24,11 @@ commands or functionality in the future.
 
 `checkout.workers`::
 	The number of parallel workers to use when updating the working tree.
-	The default is one, i.e. sequential execution. If set to a value less
-	than one, Git will use as many workers as the number of logical cores
-	available. This setting and `checkout.thresholdForParallelism` affect
-	all commands that perform checkout. E.g. checkout, clone, reset,
-	sparse-checkout, etc.
+	The default is one, i.e. sequential execution, except on Windows,
+	where it is four. If set to a value less than one, Git will use as
+	many workers as the number of logical cores available. This setting
+	and `checkout.thresholdForParallelism` affect all commands that
+	perform checkout. E.g. checkout, clone, reset, sparse-checkout, etc.
 +
 On Windows the number of workers is capped at 62, because the `poll()`
 emulation cannot wait on more worker pipes than that. A higher configured
@@ -46,4 +46,4 @@ well the parallel version performs.
 	of subprocess spawning and inter-process communication might outweigh
 	the parallelization gains. This setting allows you to define the minimum
 	number of files for which parallel checkout should be attempted. The
-	default is 100.
+	default is 100 (500 on Windows).

compat/mingw.h

@@ -87,6 +87,15 @@ int is_valid_win32_path(const char *path, int allow_literal_nul);
  */
 #define MAX_LONG_PATH 4096
 
+/*
+ * Replacing a file in the worktree (unlink, create, write, close) costs
+ * a few hundred microseconds on Windows, and parallel checkout overlaps
+ * that cost well. Starting a worker costs tens of milliseconds, though,
+ * so only use workers for checkouts that touch at least 500 files.
+ */
+#define PARALLEL_CHECKOUT_DEFAULT_WORKERS 4
+#define PARALLEL_CHECKOUT_DEFAULT_THRESHOLD 500
+
 /**
  * Handles paths that would exceed the MAX_PATH limit of Windows Unicode APIs.
  *

parallel-checkout.c

@@ -38,8 +38,16 @@ enum pc_status parallel_checkout_status(void)
 	return parallel_checkout.status;
 }
 
-static const int DEFAULT_THRESHOLD_FOR_PARALLELISM = 100;
-static const int DEFAULT_NUM_WORKERS = 1;
+#ifndef PARALLEL_CHECKOUT_DEFAULT_THRESHOLD
+#define PARALLEL_CHECKOUT_DEFAULT_THRESHOLD 100
+#endif
+
+#ifndef PARALLEL_CHECKOUT_DEFAULT_WORKERS
+#define PARALLEL_CHECKOUT_DEFAULT_WORKERS 1
+#endif
+
+static const int DEFAULT_THRESHOLD_FOR_PARALLELISM = PARALLEL_CHECKOUT_DEFAULT_THRESHOLD;
+static const int DEFAULT_NUM_WORKERS = PARALLEL_CHECKOUT_DEFAULT_WORKERS;
 
 void get_parallel_checkout_configs(int *num_workers, int *threshold)
 {

t/t2080-parallel-checkout-basics.sh

@@ -356,4 +356,49 @@ test_expect_success MINGW 'checkout caps workers at the poll limit' '
 	verify_checkout many-workers
 '
 
+# On Windows, parallel checkout is on by default (four workers), but only
+# for checkouts of at least 500 files; elsewhere it is off by default.
+test_expect_success 'default workers and threshold' '
+	test_when_finished "rm -rf default-workers" &&
+	git init default-workers &&
+	(
+		cd default-workers &&
+		mkdir dir &&
+		for i in $(test_seq 1 500)
+		do
+			echo "content $i" >dir/file$i || return 1
+		done &&
+		git add -A &&
+		git commit -q -m base &&
+		git tag base &&
+
+		git checkout -q -b few &&
+		for i in $(test_seq 1 499)
+		do
+			echo "changed $i" >dir/file$i || return 1
+		done &&
+		git commit -q -a -m few &&
+
+		git checkout -q -b many base &&
+		for i in $(test_seq 1 500)
+		do
+			echo "changed $i" >dir/file$i || return 1
+		done &&
+		git commit -q -a -m many &&
+		git checkout -q base
+	) &&
+
+	if test_have_prereq MINGW
+	then
+		expected_workers=4
+	else
+		expected_workers=0
+	fi &&
+	test_checkout_workers 0 git -C default-workers checkout few &&
+	verify_checkout default-workers &&
+	git -C default-workers checkout -q base &&
+	test_checkout_workers $expected_workers git -C default-workers checkout many &&
+	verify_checkout default-workers
+'
+
 test_done