* [RFC] change dst_entry padding from using an array to using __attribute__
@ 2010-04-07 16:46 Laurent Chavey
2010-04-07 16:55 ` Eric Dumazet
0 siblings, 1 reply; 3+ messages in thread
From: Laurent Chavey @ 2010-04-07 16:46 UTC (permalink / raw)
To: netdev; +Cc: laurent chavey
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 */
};
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [RFC] change dst_entry padding from using an array to using __attribute__
2010-04-07 16:46 [RFC] change dst_entry padding from using an array to using __attribute__ Laurent Chavey
@ 2010-04-07 16:55 ` Eric Dumazet
2010-04-07 16:56 ` Eric Dumazet
0 siblings, 1 reply; 3+ messages in thread
From: Eric Dumazet @ 2010-04-07 16:55 UTC (permalink / raw)
To: Laurent Chavey; +Cc: netdev
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
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [RFC] change dst_entry padding from using an array to using __attribute__
2010-04-07 16:55 ` Eric Dumazet
@ 2010-04-07 16:56 ` Eric Dumazet
0 siblings, 0 replies; 3+ messages in thread
From: Eric Dumazet @ 2010-04-07 16:56 UTC (permalink / raw)
To: Laurent Chavey; +Cc: netdev
Le mercredi 07 avril 2010 à 18:55 +0200, Eric Dumazet a écrit :
> We dont want a huge hole (say.. 120 bytes) being not noticed.
sorry, 60 bytes of course.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2010-04-07 16:46 [RFC] change dst_entry padding from using an array to using __attribute__ Laurent Chavey
2010-04-07 16:55 ` Eric Dumazet
2010-04-07 16:56 ` Eric Dumazet
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