Re: [patch V2 3/7] futex: Add op for hash preallocation

From: Thomas Gleixner
Date: Sat May 07 2016 - 04:49:25 EST

On Fri, 6 May 2016, Darren Hart wrote:
> On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote:
> > From: Sebastian Siewior <bigeasy@xxxxxxxxxxxxx>
> >
> > The per process hash is allocated on the fly at the first futex operation of a
> > process. The size of the hash is determined by a system wide default setting
> > controlled by the sys admin, This is suboptimal for RT applications and
> > applications with pathological futex abuse,
> >
> > - For RT applications its important to allocate the per process hash before the
> > first futex operation to avoid the allocation on the first futex operation.
> >
> > - For pathological applications which use gazillions of futexes its useful to
> > allocate a hash greater than the default hash size.
> "it's" or preferably "it is"
> >
> > Add a futex op which allows to preallocate the hash with the requested
> "allows for preallocating"
> > size. The size is limited by the systemwide maximum hash size, which can be
> system-wide
> > set by the admin. The requested size is rounded up to the next order of 2.
> >
> > The function can be called several times, but ony the first call results in a
> > hash allocation of the requested size as there is no non-intrusive way to
> > reallocate/rehash in a multithreaded application.
> >
> > Note, that this call must be issued before the first futex operation in the
> > process because that would automatically allocate the default sized hash.
> So this seems like it could be tricky for the user as system libraries, like
> glibc, make use of futexes. Can we guarantee that "sys_futex" is not called by
> the time main() is called?

To the extent of my testing I never observed that the hash was automatically
created when I called futex(PREALLOC) right away in main. But yes, that might
need some thought.