tcp: Fit struct tcp_conn into a single 64-byte cacheline

...by:

- storing the chained-hash next connection pointer as numeric
  reference rather than as pointer

- storing the MSS as 14-bit value, and rounding it

- using only the effective amount of bits needed to store the hash
  bucket number

- explicitly limiting window scaling factors to 4-bit values
  (maximum factor is 14, from RFC 7323)

- scaling SO_SNDBUF values, and using a 8-bit representation for
  the duplicate ACK sequence

- keeping window values unscaled, as received and sent

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

tcp.h

@@ -6,11 +6,12 @@
 #ifndef TCP_H
 #define TCP_H
 
-#define REFILL_INTERVAL			1000 /* ms */
+#define REFILL_INTERVAL			1000	/* ms */
 #define PORT_DETECT_INTERVAL		1000
 #define TCP_TIMER_INTERVAL	MIN(REFILL_INTERVAL, PORT_DETECT_INTERVAL)
 
-#define TCP_MAX_CONNS			(128 * 1024)
+#define TCP_CONN_INDEX_BITS		17	/* 128k */
+#define TCP_MAX_CONNS			(1 << TCP_CONN_INDEX_BITS)
 #define TCP_MAX_SOCKS			(TCP_MAX_CONNS + USHRT_MAX * 2)
 
 #define TCP_SOCK_POOL_SIZE		32