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

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Message-ID: <20160519193819.GB1872@f23x64.localdomain>
Date:	Thu, 19 May 2016 12:38:19 -0700
From:	Darren Hart <dvhart@...radead.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	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 Thu, May 19, 2016 at 02:24:57PM +0200, Peter Zijlstra 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.
> > 
> > Add a futex op which allows to preallocate the hash with the requested
> > size. The size is limited by the systemwide maximum hash size, which can be
> > 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.
> > 
> > The function returns the actual hash size or 0 if the global hash is used. The
> > latter is the case on UP and in the rare case that the allocation failed and
> > the global hash is used as a fallback.
> 
> OK, so no on-line rehashing possible?

It doesn't do it currently... did you see something that makes it impossible to
add?


-- 
Darren Hart
Intel Open Source Technology Center