Re: [RFC PATCH v2 3/5] futex: Throughput-optimized (TO) futexes

From: Thomas Gleixner
Date: Thu Sep 22 2016 - 16:28:43 EST

On Thu, 22 Sep 2016, Waiman Long wrote:
> On 09/22/2016 09:34 AM, Thomas Gleixner wrote:
> > On Thu, 22 Sep 2016, Peter Zijlstra wrote:
> > >
> > > I'd leave out the TO part entirely (or only mention it in changelogs).
> > >
> > > That is, I'd call the futex ops: FUTEX_LOCK and FUTEX_UNLOCK.
> > That brings me to a different question:
> >
> > How is user space going to support this, i.e. is this some extra magic for
> > code which implements its own locking primitives or is there going to be a
> > wide use via e.g. glibc.
> There are some applications that use futex(2) directly to implement their
> synchronization primitives. For those applications, they will need to modify
> their code to detect the presence of the new futexes. They can then use the
> new futexes if available and use wait-wake futexes if not.

That's what I suspected. Did you talk to the other folks who complain about
futex performance (database, JVM, etc.) and play their own games with user
space spinlocks and whatever?

> I am also planning to take a look at the pthread_mutex* APIs to see if they
> can be modified to use the new futexes later on when the patch becomes more
> mature.

Please involve glibc people who are interested in the futex stuff early and
discuss the concept before it's set in stone for your particular usecase.

> > Also what's the reason that we can't do probabilistic spinning for
> > FUTEX_WAIT and have to add yet another specialized variant of futexes?
> >
> The main reason is that a FUTEX_WAIT waiter has no idea who the owner of the
> futex is. We usually do spinning when the lock owner is running and abort when
> it goes to sleep. We can't do that for FUTEX_WAIT.

Fair enough. This wants to be spelled out in the changelog and explained a
bit more detailed.