sideband: use writev(3p) to send pktlines
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> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Patrick Steinhardt committed
Mar 13, 2026 at 07:45 UTC
26986f4cbaf38d84a82b0b35da211389ce49552c
1 file changed
+11
-3
sideband.c
+11
-3
@@ -264,6 +264,7 @@ void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_ma
264
const char *p = data;
265
266
while (sz) {
267
+ struct iovec iov[2];
268
unsigned n;
269
char hdr[5];
270
@@ -273,12 +274,19 @@ void send_sideband(int fd, int band, const char *data, ssize_t sz, int packet_ma
274
if (0 <= band) {
275
xsnprintf(hdr, sizeof(hdr), "%04x", n + 5);
276
hdr[4] = band;
276
- write_or_die(fd, hdr, 5);
277
+ iov[0].iov_base = hdr;
278
+ iov[0].iov_len = 5;
279
} else {
280
xsnprintf(hdr, sizeof(hdr), "%04x", n + 4);
279
- write_or_die(fd, hdr, 4);
281
+ iov[0].iov_base = hdr;
282
+ iov[0].iov_len = 4;
283
}
281
- write_or_die(fd, p, n);
284
+
285
+ iov[1].iov_base = (void *) p;
286
+ iov[1].iov_len = n;
287
+
288
+ writev_or_die(fd, iov, ARRAY_SIZE(iov));
289
+
290
p += n;
291
sz -= n;
292
}