[PATCH v2 3/5] parallel-checkout: add configuration options

From: Matheus Tavares <matheus.bernardino@usp.br>
Date: Apr 8, 2021, 16:17 UTC
Message-ID: <6ea057f9c555882d8aa67bc9a2952f7d5a79548d.1617893234.git.matheus.bernardino@usp.br>
In-Reply-To: <cover.1617893234.git.matheus.bernardino@usp.br>

Make parallel checkout configurable by introducing two new settings: checkout.workers and checkout.thresholdForParallelism. The first defines the number of workers (where one means sequential checkout), and the second defines the minimum number of entries to attempt parallel checkout.

To decide the default value for checkout.workers, the parallel version was benchmarked during three operations in the linux repo, with cold cache: cloning v5.8, checking out v5.8 from v2.6.15 (checkout I) and checking out v5.8 from v5.7 (checkout II). The four tables below show the mean run times and standard deviations for 5 runs in: a local file system on SSD, a local file system on HDD, a Linux NFS server, and Amazon EFS (all on Linux). Each parallel checkout test was executed with the number of workers that brings the best overall results in that environment.

Local SSD:
             Sequential             10 workers            Speedup
Clone        8.805 s ± 0.043 s      3.564 s ± 0.041 s     2.47 ± 0.03
Checkout I   9.678 s ± 0.057 s      4.486 s ± 0.050 s     2.16 ± 0.03
Checkout II  5.034 s ± 0.072 s      3.021 s ± 0.038 s     1.67 ± 0.03

Local HDD:
             Sequential             10 workers             Speedup
Clone        32.288 s ± 0.580 s     30.724 s ± 0.522 s    1.05 ± 0.03
Checkout I   54.172 s ±  7.119 s    54.429 s ± 6.738 s    1.00 ± 0.18
Checkout II  40.465 s ± 2.402 s     38.682 s ± 1.365 s    1.05 ± 0.07

Linux NFS server (v4.1, on EBS, single availability zone):

             Sequential             32 workers            Speedup
Clone        240.368 s ± 6.347 s    57.349 s ± 0.870 s    4.19 ± 0.13
Checkout I   242.862 s ± 2.215 s    58.700 s ± 0.904 s    4.14 ± 0.07
Checkout II  65.751 s ± 1.577 s     23.820 s ± 0.407 s    2.76 ± 0.08

EFS (v4.1, replicated over multiple availability zones):

             Sequential             32 workers            Speedup
Clone        922.321 s ± 2.274 s    210.453 s ± 3.412 s   4.38 ± 0.07
Checkout I   1011.300 s ± 7.346 s   297.828 s ± 0.964 s   3.40 ± 0.03
Checkout II  294.104 s ± 1.836 s    126.017 s ± 1.190 s   2.33 ± 0.03

The above benchmarks show that parallel checkout is most effective on repositories located on an SSD or over a distributed file system. For local file systems on spinning disks, and/or older machines, the parallelism does not always bring a good performance. For this reason, the default value for checkout.workers is one, a.k.a. sequential checkout.

To decide the default value for checkout.thresholdForParallelism, another benchmark was executed in the "Local SSD" setup, where parallel checkout showed to be beneficial. This time, we compared the runtime of a `git checkout -f`, with and without parallelism, after randomly removing an increasing number of files from the Linux working tree. The "sequential fallback" column below corresponds to the executions where checkout.workers was 10 but checkout.thresholdForParallelism was equal to the number of to-be-updated files plus one (so that we end up writing sequentially). Each test case was sampled 15 times, and each sample had a randomly different set of files removed. Here are the results:

