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, 20 insertions, 16 deletions

diff --git a/tcp.c b/tcp.c
index e4fac22..a3a9dfd 100644
--- a/tcp.c
+++ b/tcp.c
@@ -343,7 +343,9 @@
 #define MAX_TAP_CONNS			(128 * 1024)
 #define MAX_SPLICE_CONNS		(128 * 1024)
 
-#define TCP_TAP_FRAMES			256
+#define TCP_TAP_FRAMES_MEM		256
+#define TCP_TAP_FRAMES							\
+	(c->mode == MODE_PASST ? TCP_TAP_FRAMES_MEM : 1)
 
 #define MAX_PIPE_SIZE			(2UL * 1024 * 1024)
 
@@ -609,7 +611,7 @@ static struct tcp4_l2_buf_t {
 #else
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))))
 #endif
-tcp4_l2_buf[TCP_TAP_FRAMES];
+tcp4_l2_buf[TCP_TAP_FRAMES_MEM];
 
 static unsigned int tcp4_l2_buf_used;
 static size_t tcp4_l2_buf_bytes;
@@ -640,21 +642,21 @@ struct tcp6_l2_buf_t {
 #else
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))))
 #endif
-tcp6_l2_buf[TCP_TAP_FRAMES];
+tcp6_l2_buf[TCP_TAP_FRAMES_MEM];
 
 static unsigned int tcp6_l2_buf_used;
 static size_t tcp6_l2_buf_bytes;
 
 /* recvmsg()/sendmsg() data for tap */
 static char 		tcp_buf_discard		[MAX_WINDOW];
-static struct iovec	iov_sock		[TCP_TAP_FRAMES + 1];
+static struct iovec	iov_sock		[TCP_TAP_FRAMES_MEM + 1];
 
-static struct iovec	tcp4_l2_iov_tap		[TCP_TAP_FRAMES];
-static struct iovec	tcp6_l2_iov_tap		[TCP_TAP_FRAMES];
-static struct iovec	tcp4_l2_flags_iov_tap	[TCP_TAP_FRAMES];
-static struct iovec	tcp6_l2_flags_iov_tap	[TCP_TAP_FRAMES];
+static struct iovec	tcp4_l2_iov_tap		[TCP_TAP_FRAMES_MEM];
+static struct iovec	tcp6_l2_iov_tap		[TCP_TAP_FRAMES_MEM];
+static struct iovec	tcp4_l2_flags_iov_tap	[TCP_TAP_FRAMES_MEM];
+static struct iovec	tcp6_l2_flags_iov_tap	[TCP_TAP_FRAMES_MEM];
 
-static struct mmsghdr	tcp_l2_mh_tap		[TCP_TAP_FRAMES];
+static struct mmsghdr	tcp_l2_mh_tap		[TCP_TAP_FRAMES_MEM];
 
 /* sendmsg() to socket */
 static struct iovec	tcp_tap_iov		[UIO_MAXIOV];
@@ -688,7 +690,7 @@ static struct tcp4_l2_flags_buf_t {
 #else
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))))
 #endif
-tcp4_l2_flags_buf[TCP_TAP_FRAMES];
+tcp4_l2_flags_buf[TCP_TAP_FRAMES_MEM];
 
 static int tcp4_l2_flags_buf_used;
 
@@ -717,7 +719,7 @@ static struct tcp6_l2_flags_buf_t {
 #else
 } __attribute__ ((packed, aligned(__alignof__(unsigned int))))
 #endif
-tcp6_l2_flags_buf[TCP_TAP_FRAMES];
+tcp6_l2_flags_buf[TCP_TAP_FRAMES_MEM];
 
 static int tcp6_l2_flags_buf_used;
 
@@ -916,7 +918,7 @@ void tcp_update_l2_buf(unsigned char *eth_d, unsigned char *eth_s,
 {
 	int i;
 
-	for (i = 0; i < TCP_TAP_FRAMES; i++) {
+	for (i = 0; i < TCP_TAP_FRAMES_MEM; i++) {
 		struct tcp4_l2_flags_buf_t *b4f = &tcp4_l2_flags_buf[i];
 		struct tcp6_l2_flags_buf_t *b6f = &tcp6_l2_flags_buf[i];
 		struct tcp4_l2_buf_t *b4 = &tcp4_l2_buf[i];
@@ -982,12 +984,13 @@ static void tcp_sock4_iov_init(void)
 		};
 	}
 
-	for (i = 0, iov = tcp4_l2_iov_tap; i < TCP_TAP_FRAMES; i++, iov++) {
+	for (i = 0, iov = tcp4_l2_iov_tap; i < TCP_TAP_FRAMES_MEM; i++, iov++) {
 		iov->iov_base = &tcp4_l2_buf[i].vnet_len;
 		iov->iov_len = MSS_DEFAULT;
 	}
 
-	for (i = 0, iov = tcp4_l2_flags_iov_tap; i < TCP_TAP_FRAMES; i++, iov++)
+	for (i = 0, iov = tcp4_l2_flags_iov_tap; i < TCP_TAP_FRAMES_MEM;
+	     i++, iov++)
 		iov->iov_base = &tcp4_l2_flags_buf[i].vnet_len;
 }
 
@@ -1015,12 +1018,13 @@ static void tcp_sock6_iov_init(void)
 		};
 	}
 
-	for (i = 0, iov = tcp6_l2_iov_tap; i < TCP_TAP_FRAMES; i++, iov++) {
+	for (i = 0, iov = tcp6_l2_iov_tap; i < TCP_TAP_FRAMES_MEM; i++, iov++) {
 		iov->iov_base = &tcp6_l2_buf[i].vnet_len;
 		iov->iov_len = MSS_DEFAULT;
 	}
 
-	for (i = 0, iov = tcp6_l2_flags_iov_tap; i < TCP_TAP_FRAMES; i++, iov++)
+	for (i = 0, iov = tcp6_l2_flags_iov_tap; i < TCP_TAP_FRAMES_MEM;
+	     i++, iov++)
 		iov->iov_base = &tcp6_l2_flags_buf[i].vnet_len;
 }