tcp, udp: Receive batching doesn't pay off when writing single frames to tap

In pasta mode, when we get data from sockets and write it as single
frames to the tap device, we batch receive operations considerably,
and then (conceptually) split the data in many smaller writes.

It looked like an obvious choice, but performance is actually better
if we receive data in many small frame-sized recvmsg()/recvmmsg().

The syscall overhead with the previous behaviour, observed by perf,
comes predominantly from write operations, but receiving data in
shorter chunks probably improves cache locality by a considerable
amount.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 17 insertions, 16 deletions

diff --git a/udp.c b/udp.c
index 8129a89..d4f3714 100644
--- a/udp.c
+++ b/udp.c
@@ -118,7 +118,8 @@
 
 #define UDP_CONN_TIMEOUT	180 /* s, timeout for ephemeral or local bind */
 #define UDP_SPLICE_FRAMES	128
-#define UDP_TAP_FRAMES		128
+#define UDP_TAP_FRAMES_MEM	128
+#define UDP_TAP_FRAMES		(c->mode == MODE_PASST ? UDP_TAP_FRAMES_MEM : 1)
 
 /**
  * struct udp_tap_port - Port tracking based on tap-facing source port
@@ -204,7 +205,7 @@ static struct udp4_l2_buf_t {
 	uint8_t data[USHRT_MAX -
 		     (sizeof(struct iphdr) + sizeof(struct udphdr))];
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))))
-udp4_l2_buf[UDP_TAP_FRAMES];
+udp4_l2_buf[UDP_TAP_FRAMES_MEM];
 
 /**
  * udp6_l2_buf_t - Pre-cooked IPv6 packet buffers for tap connections
@@ -234,23 +235,23 @@ struct udp6_l2_buf_t {
 #else
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))))
 #endif
-udp6_l2_buf[UDP_TAP_FRAMES];
+udp6_l2_buf[UDP_TAP_FRAMES_MEM];
 
 static struct sockaddr_storage udp_splice_namebuf;
 static uint8_t udp_splice_buf[UDP_SPLICE_FRAMES][USHRT_MAX];
 
 /* recvmmsg()/sendmmsg() data for tap */
-static struct iovec	udp4_l2_iov_sock	[UDP_TAP_FRAMES];
-static struct iovec	udp6_l2_iov_sock	[UDP_TAP_FRAMES];
+static struct iovec	udp4_l2_iov_sock	[UDP_TAP_FRAMES_MEM];
+static struct iovec	udp6_l2_iov_sock	[UDP_TAP_FRAMES_MEM];
 
-static struct iovec	udp4_l2_iov_tap		[UDP_TAP_FRAMES];
-static struct iovec	udp6_l2_iov_tap		[UDP_TAP_FRAMES];
+static struct iovec	udp4_l2_iov_tap		[UDP_TAP_FRAMES_MEM];
+static struct iovec	udp6_l2_iov_tap		[UDP_TAP_FRAMES_MEM];
 
-static struct mmsghdr	udp4_l2_mh_sock		[UDP_TAP_FRAMES];
-static struct mmsghdr	udp6_l2_mh_sock		[UDP_TAP_FRAMES];
+static struct mmsghdr	udp4_l2_mh_sock		[UDP_TAP_FRAMES_MEM];
+static struct mmsghdr	udp6_l2_mh_sock		[UDP_TAP_FRAMES_MEM];
 
-static struct mmsghdr	udp4_l2_mh_tap		[UDP_TAP_FRAMES];
-static struct mmsghdr	udp6_l2_mh_tap		[UDP_TAP_FRAMES];
+static struct mmsghdr	udp4_l2_mh_tap		[UDP_TAP_FRAMES_MEM];
+static struct mmsghdr	udp6_l2_mh_tap		[UDP_TAP_FRAMES_MEM];
 
 /* recvmmsg()/sendmmsg() data for "spliced" connections */
 static struct iovec	udp_splice_iov_recv	[UDP_SPLICE_FRAMES];
@@ -310,7 +311,7 @@ void udp_update_l2_buf(unsigned char *eth_d, unsigned char *eth_s,
 {
 	int i;
 
-	for (i = 0; i < UDP_TAP_FRAMES; i++) {
+	for (i = 0; i < UDP_TAP_FRAMES_MEM; i++) {
 		struct udp4_l2_buf_t *b4 = &udp4_l2_buf[i];
 		struct udp6_l2_buf_t *b6 = &udp6_l2_buf[i];
 
@@ -354,7 +355,7 @@ static void udp_sock4_iov_init(void)
 		};
 	}
 
-	for (i = 0, h = udp4_l2_mh_sock; i < UDP_TAP_FRAMES; i++, h++) {
+	for (i = 0, h = udp4_l2_mh_sock; i < UDP_TAP_FRAMES_MEM; i++, h++) {
 		struct msghdr *mh = &h->msg_hdr;
 
 		mh->msg_name			= &udp4_l2_buf[i].s_in;
@@ -366,7 +367,7 @@ static void udp_sock4_iov_init(void)
 		mh->msg_iovlen			= 1;
 	}
 
-	for (i = 0, h = udp4_l2_mh_tap; i < UDP_TAP_FRAMES; i++, h++) {
+	for (i = 0, h = udp4_l2_mh_tap; i < UDP_TAP_FRAMES_MEM; i++, h++) {
 		struct msghdr *mh = &h->msg_hdr;
 
 		udp4_l2_iov_tap[i].iov_base	= &udp4_l2_buf[i].vnet_len;
@@ -394,7 +395,7 @@ static void udp_sock6_iov_init(void)
 		};
 	}
 
-	for (i = 0, h = udp6_l2_mh_sock; i < UDP_TAP_FRAMES; i++, h++) {
+	for (i = 0, h = udp6_l2_mh_sock; i < UDP_TAP_FRAMES_MEM; i++, h++) {
 		struct msghdr *mh = &h->msg_hdr;
 
 		mh->msg_name			= &udp6_l2_buf[i].s_in6;
@@ -406,7 +407,7 @@ static void udp_sock6_iov_init(void)
 		mh->msg_iovlen			= 1;
 	}
 
-	for (i = 0, h = udp6_l2_mh_tap; i < UDP_TAP_FRAMES; i++, h++) {
+	for (i = 0, h = udp6_l2_mh_tap; i < UDP_TAP_FRAMES_MEM; i++, h++) {
 		struct msghdr *mh = &h->msg_hdr;
 
 		udp6_l2_iov_tap[i].iov_base	= &udp6_l2_buf[i].vnet_len;