buf_ring(9) API precisions

[Arnaud Lacombe]

Hi,

On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe <lacombar at gmail.com> wrote:
> 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;
shouldn't those 3 fields be updated atomically, especially on 32bits
platforms ? That might pose a problem as, AFAIK, FreeBSD do not have
MI 64bits atomics operations...

 - Arnaud

>        /*
>         * 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];
> #ifdef DEBUG_BUFRING
>        struct mtx              *br_lock;
> #endif
>        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
>