On Wed, Apr 7, 2010 at 11:46 PM, Mulyadi Santosa
<mulyadi.santosa@xxxxxxxxx> wrote:
> Hi Onkar....
>
> On Wed, Apr 7, 2010 at 17:56, Onkar Mahajan <kern.devel@xxxxxxxxx> wrote:
>> static struct hlist_head *inode_hashtable __read_mostly;
>>
>>
>> what is the use of __read_mostly ?
>>
>> I did not find any detailed documentation for this
>> on GCC website , Is it GCC optimization feature ?
>
> I failed to search the definition in kernel source or gcc docs.
> However, I am pretty confidence that this is somekind of definition to
> mark the related variable/structure/whatever and put them into the
> same memory segment.
>
> By doing so, the programmers are hoping that these data are grouped
> into the same cache line. And since they are mostly read only, cache
> won't be invalidated too much (maybe zero in some cases) if there is
> update into that cache line.
>
I think u are right too....
>From here is the definition:
arch/x86/include/asm/cache.h:
#define __read_mostly __attribute__((__section__(".data.read_mostly")))
and then the location is defined in the LDS linker script
(arch/x86/kernel/vmlinux.lds):
CONSTRUCTORS
/* rarely changed data like cpu maps */
. = ALIGN((1 << 6)); *(.data.read_mostly)
/* End of data section */
_edata = .;
} :data
which defined how vmlinux is created.
but ultimately it is a special keyword recognized by GCC.
(http://www.jauu.net/data/pdf/effective-c-with-gcc.pdf)
--
Regards,
Peter Teoh
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