buf_ring(9) API precisions

Arnaud Lacombe lacombar at gmail.com
Thu Sep 15 03:21:47 UTC 2011

Hi Kip,

I've got a few question about the buf_ring(9) API.

1) what means the 'drbr_' prefix. I can guess the two last letter, 'b'
and 'r', for Buffer Ring, but what about 'd' and 'r' ?

2) in `sys/sys/buf_ring.h', you defined 'struct buf_ring' as:

struct buf_ring {
        volatile uint32_t       br_prod_head;
        volatile uint32_t       br_prod_tail;
        int                     br_prod_size;
        int                     br_prod_mask;
        uint64_t                br_drops;
        uint64_t                br_prod_bufs;
        uint64_t                br_prod_bytes;
         * Pad out to next L2 cache line
        uint64_t                _pad0[11];

        volatile uint32_t       br_cons_head;
        volatile uint32_t       br_cons_tail;
        int                     br_cons_size;
        int                     br_cons_mask;

         * Pad out to next L2 cache line
        uint64_t                _pad1[14];
        struct mtx              *br_lock;
        void                    *br_ring[0];

Why are you making an MD guess, the amount of padding to fit the size
of a cache line, in MI API ? Strangely enough, you did not make this
assumption in, say r205488 (picked randomly).

Thanks in advance,
 - Arnaud

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