             sequential fallback   10 workers           speedup
10   files    772.3 ms ± 12.6 ms   769.0 ms ± 13.6 ms   1.00 ± 0.02
20   files    780.5 ms ± 15.8 ms   775.2 ms ±  9.2 ms   1.01 ± 0.02
50   files    806.2 ms ± 13.8 ms   767.4 ms ±  8.5 ms   1.05 ± 0.02
100  files    833.7 ms ± 21.4 ms   750.5 ms ± 16.8 ms   1.11 ± 0.04
200  files    897.6 ms ± 30.9 ms   730.5 ms ± 14.7 ms   1.23 ± 0.05
500  files   1035.4 ms ± 48.0 ms   677.1 ms ± 22.3 ms   1.53 ± 0.09
1000 files   1244.6 ms ± 35.6 ms   654.0 ms ± 38.3 ms   1.90 ± 0.12
2000 files   1488.8 ms ± 53.4 ms   658.8 ms ± 23.8 ms   2.26 ± 0.12

From the above numbers, 100 files seems to be a reasonable default value for the threshold setting.

Note: Up to 1000 files, we observe a drop in the execution time of the
parallel code with an increase in the number of files. This is a rather
odd behavior, but it was observed in multiple repetitions. Above 1000
files, the execution time increases according to the number of files, as
one would expect.

About the test environments: Local SSD tests were executed on an i7-7700HQ (4 cores with hyper-threading) running Manjaro Linux. Local HDD tests were executed on an Intel(R) Xeon(R) E3-1230 (also 4 cores with hyper-threading), HDD Seagate Barracuda 7200.14 SATA 3.1, running Debian. NFS and EFS tests were executed on an Amazon EC2 c5n.xlarge instance, with 4 vCPUs. The Linux NFS server was running on a m6g.large instance with 2 vCPUSs and a 1 TB EBS GP2 volume. Before each timing, the linux repository was removed (or checked out back to its previous state), and `sync && sysctl vm.drop_caches=3` was executed.

Co-authored-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
---
 Documentation/config/checkout.txt | 21 +++++++++++++++++++++
 parallel-checkout.c               | 24 +++++++++++++++++++-----
 parallel-checkout.h               |  9 +++++++--
 unpack-trees.c                    | 10 +++++++---
 4 files changed, 54 insertions(+), 10 deletions(-)

diff --git a/Documentation/config/checkout.txt b/Documentation/config/checkout.txt
index 2cddf7b4b4..bfbca90f0e 100644
--- a/Documentation/config/checkout.txt
+++ b/Documentation/config/checkout.txt
@@ -21,3 +21,24 @@ checkout.guess::
 	Provides the default value for the `--guess` or `--no-guess`
 	option in `git checkout` and `git switch`. See
 	linkgit:git-switch[1] and linkgit:git-checkout[1].
+
+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.
++
+Note: parallel checkout usually delivers better performance for repositories
+located on SSDs or over NFS. For repositories on spinning disks and/or machines
+with a small number of cores, the default sequential checkout often performs
+better. The size and compression level of a repository might also influence how
+well the parallel version performs.
+
+checkout.thresholdForParallelism::
+	When running parallel checkout with a small number of files, the cost
+	of subprocess spawning and inter-process communication might outweigh
+	the parallelization gains. This setting allows to define the minimum
+	number of files for which parallel checkout should be attempted. The
+	default is 100.
diff --git a/parallel-checkout.c b/parallel-checkout.c
index 41c301bbda..d6a0f31664 100644
--- a/parallel-checkout.c
+++ b/parallel-checkout.c
@@ -1,10 +1,12 @@
 #include "cache.h"
+#include "config.h"
 #include "entry.h"
 #include "parallel-checkout.h"
 #include "pkt-line.h"
 #include "run-command.h"
 #include "sigchain.h"
 #include "streaming.h"
+#include "thread-utils.h"
 
 struct pc_worker {
 	struct child_process cp;
@@ -24,6 +26,20 @@ 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;
+
+void get_parallel_checkout_configs(int *num_workers, int *threshold)
+{
+	if (git_config_get_int("checkout.workers", num_workers))
+		*num_workers = DEFAULT_NUM_WORKERS;
+	else if (*num_workers < 1)
+		*num_workers = online_cpus();
+
+	if (git_config_get_int("checkout.thresholdForParallelism", threshold))
+		*threshold = DEFAULT_THRESHOLD_FOR_PARALLELISM;
+}
+
 void init_parallel_checkout(void)
 {
 	if (parallel_checkout.status != PC_UNINITIALIZED)
@@ -587,11 +603,9 @@ static void write_items_sequentially(struct checkout *state)
 		write_pc_item(&parallel_checkout.items[i], state);
 }
 
