[PATCH v4 07/10] sideband: use writev(3p) to send pktlines

From: Patrick Steinhardt <ps@pks.im>
Date: Mar 13, 2026, 06:45 UTC
Message-ID: <20260313-pks-upload-pack-write-contention-v4-7-7a9668061f7f@pks.im>
In-Reply-To: <20260313-pks-upload-pack-write-contention-v4-0-7a9668061f7f@pks.im>

Every pktline that we send out via `send_sideband()` currently requires two syscalls: one to write the pktline's length, and one to send its data. This typically isn't all that much of a problem, but under extreme load the syscalls may cause contention in the kernel.

Refactor the code to instead use the newly introduced writev(3p) infra so that we can send out the data with a single syscall. This reduces the number of syscalls from around 133,000 calls to write(3p) to around 67,000 calls to writev(3p).

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 sideband.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/sideband.c b/sideband.c
index ea7c25211e..1ed6614eaf 100644
--- a/sideband.c
+++ b/sideband.c
@@ -264,6 +264,7 @@ void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_ma
 	const char *p = data;
 
 	while (sz) {
+		struct iovec iov[2];
 		unsigned n;
 		char hdr[5];
 
@@ -273,12 +274,19 @@ void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_ma
 		if (0 <= band) {
 			xsnprintf(hdr, sizeof(hdr), "%04x", n + 5);
 			hdr[4] = band;
-			write_or_die(fd, hdr, 5);
+			iov[0].iov_base = hdr;
+			iov[0].iov_len = 5;
 		} else {
 			xsnprintf(hdr, sizeof(hdr), "%04x", n + 4);
-			write_or_die(fd, hdr, 4);
+			iov[0].iov_base = hdr;
+			iov[0].iov_len = 4;
 		}
-		write_or_die(fd, p, n);
+
+		iov[1].iov_base = (void *) p;
+		iov[1].iov_len = n;
+
+		writev_or_die(fd, iov, ARRAY_SIZE(iov));
+
 		p += n;
 		sz -= n;
 	}

-- 
2.53.0.904.g2727be2e99.dirty