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b178bb3dfc
Right now, fields in struct sock are not optimally ordered, because each path (RX softirq, TX completion, RX user, TX user) has to touch fields that are contained in many different cache lines. The really critical thing is to shrink number of cache lines that are used at RX softirq time : CPU handling softirqs for a device can receive many frames per second for many sockets. If load is too big, we can drop frames at NIC level. RPS or multiqueue cards can help, but better reduce latency if possible. This patch starts with UDP protocol, then additional patches will try to reduce latencies of other ones as well. At RX softirq time, fields of interest for UDP protocol are : (not counting ones in inet struct for the lookup) Read/Written: sk_refcnt (atomic increment/decrement) sk_rmem_alloc & sk_backlog.len (to check if there is room in queues) sk_receive_queue sk_backlog (if socket locked by user program) sk_rxhash sk_forward_alloc sk_drops Read only: sk_rcvbuf (sk_rcvqueues_full()) sk_filter sk_wq sk_policy[0] sk_flags Additional notes : - sk_backlog has one hole on 64bit arches. We can fill it to save 8 bytes. - sk_backlog is used only if RX sofirq handler finds the socket while locked by user. - sk_rxhash is written only once per flow. - sk_drops is written only if queues are full Final layout : [1] One section grouping all read/write fields, but placing rxhash and sk_backlog at the end of this section. [2] One section grouping all read fields in RX handler (sk_filter, sk_rcv_buf, sk_wq) [3] Section used by other paths I'll post a patch on its own to put sk_refcnt at the end of struct sock_common so that it shares same cache line than section [1] New offsets on 64bit arch : sizeof(struct sock)=0x268 offsetof(struct sock, sk_refcnt) =0x10 offsetof(struct sock, sk_lock) =0x48 offsetof(struct sock, sk_receive_queue)=0x68 offsetof(struct sock, sk_backlog)=0x80 offsetof(struct sock, sk_rmem_alloc)=0x80 offsetof(struct sock, sk_forward_alloc)=0x98 offsetof(struct sock, sk_rxhash)=0x9c offsetof(struct sock, sk_rcvbuf)=0xa4 offsetof(struct sock, sk_drops) =0xa0 offsetof(struct sock, sk_filter)=0xa8 offsetof(struct sock, sk_wq)=0xb0 offsetof(struct sock, sk_policy)=0xd0 offsetof(struct sock, sk_flags) =0xe0 Instead of : sizeof(struct sock)=0x270 offsetof(struct sock, sk_refcnt) =0x10 offsetof(struct sock, sk_lock) =0x50 offsetof(struct sock, sk_receive_queue)=0xc0 offsetof(struct sock, sk_backlog)=0x70 offsetof(struct sock, sk_rmem_alloc)=0xac offsetof(struct sock, sk_forward_alloc)=0x10c offsetof(struct sock, sk_rxhash)=0x128 offsetof(struct sock, sk_rcvbuf)=0x4c offsetof(struct sock, sk_drops) =0x16c offsetof(struct sock, sk_filter)=0x198 offsetof(struct sock, sk_wq)=0x88 offsetof(struct sock, sk_policy)=0x98 offsetof(struct sock, sk_flags) =0x130 Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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