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>

diff --git a/sideband.c b/sideband.c
index ea7c25211ef7e1c8a80cb60475ee43457c8c13f1..1ed6614eaf1baf8747ef613933433060cc1a8be4 100644 (file)
--- 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;
        }