"Chris M. Thomasson" <no@[EMAIL PROTECTED]
> wrote in message
news:5YLGk.29873$gY7.19956@[EMAIL PROTECTED]
> "Hallvard B Furuseth" <h.b.furuseth@[EMAIL PROTECTED]
> wrote in message
> news:hbf.20081007bpkh@[EMAIL PROTECTED]
>> Hello again... thanks for the answers so far:
>>
>> Dave Butenhof writes:
>>>Hallvard B Furuseth wrote:
>>>> What do POSIX and other thread APIs have to say about memory layout
>>>> of data accessed by different treads? E.g.:
>>>>
>>>> struct Foo {
>>>> pthread_mutex_t ma, mb;
>>>> int a, b; /* protected by ma and mb respectively */
>>>> int c; /* not mutex-protected, only accessed by 1 thread */
>>>> };
>>> (...)
>>> Generally int will be safe. Although on some machines, an UNALIGNED
int
>>> is legal but not necessarily atomic.
>>
>>> Both C and C++ are working now on detailed memory models that will
nail
>>> down standard and ****table answers to this sort of
>>> question. Unfortunately it's all well below the level that could be
>>> resolved in POSIX.
>>>
>>> As David Schwartz suggested, however, you're generally going to be
>>> better off keeping independently accessed data as "distinct objects"
>>> rather than compacting them into a common container. If you keep them
>>> distinctly separate, you have a much better chance that your code, the
>>> compiler, the memory allocator in your runtime, and the hardware will
>>> all get along.
>>
>> Makes sense. What would you do if you want data structures with
>> differently-protected data in the same struct though?
>
>
> Here is possible contrived solution:
>
> struct data {
> int part1;
> char l2pad1[L2_CACHE_LINE_SIZE - sizeof(int)];
> int part2;
> char l2pad2[L2_CACHE_LINE_SIZE - sizeof(int)];
> int part3;
> char l2pad3[L2_CACHE_LINE_SIZE - sizeof(int)];
> };
>
>
> If you align this struct on a cache line boundary `data::part1/2/3' will
> never be false-shared with each other.
[...]
A much better solution would be to do something like:
union data_l2pad {
int data;
char l2pad[L2_CACHE_LINE_SIZE];
};
struct data {
union data_l2pad part1;
union data_l2pad part2;
union data_l2pad part3;
};
:^)
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