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[v3,2/8] test-tool genzeros: generate large amounts of data more efficiently

Message ID 052197200141c321118b7766f5615a61f951e59f.1635515959.git.gitgitgadget@gmail.com (mailing list archive)
State New, archived
Headers show
Series Allow clean/smudge filters to handle huge files in the LLP64 data model | expand

Commit Message

Johannes Schindelin Oct. 29, 2021, 1:59 p.m. UTC 
From: Johannes Schindelin <johannes.schindelin@gmx.de>

In this developer's tests, producing one gigabyte worth of NULs in a
busy loop that writes out individual bytes, unbuffered, took ~27sec.
Writing chunked 256kB buffers instead only took ~0.6sec

This matters because we are about to introduce a pair of test cases that
want to be able to produce 5GB of NULs, and we cannot use `/dev/zero`
because of the HP NonStop platform's lack of support for that device.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 t/helper/test-genzeros.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

Comments

Junio C Hamano Oct. 29, 2021, 10:50 p.m. UTC | #1 
"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> @@ -12,9 +15,19 @@ int cmd__genzeros(int argc, const char **argv)
>  
>  	count = argc > 1 ? strtoimax(argv[1], NULL, 0) : -1;
>  
> -	while (count < 0 || count--) {
> -		if (putchar(0) == EOF)
> +	/* Writing out individual NUL bytes is slow... */
> +	while (count < 0)
> +		if (write(1, zeros, ARRAY_SIZE(zeros)) < 0)
>  			return -1;
> +
> +	while (count > 0) {
> +		n = write(1, zeros, count < ARRAY_SIZE(zeros) ?
> +			  count : ARRAY_SIZE(zeros));
> +
> +		if (n < 0)
> +			return -1;
> +
> +		count -= n;
>  	}
>  
>  	return 0;

This round looks OK to me.
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Patch

diff --git a/t/helper/test-genzeros.c b/t/helper/test-genzeros.c
index b1197e91a89..8ca988d6216 100644
--- a/t/helper/test-genzeros.c
+++ b/t/helper/test-genzeros.c
@@ -3,7 +3,10 @@ 
 
 int cmd__genzeros(int argc, const char **argv)
 {
+	/* static, so that it is NUL-initialized */
+	static const char zeros[256 * 1024];
 	intmax_t count;
+	ssize_t n;
 
 	if (argc > 2) {
 		fprintf(stderr, "usage: %s [<count>]\n", argv[0]);
@@ -12,9 +15,19 @@  int cmd__genzeros(int argc, const char **argv)
 
 	count = argc > 1 ? strtoimax(argv[1], NULL, 0) : -1;
 
-	while (count < 0 || count--) {
-		if (putchar(0) == EOF)
+	/* Writing out individual NUL bytes is slow... */
+	while (count < 0)
+		if (write(1, zeros, ARRAY_SIZE(zeros)) < 0)
 			return -1;
+
+	while (count > 0) {
+		n = write(1, zeros, count < ARRAY_SIZE(zeros) ?
+			  count : ARRAY_SIZE(zeros));
+
+		if (n < 0)
+			return -1;
+
+		count -= n;
 	}
 
 	return 0;