Hi Peter,
Às 02:44 de 04/03/21, Peter Oskolkov escreveu:
> On Wed, Mar 3, 2021 at 5:22 PM André Almeida <[email protected]> wrote:
>>
>> Hi,
>>
>> This patch series introduces the futex2 syscalls.
>>
>> * FAQ
>>
>> ** "And what's about FUTEX_64?"
>>
>> By supporting 64 bit futexes, the kernel structure for futex would
>> need to have a 64 bit field for the value, and that could defeat one of
>> the purposes of having different sized futexes in the first place:
>> supporting smaller ones to decrease memory usage. This might be
>> something that could be disabled for 32bit archs (and even for
>> CONFIG_BASE_SMALL).
>>
>> Which use case would benefit for FUTEX_64? Does it worth the trade-offs?
>
> The ability to store a pointer value on 64bit platforms is an
> important use case.
> Imagine a simple producer/consumer scenario, with the producer updating
> some shared memory data and waking the consumer. Storing the pointer
> in the futex makes it so that only one shared memory location needs to be
> accessed "atomically", etc. With two atomics synchronization becomes
> more involved (= slower).
>
So the idea is to, instead of doing this:
T1:
atomic_set(&shm_addr, buffer_addr);
atomic_set(&futex, 0);
futex_wake(&futex, 1);
T2:
consume(shm_addr);
To do that:
T1:
atomic_set(&futex, buffer_addr);
futex_wake(&futex, 1);
T2:
consume(futex);
Right?
I'll try to write a small test to see how the perf numbers looks like.