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test-tool genzeros: generate large amounts of data more efficiently

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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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Johannes Schindelin 2021-11-02 15:46:05 +00:00 committed by Junio C Hamano
parent cbc985a1f4
commit df7000cd91
1 changed files with 15 additions and 2 deletions
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17
t/helper/test-genzeros.c
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@ -3,7 +3,10 @@
int cmd__genzeros(int argc, const char **argv) int cmd__genzeros(int argc, const char **argv)
{ {
/* static, so that it is NUL-initialized */
static const char zeros[256 * 1024];
intmax_t count; intmax_t count;
ssize_t n;
if (argc > 2) { if (argc > 2) {
fprintf(stderr, "usage: %s [<count>]\n", argv[0]); 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; count = argc > 1 ? strtoimax(argv[1], NULL, 0) : -1;
while (count < 0 || count--) { /* Writing out individual NUL bytes is slow... */
if (putchar(0) == EOF) while (count < 0)
if (write(1, zeros, ARRAY_SIZE(zeros)) < 0)
return -1; 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; return 0;