Re: [PATCH] strbuf_write: omit system call when length is zero

Duy Nguyen [off-list ref] writes:

On Fri, Feb 26, 2016 at 5:34 AM, Stefan Beller [off-list ref] wrote:

quoted

In case the length of the buffer is zero, we do not need to call the
fwrite system call as a performance improvement.

fwrite is a libc call, not system call. Are you sure it always calls
write() (assuming buffering is off)?

I do not think so, but I suspect that the patch misstates its
rationale (I said I get uncomfortable every time I see a function
that takes size and nelem separately used by a caller that can
potentially pass nleme=0, when I wondered it it is OK that no caller
of this funtion checks its return value).

quoted

Signed-off-by: Stefan Beller <redacted>
---

 This applies on top of v17 for origin/sb/submodule-parallel-update.

 In case there are other reasons for origin/sb/submodule-parallel-update
 to need a reroll I'll squash it in. But as this is a pure performance
 optimization in a case we are not running into with that series and that
 series is clashing with Davids refs backend series, I figure we may not
 want to have a reroll for this fix alone.

 Thanks,
 Stefan


 strbuf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/strbuf.c b/strbuf.c
index 71345cd..5f6da82 100644
--- a/strbuf.c
+++ b/strbuf.c

@@ -397,7 +397,7 @@ ssize_t strbuf_read_once(struct strbuf *sb, int fd, size_t hint)

 ssize_t strbuf_write(struct strbuf *sb, FILE *f)
 {
-       return fwrite(sb->buf, 1, sb->len, f);
+       return sb->len ? fwrite(sb->buf, 1, sb->len, f) : 0;
 }


--

2.7.2.374.ga5f0819.dirty

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On Thu, Feb 25, 2016 at 5:40 PM, Junio C Hamano [off-list ref] wrote:

Duy Nguyen [off-list ref] writes:

quoted

On Fri, Feb 26, 2016 at 5:34 AM, Stefan Beller [off-list ref] wrote:

quoted

In case the length of the buffer is zero, we do not need to call the
fwrite system call as a performance improvement.

fwrite is a libc call, not system call. Are you sure it always calls
write() (assuming buffering is off)?

I do not think so, but I suspect that the patch misstates its
rationale (I said I get uncomfortable every time I see a function
that takes size and nelem separately used by a caller that can
potentially pass nleme=0, when I wondered it it is OK that no caller
of this funtion checks its return value).

This is exactly what happened. I assumed fwrite would be a system call
eventually.