-static const int DEFAULT_NUM_WORKERS = 2;
-
-int run_parallel_checkout(struct checkout *state)
+int run_parallel_checkout(struct checkout *state, int num_workers, int threshold)
 {
-	int ret, num_workers = DEFAULT_NUM_WORKERS;
+	int ret;
 
 	if (parallel_checkout.status != PC_ACCEPTING_ENTRIES)
 		BUG("cannot run parallel checkout: uninitialized or already running");
@@ -601,7 +615,7 @@ int run_parallel_checkout(struct checkout *state)
 	if (parallel_checkout.nr < num_workers)
 		num_workers = parallel_checkout.nr;
 
-	if (num_workers <= 1) {
+	if (num_workers <= 1 || parallel_checkout.nr < threshold) {
 		write_items_sequentially(state);
 	} else {
 		struct pc_worker *workers = setup_workers(state, num_workers);
diff --git a/parallel-checkout.h b/parallel-checkout.h
index ec58716519..2a68ab954d 100644
--- a/parallel-checkout.h
+++ b/parallel-checkout.h
@@ -17,6 +17,7 @@ enum pc_status {
 };
 
 enum pc_status parallel_checkout_status(void);
+void get_parallel_checkout_configs(int *num_workers, int *threshold);
 
 /*
  * Put parallel checkout into the PC_ACCEPTING_ENTRIES state. Should be used
@@ -31,8 +32,12 @@ void init_parallel_checkout(void);
  */
 int enqueue_checkout(struct cache_entry *ce, struct conv_attrs *ca);
 
-/* Write all the queued entries, returning 0 on success.*/
-int run_parallel_checkout(struct checkout *state);
+/*
+ * Write all the queued entries, returning 0 on success. If the number of
+ * entries is smaller than the specified threshold, the operation is performed
+ * sequentially.
+ */
+int run_parallel_checkout(struct checkout *state, int num_workers, int threshold);
 
 /****************************************************************
  * Interface with checkout--worker
diff --git a/unpack-trees.c b/unpack-trees.c
index f0430d458d..0669748f21 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -399,7 +399,7 @@ static int check_updates(struct unpack_trees_options *o,
 	int errs = 0;
 	struct progress *progress;
 	struct checkout state = CHECKOUT_INIT;
-	int i;
+	int i, pc_workers, pc_threshold;
 
 	trace_performance_enter();
 	state.force = 1;
@@ -465,8 +465,11 @@ static int check_updates(struct unpack_trees_options *o,
 		oid_array_clear(&to_fetch);
 	}
 
+	get_parallel_checkout_configs(&pc_workers, &pc_threshold);
+
 	enable_delayed_checkout(&state);
-	init_parallel_checkout();
+	if (pc_workers > 1)
+		init_parallel_checkout();
 	for (i = 0; i < index->cache_nr; i++) {
 		struct cache_entry *ce = index->cache[i];
 
@@ -480,7 +483,8 @@ static int check_updates(struct unpack_trees_options *o,
 		}
 	}
 	stop_progress(&progress);
-	errs |= run_parallel_checkout(&state);
+	if (pc_workers > 1)
+		errs |= run_parallel_checkout(&state, pc_workers, pc_threshold);
 	errs |= finish_delayed_checkout(&state, NULL);
 	git_attr_set_direction(GIT_ATTR_CHECKIN);

-- 
2.30.1