netdev - Re: [RFC] change dst_entry padding from using an array to using __attribute_

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Message-ID: <1270659329.8141.40.camel@edumazet-laptop>
Date:	Wed, 07 Apr 2010 18:55:29 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Laurent Chavey <chavey@...gle.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] change dst_entry padding from using an array to using 
 __attribute__

Le mercredi 07 avril 2010 à 09:46 -0700, Laurent Chavey a écrit :
> what are the benefit(s)  of using an array to force a struct
> element to be aligned on 64 bytes / 32 bytes boundaries
> versus using gcc __attribute__.
> 
> 
> ------------------------------------
> 
> struct dst_entry {
> 
>        /*
>         * Align __refcnt to a 64 bytes alignment
>         * (L1_CACHE_SIZE would be too much)
>         */
> - #ifdef CONFIG_64BIT
> -       long                    __pad_to_align_refcnt[1];
> - #else
> -       long                    __pad_to_align_refcnt[0];
> - #endif
> +       atomic_t                __refcnt __attribute__
> ((aligned(64))); /* client references    */
> #undef __padding__
>        /*
>         * __refcnt wants to be on a different cache line from
>         * input/output/ops or performance tanks badly
>         */
>        atomic_t                __refcnt;       /* client references    */
> };

We dont want a huge hole (say.. 120 bytes) being not noticed.

Some machines around have 4 millions dst entries.

This deserve us being not lazy :)

Thanks


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