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Message-ID: <alpine.DEB.2.11.1605071046130.3540@nanos>
Date: Sat, 7 May 2016 10:47:38 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Darren Hart <dvhart@...radead.org>
cc: LKML <linux-kernel@...r.kernel.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Darren Hart <darren@...art.com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Michael Kerrisk <mtk.manpages@...glemail.com>,
Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
Carlos O'Donell <carlos@...hat.com>,
Torvald Riegel <triegel@...hat.com>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation
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@...utronix.de>
> >
> > 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.
Thanks,
tglx
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