From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754881AbcESMZI (ORCPT ); Thu, 19 May 2016 08:25:08 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:46303 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754818AbcESMZG (ORCPT ); Thu, 19 May 2016 08:25:06 -0400 Date: Thu, 19 May 2016 14:24:57 +0200 From: Peter Zijlstra To: Thomas Gleixner Cc: LKML , Sebastian Andrzej Siewior , Linus Torvalds , Darren Hart , Ingo Molnar , Michael Kerrisk , Davidlohr Bueso , Chris Mason , "Carlos O'Donell" , Torvald Riegel , Eric Dumazet Subject: Re: [patch V2 3/7] futex: Add op for hash preallocation Message-ID: <20160519122457.GB3192@twins.programming.kicks-ass.net> References: <20160505204230.932454245@linutronix.de> <20160505204354.051976225@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160505204354.051976225@linutronix.de> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 05, 2016 at 08:44:05PM -0000, Thomas Gleixner wrote: > From: Sebastian Siewior > > 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?