Re: glibc 2.21 - Machine maintainers, please test your machines.

[Torvald Riegel <triegel at redhat dot com>]

On Mon, 2015-01-26 at 16:17 -0800, H.J. Lu wrote:
> What is the drawback with 32-bit atomics for x32? If there is no real
> advantage for 64-bit atomics, x32 will use 32-bit atomics.  If x32 needs
> to use 64-bit atomics, I need to investigate which way to go.
> 

64b atomics do not give any advantage in sem_post nor on the fast path
of sem_wait (ie, if sem_wait does not use a futex to block, but just can
grab a token, or spin-waits (not implemented yet)).

64b atomics allows for a more efficient sem_wait slow path; that is,
when we use a futex to block.  In a nutshell, we don't need to do what's
in sem_waitcommon.c:__sem_wait_32_finish() and another atomic
read-modify-write operation before the futex_wait.

The additional overhead around the futex_wait call is not a really big
problem I would say, as futexes are relatively slow anyway and we
shouldn't use them for short periods of waiting (that's where the
missing spin-waiting would come into play).

The disadvantage that the slow path can have however is potentially
decreased scalability, because the additional atomic ops might slow down
other threads.  But I wouldn't consider this a really critical issue,
especially not once we add some spin-waiting.  There could also be
pathological situations in which we have a high rate of sem_wait /
sem_post calls, and the sem_wait's can't agree on who's last, so we
might get lots of unnecessary futex_wake calls in
__sem_wait_32_finish().