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
- 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]
projects / linux-beck.git / commit