From: Johannes Schindelin via GitGitGadget Date: Thu, 28 Oct 2021 20:50:32 GMT Subject: [PATCH v2 2/7] test-tool genzeros: generate large amounts of data more efficiently Message-ID: <6edcbae372ef63bd75ca6cc2d181f7506f35880f.1635454237.git.gitgitgadget@gmail.com> In-Reply-To: From: Johannes Schindelin 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 --- t/helper/test-genzeros.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/t/helper/test-genzeros.c b/t/helper/test-genzeros.c index b1197e91a89..c061c429da9 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 char zeros[256 * 1024]; intmax_t count; + ssize_t n; if (argc > 2) { fprintf(stderr, "usage: %s []\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; -- gitgitgadget